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From the Bench
Gas Monitoring and Sensing (Part 1)
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Fun with Fragrant Analysis
Gas sensing ttec technology has come long way since the days of canaries in ccoal oa mines. This month columnist Jeff covers the background iiss issues surrounding gas monitoring and sensing. Then he desc describes crib how he uses sensors, A/D conversion and Arduino tech technologies hno to do oxygen measurement. By Jeff Bachioc Bachiochi chi
W
hen coal miners began dropping like flies, it was determined that poisonous gas was the culprit. To date there was no test to detect the presence of this odorless ghost. Sacrificial canaries became the guinea pigs, giving up their lives to save the miners. These birds are especially sensitive to methane and carbon monoxide. When the song bird stopped singing, miners headed for a breath of fresh air until the mine could be cleared of the silent killer. Seemingly ripe for disaster, the flame height of an oil lamp was used for detecting dangerous conditions in the 1800s. A shrinking flame indicated reduced oxygen, while a stronger flame indicated the presence of methane—or other combustible gas. Flame arrestors kept the combustion internal to the lamp, preventing external gas ignition unless it was dropped. In the 1900s, it was discovered that the current through an electric heater was affected when nearby combustible gases increased in temperature. The use of a catalytic material—such as palladium—lowers the temperature at which combustion takes place. Using these heaters in a Whetstone bridge configuration—where one leg is
exposed to the gas—can create an easily measured imbalance proportional to the concentration of the combustible gas. Infrared light can be used to measure the concentration of many hydrocarbon gases. When compared to a gas-free path, the IR absorption through a gas can indicate the concentration of hydrocarbon molecules. Gases can be identified by their molecular makeup. That is the amount of each element present. Absorption bands can be identified by dispersion through diffraction or nondispersion through filtration. Concentration is the relationship of a particular wavelength between a reference path and a gas absorption path. There are many techniques available today for monitoring gases. Refer to Table 1 for a breakdown of gas monitoring methods and their associated advantages and disadvantages. HAZMAT Class 2 in United States identifies all gases which can be compressed and stored for transportation. Even though we are not directly dealing with storage or transportation, the class is further defined by three groups of gases: flammable, toxic and others (non-flammable). You can see how a gas of interest is classified under HAZMAT rules in Table 2.
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CATALYTIC BEAD SENSORS The catalytic bead sensor comprises a bead or ‘pellistor’ made of a platinum coil with a ceramic coating (alumina) soaked with a special palladium catalyst. A catalyst enables
combustion to occur at lower temperatures without affecting the chemical equilibrium of a reaction. A Nickel-Chromium heating element passes through the pellistor raising its temperature and oxidizes the gas. The pellistor is supported within a flameproof body that allows the gas to enter via a stainlesssteel mesh and prevents combustion from exiting the sensor. A good tear down of this sensor can be seen at www.engineersgarage. com/insight/how-gas-sensor-works. NOTE: A slightly more complex sensor will have two beads. One is exposed to the gas in question, while the second is not—it is used instead as a reference. Catalytic gas sensors are calorimetric in nature. The platinum heating coil raises the internal temperature of the catalytic layer to where any available combustible gas will
Materials
Advantages
Disadvantages
Target Gases and Application Fields
Metal Oxide Semiconductor
Low cost Short response time Wide range of target gases Long lifetime
Relatively low sensitivity and selectivity Sensitive to environmental factors High energy consumption
Industrial applications and civil use
Polymer
High sensitivity Short response time Low cost of fabrication Simple and portable structure Low energy consumption
Long-time instability Irreversibility Poor selectivity
Indoor air monitoring Storage place of synthetic products as paints, wax or fuels Workplaces like chemical industries
Carbon Nanotubes
Ultra-sensitive Great adsorptive capacity Quick response time Low weight
Difficulties in fabrication and repeatability High cost
Detection of partial discharge (PD)
Moisture Absorbing Material
Low cost Low weight High selectivity to water vapor
Vulnerable to friction Potential irreversibility in high humidity
Humidity monitoring
Optical Methods
High sensitivity, selectivity and stability Long lifetime Insensitive to environment change
Difficulty in miniaturization High cost
Remote air quality monitoring Gas leak detection systems with high accuracy and safety High-end market applications.
Calorimetric Methods
Stable at ambient temperature Low cost Adequate sensitivity for industrial detection (ppth range)
Risk of catalyst poisoning and explosion Intrinsic deficiencies in selectivity
Most combustible gases under industrial environment Petrochemical plants Mine tunnels Kitchens
Gas Chromatograph
Excellent separation performance High sensitivity and selectivity
High cost Difficulty in miniaturization for portable applications
Typical laboratory analysis
Accoustic Methods
Long lifetime Avoiding secondary pollution
Low sensitivity Sensitive to environmental change
Components of Wireless Sensor Networks
TABLE 1 This summary of the basic gas sensing methods includes their advantages, disadvantages and general areas of use. (Source: www.equipcoservices.com/support/reference/ ionization-potentials-of-common-chemicals)
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Basically, a gas detector is a device which detects the presence of various gases within an area. This is usually part of a safety system that can indicate a hazardous condition. It might sound an alarm or otherwise alert humans to leave the area as many gases are harmful to organic life. This project delves into the catalytic gas sensors most often used to detect levels of combustible gasses—for example the mandatory carbon monoxide detector in your home. Catalytic gas sensors fall under the ‘calorimetric methods’ category of gas sensing techniques.
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burn on its surface. The additional heat from this combustion changes the resistance of the coil, which can be measured electronically. The limit of detection (LOD) for calorimetric sensors is typically in the low parts-perthousand (ppth) range. You may recall from elementary school fire prevention classes that there are three ways to extinguish a fire: eliminate the fuel, the air or the heat. A fire must have an adequate supply of each of these to sustain combustion. And our sensors must have these to operate as well. Let’s takes a closer look at this. We’ve just discussed how catalytic sensors use a heating element in conjunction with a catalyst
Gases
Nonliquefied Compressed Gas
Acetylene
(2)
Air
X
Liquefied Gas
to provide an adequate amount of heat to sustain combustion. We must also have a supply of oxygen (air) for our fuel to burn. By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide and small amounts of other gases. Air also contains a variable amount of water vapor—on average around 1% at sea level. Note: The minimum oxygen concentration for normal human breathing is 19.5%. That’s not much wiggle room! Obviously in an area where the typical atmosphere is artificially altered by the introduction of a combustible gas, the ratio of that gas to the oxygen content is changed.
Physical Hazards
Flammable Limits in Air (1) Vol %
Flammable
2.5 - 100
Inert
Corrosive
Toxic
Oxidizer
Allene
X
Flammable
2.2 - n/a
Ammonia
X
Nonflammable
15 - 28
Argon
Additional Gas Properties
X
Nonflammable
X X
Arsine
X
Flammable
Boron Trichloride
X
Nonflammable
X
X
Nonflammable
X
(4)
(3)
X
Boron Trifluoride
X
1,3-Butadiene
(5)
Flammable
5.1 - 78
2 - 11.5
Butane
X
Flammable
1.8 - 8.4
Butenes
X
Flammable
1.6 - 10
Carbon Dioxide
X
Nonflammable Flammable
12.5 - 74
X
Flammable
11.9 - 28.5
Carbon Monoxide
X
Carbonyl Sulfide Chlorine
X
Oxidizer
Cyanogen
X
Flammable
Cyclopropane
X
Deuterium
X
Diborane
X
Dimethylamine
X
X
(3)
Flammable
2.4 - 10.4
Flammable
4.9 - 75
Flammable
0.8 - 98
Flammable
2.8 - 14.4
Dimethyl Ether
X
Flammable
3.4 - 27
X
Flammable
3 - 12.4
Ethyl Acetylene
X
Flammable
(7)
X
Flammable
3.8 - 15.4
Flammable
2.7 - 36
Flammable
3.6 - 100
Ethyl Chloride X
Ethylene Oxide
(6)
Fluorine
X
Germane
X
Halocarbon 12 Halocarbon 13 Halocarbon 14
Flammable
(4) X
X (4)
(7)
(4)
X
Nonflammable
X
X
Nonflammable
X
Nonflammable
X
Nonflammable
X
X
Helium
X
Nonflammable
Hydrogen
X
Flammable
(4) (4)
Oxidizer
X
Halocarbon 22
X
6.6 - 32
Ethane
Ethylene
(4)
X 4 - 75
Hydrogen Bromide
X
Nonflammable
(3)
(4)
Hydrogen Chloride
X
Nonflammable
(3)
(4)
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OXYGEN SENSOR I chose the Winsen Electronics Technologies ME2-O2 Electrochemical Sensor
LiqueďŹ ed Gas
Physical Hazards
Hydrogen Fluoride
X
Nonammable
Hydrogen SulďŹ de
X
Flammable
4 - 44
Isobutane
X
Flammable
1.8 - 9.6
X
Flammable
1.8 - 9.6
Gases
NonliqueďŹ ed Compressed Gas
considered hazardous once 5% to 10% of the LEL has been reached. We just saw how our atmospheric content is critical to our existence. Given that the same is true of combustion, and that the LEL is based on knowing the oxygen level, we can either assume it is normal or measure it. In open air or ventilated areas, you may be able to assume the oxygen content. In closed areas, this is not the case. With that in mind, let’s look at oxygen measurement.
Isobutylene
Flammable Limits in Air (1) Vol %
Krypton
X
Nonammable
Methane
X
Flammable
5 - 15
Additional Gas Properties Inert
Corrosive X
(4)
(3)
(3)
(4)
X
Flammable
10.7 - 17.4
Methyl Mercaptan
X
Flammable
3.9 - 22
Monoethylamine
X
Flammable
3.5 - 14
X
Monomethylamine
X
Flammable
4.9 - 20.7
X
Neon
X
Nonammable
Nitric Oxide
X
Oxidizer
Nitrogen
X
Nonammable
Toxic
X
Methyl Chloride
(4)
X (3)
(4)
X
Nitrogen Dioxide
X
Oxidizer
(3)
(4)
Nitrogen Trioxide
X
Oxidizer
(3)
(4)
Nitrosyl Chloride
X
Oxidizer
(3)
(4)
Nitrous Oxide
X
Oxidizer
X
(4)
Oxygen
X
Oxidizer
Phosgene
X
Nonammable
Phosphine
X
Flammable
1.6 - 99
Propane
X
Flammable
2.1 - 9.5
X
Flammable
2 - 11
Flammable
1.5 - 98
Propylene Silane
X
Sulfur Dioxide
X
Nonammable
Sulfur Hexauoride
X
Nonammable
X
Nonammable
Trimethylamine
X
Flammable
2 - 12
Vinyl Bromide
X
Flammable
9 - 15
Vinyl Chloride
(5)
Flammable
3.6 - 33
X
Nonammable
(4)
(3)
(4)
X
(4)
X
Sulfur Tetrauoride
Xenon
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If the ratio is high in oxygen, there will be too little gas to support combustion—a lean mixture. On the opposite extreme, if the ratio is high in gas, that means there is too little oxygen to support combustion—a rich mixture. That all illustrates that the ratio has a sweet spot, outside of which no combustion can take place. Each gas has its own sweet spot or flammability range. The flammability of any gas has both a Lower Energy Level (LEL) point and an Upper Energy Level (UEL) point. The gas concentration between these two points is known as the Flammability Range. This range is different for each gas. A gas is
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X
X
(1) Flammable limits are normal atmospheric pressure and temperature. Other conditions will change the limits. (2) Dissolved in solvent under pressure. Gas may be unstable and explosive above 15 psig (1 bar). (3) Corrosive in the presence of moisture. (4) Toxic: It is recommended that the user be thoroughly familiar with the toxicity and other properties of this gas. (5) Cancer suspect agent. (6) Recognized human carcinogen. (7) Flammable, however, limits are not known. !
TABLE 2 Gas mixtures assume the categories of the components of the mixture, with the predominant component determining the final classification of the mixture. The exception for gases is when a component is toxic to a degree sufficient to influence the final classification. Table Copyright 2012 by the authors; licensee MDPI, Basel, Switzerland. www.ncbi.nlm.nih.gov/ pmc/articles/PMC3444121
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FIGURE 1 The physical layout of the connectors in this schematic place all of the gas sensors on the same plane. The PCB plugs onto an analog port of most Arduinos, with power and one DIO coming from the Arduino’s SPI connector.
to monitor oxygen level. This type of sensor generates a current and can be considered a micro fuel cell. It consists of an electrolyte speciďŹ cally chosen for its reaction to the gas of interest. Gas enters the sensor through a hydrophobic barrier—to eliminate most
//***************************************** // ME2-O2 Declare Start //***************************************** const float VRef = 5.0; // voltage of adc reference const int pinO2ADC = A0; // pin 54 is the first analog pin const String SignOnO2=�ME2-O2 5/22/2017�; //***************************************** // ME2-O2 Declare Start //***************************************** byte debug=1; // void setup() { Serial.begin(9600); //***************************************** // ME2-O2 setup Start //***************************************** Serial.println(SignOnO2); Serial.println(“Oxygen Sensor on pin:� + String(pinO2ADC)); //***************************************** // ME2-O2 setup End //***************************************** (continued) LISTING 1 The oxygen sensor application is in a generic form which will allow easy inclusion of additional analog sensors as necessary.
water from entering the sensor. The gas then eventually reaches the electrode—a cathode made of platinum, for example. Platinum is a catalyst for the reduction of oxygen. Oxygen atoms react with the electrolyte—potassium hydroxide for example—producing hydroxyl ions. Each resultant anion OH molecule has a negative charge attributable to reduction. O2 + 2H2O + 4e-- → 4OH-These hydroxyl ions migrate through the electrolyte carrying their negative charge to the lead anode. They react with the lead anode which is then oxidized into lead oxide. The charge is lost through oxidation. 2Pb + 4OH-- → 2PbO + 4e-- + 2H2O The water remains in the electrolyte. The charge will produce a current when the sensor’s cathode is connected to its anode with an external resistor. The current—and voltage—through the resistor is proportional to the amount of O2 (OH) that moves through the electrolyte. I purchased an oxygen sensor with a carrier PCB from LinkSprite that is similar to other gas sensors—although the pin out is not the same. I added a 4-pin header to make all gas sensor connections the same. This project will be based on the Arduino architecture. Arduino provides lots of analog and digital I/O and can handle oating point math, which we’ll need to present concentration levels for each sensor. The schematic I used in this initial experiment can be seen in Figure 1. It was designed to take advantage of one analog
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(Listing 1 continued) } // void loop() { //***************************************** // ME2-O2 loop Start //***************************************** float Vout =0; Vout = readO2Data(); if(debug && 1) { Serial.print(“Vout =”); Serial.print(Vout); Serial.print(“ V, “); } // float Concentration=0; Concentration = calcO2Concentration(Vout); Serial.print(“O2 is “); Serial.print(Concentration); Serial.println(“%”); //***************************************** // ME2-O2 loop Start //***************************************** delay(500); } //***************************************** // ME2-O2 Support Start //***************************************** float readO2Data() // average 32 samples { long sum = 0; for(int i=0; i<32; i++) { sum += analogRead(pinO2ADC); } sum >>= 5; float MeasuredVout = sum * (VRef / 1023.0); return MeasuredVout; } float calcO2Concentration(float MeasuredVout) { // Vout samples are with reference to 5.0V //float Concentration = 21%, when its output voltage is 2.0V, float Concentration = MeasuredVout * 0.21 / 2.0; float Concentration_Percentage=Concentration*100; return Concentration_Percentage; } //***************************************** // ME2-O2 Support End //*****************************************
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port (8 sensors) on the Arduino. Power— and a single digital I/O—comes from the ICSP 2x3 header. While all the gas sensors have identical 4-pin connections, there is a separate 4-pin connector for a humidity and temperature sensor. We’ll get to this shortly. Let’s begin with this oxygen sensor connected to A0. The 4-pin sensor connector will contain the following signals: VCC , ground, digital out and analog out. While many of the sensor carrier PCBs contain support circuitry with a comparator for level sensing (digital) output, I won’t be using this pin. The switching level can be set via an on-board pot. Basically, each sensor will provide an analog output to the 10-bit A/D converter of the Arduino. Each 4-pin sensor connector has its analog output connected to a different analog input pin. Support for each sensor will therefore have three parts: definition, setup and loop—and support routines. Listing 1 shows these for the oxygen sensor in our first Arduino program. The declare section initializes the floating point 5.0 reference voltage we’ll be using in all ADC calculations. We are designating a connection to analog channel 0 (pin 54 on the MEGA 2560) for the Oxygen sensor. I like to use a SignOn message which includes the program name and date in all my Arduino programs. Once an Arduino has been programmed with an application and gets set aside, it is impossible to figure out what it’s been programmed with unless you leave yourself a trail of crumbs! In the setup section, the serial port is initialized for display (and debug) and the SignOn message is sent out. For most applications, the USB console port makes it simple to display information. The main loop handles access to the sensor support routines for collecting data. This loop can be used to continuously send formatted data to the console. Here I use a debug flag to include or skip intermediate data—such as analog voltage measured from the port. Two support routines are used for the oxygen sensor: collect and concentration. The collection routine handles requesting the ADC’s representation of the analog input voltage and converting this back to a voltage. We know the ADC breaks the reference voltage into 1,024 distinct voltage levels and will compare each of these to the analog input to find the closest match. You can query the ADC to find out which one of those levels it has determined is the closest to the actual input. To convert this 10-bit digital representation of the input voltage back into a voltage, we need to know what voltage value each bit represents. In this case the reference voltage was defined as 5.0 V. This is divided by the ADC
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This means its output must be less than 3.3 V. In this case 2.0 V indicates a 21% concentration of Oxygen. The equation using this fact is:
concentration =
ADC voltage × 21% 2.0 volts
or
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ADC voltage × 0.21 2.0 volts Notice when ADC voltage = 2.0 V, the 2.0s cancel leaving 0.21 or 21%. And when ADC voltage = 0 V we get 0/2.0 or 0%. The sensor is rated to 25% (30% max). FIGURE 2 This graph shows how humidity and temperature affect the oxygen content of the atmosphere we breathe. As the humidity increases the concentration of oxygen falls, which makes it more difficult for us to take in adequate oxygen. (Source: "Effect of Humidity and Temperature on Galvanic Oxygen Sensors" aii1.com/PDF/r_s_humd.pdf)
resolution 1024-1 (10-bits) to get 4.9 mV/ bit. Multiply 4.9 m times the ADC conversion number to get the input voltage represented by that ADC conversion number. Note: The calculated voltage isn’t exact, but should be within +/- one-half bit of the actual voltage. The higher the ADC resolution, the closer the conversion will be. It’s typical for an analog voltage to be a bit noisy, especially when amplifiers are involved. Averaging multiple ADC readings is a good way to smooth out that fluctuation—unless that’s your data of interest. For this sensor, we do this by taking multiple samples and dividing their sum by the number of samples. The concentration routine calculates the concentration of oxygen represented by the analog voltage. The data sheet for the oxygen sensor gives a graph that shows the sensor’s output voltage in relation to oxygen concentration. The carrier PCB has an amplifier which multiplies this output to a level that we can more easily measure by an ADC using either a 3.3 V or 5.0 V reference.
RESOURCE Tear down of catalytic sensor technology (pages 1-3) www.engineersgarage.com/insight/ how-gas-sensor-works circuitcellar.com/ccmaterials
HUMIDITY/TEMPERATURE Ever notice how it gets more difficult to breathe as the humidity goes up? For many with chronic breathing issues, the only relief is moving to a more suitable climate. The level of discomfort we feel in high temperatures is closely associated with the dew point. The dew point is the temperature at which the water vapor in a sample of air condenses into liquid water at the same rate at which it evaporates. At temperatures higher than the dew point, evaporation is taking place while lowering humidity. A relative humidity of 100% indicates the dew point is equal to the current temperature and that the air is saturated with water. When the moisture content remains constant and temperature increases, relative humidity decreases. All this is true for any given barometric pressure. When the air temperature is high, the human body uses the evaporation of sweat to cool down. The cooling effect is directly related to how fast the perspiration evaporates. And this directly relates to how much moisture is in the air and how much moisture the air can hold. If the air is already saturated with moisture, perspiration will not evaporate and you just remain sweaty. As you might imagine, the concentration of oxygen can be affected by both humidity and temperature. Refer to the graph in Figure 2 to see how they affect O2 concentration. Knowing this, it might be good idea if we monitor both the humidity and temperature to better understand what is happening in our environment. From the graph, we can see that with a high relative humidity and temperature the O2 content can become dangerously low. I’ve chosen the combo humidity/ temperature sensor module from Seeed Studio which uses the HDT11 sensor from
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Aosong Electronics (Photo 1). You’ll note the schematic (Figure 1) has one digital I/O connection for this module as it will transmit its data serially whenever the data line is forced low for at least 18 ms. Once the sensor sees a low on its data line for greater than 18 ms—and a release of the bus—it will wake up and send an acknowledge consisting of a low sink pulse of 80 ns and a release of the bus (pull-up) for 80 µs. The data consists of 40 bits: the onebyte integer portion of the humidity followed by one byte of the decimal portion of the humidity value. The second two bytes are in the same format for temperature. The final byte is a checksum—the least significant byte (LSB) of the total of the first 4 bytes. Each bit is determined by the length of time the data bus stays high after the bus has been held low for 50 µs. A data=0 remains high for only approximately 35 µs, however a data=1 remains high for approximately 70 µs. This means the sensor will again pull down the bus to signify the end of the 40th bit. The response should therefore take less than 5 ms. The sensor samples data in around 1 second, so it won’t help to read the data any faster. Current consumption is about 1 mA while active and 100 µA in standby. While we don’t need to convert and data from the sensor, we do need to receive and interpret it. Refer to Listing 2 to see how I chose to do this using the pulseIn() command. We wake up the sensor with a low pulse of a minimum of 18 ms, followed by a release of the bus—changing the pin’s mode to INPUT_PULLUP in preparation of the senor’s response. The sensor will then send an acknowledge pulse of 80 µs low and an idle (high) of 80 µs. Here I look for HIGH pulse in using pulseIn(pinDHT11, HIGH, 200)==0). NOTE: Since the bus is now HIGH (pulled up) the command will wait for the input to go LOW and then HIGH, before beginning to measure the HIGH duration. The 200 is an optional timeout (200 µs). We know response from the sensor contains a low 80 µs pulse, so if we don’t see one in 200 µs, then we escape the routine and indicate an error (timeout) condition. If the response is seen, then we can continue on receiving the next 40 bits or 5 bytes of data. The 5 bytes of data are received via a call to the getAByte() routine. These bytes contain the integer and decimal data for the humidity and temperature, plus a checksum byte—the LSB byte of the total of the first 4 bytes. We can compare the checksum and report any error condition and then display any ‘good’ data. The display format is “Humidity: x.x%” and ”Temperature: x.xC”.
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PHOTO 1 This screenshot/photo shows the application output with the sensor array PCB hanging off the Arduino’s analog port. The ME2-O2 oxygen sensor is plugged into slot ‘0’ (of 8). The DHT-11 humidity/temperature sensor has its own digital port—sticking up with the blue box shaped sensor.
The data for each of the 5 bytes is collected by using pulseIn(pinDHT11, HIGH, 150). The data is determined by the duration of the HIGH time for each data bit. A data bit consists of a low pulse of 50 µs, followed by an idle (HIGH) time of approximately 30 µs for a data=0 and 70 µs for a data=1. I’m testing the pulseIn() result first for ‘0’ to see if the sensor is responding, and then greater than 35 µs to indicate a data=1, else data=0. I keep track of the data via the variable Value initialized to ‘0’. After each bit test, Value is shifted left 1 bit and then ‘1’ is added only if data=1. The data is received most significant bit (MSB) first, so the shift is to the left.
ENVIRONMENTAL MONITORING We’ll begin next month by looking at a few of the inexpensive gas sensors available to us as well as the general routines required
ABOUT THE AUTHOR Jeff Bachiochi (pronounced BAH-key-AHkey) has been writing for Circuit Cellar s i n c e 1 9 8 8 . H i s b a c kg ro u n d i n c l u d e s product design and manufacturing. You can reach him at: jeff.bachiochi@imaginethatnow.com or at: www.imaginethatnow.com.
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LISTING 2 The DHT-11 humidity/temperature sensor requires its own digital communication routine. The sensor is factory calibrated and reports in a two-byte integer/decimal format for humidity and temperature.
//***************************************** // DHT-11 Declare Start //***************************************** #define pinDHT11 52 // PB1 (D52) byte dataDHT11[5]; const String SignOnDHT11=”DHT-11 5/22/2017”; //***************************************** // DHT-11 Declare End //***************************************** byte debug=1; // void setup() { Serial.begin(9600); //***************************************** // setup DHT11 Start //***************************************** pinMode(pinDHT11, OUTPUT); // sets the digital pin as output digitalWrite(pinDHT11, HIGH); // sets the digital output high Serial.println(SignOnDHT11); //***************************************** // setup DHT11 End //***************************************** } // void loop() { //***************************************** // loop DHT11 Start //***************************************** if(readDHT11Data()) { Serial.print(“Current humdity = “); Serial.print(dataDHT11[0], DEC); Serial.print(“.”); Serial.print(dataDHT11[1], DEC); Serial.print(“% “); Serial.print(“temperature = “); Serial.print(dataDHT11[2], DEC); Serial.print(“.”); Serial.print(dataDHT11[3], DEC); Serial.println(“C “); } //***************************************** // loop DHT11 End //***************************************** delay(2000); } //***************************************** // DHT11 Support Start //***************************************** boolean readDHT11Data() { digitalWrite(pinDHT11, LOW); // force i/o pin low for 18ms delay(18); digitalWrite(pinDHT11, HIGH); // force i/o pin high for 40ms delayMicroseconds(40); pinMode(pinDHT11, INPUT); // 3. i/o pin now input and wait 40ms delayMicroseconds(40); byte inDHT11 = digitalRead(pinDHT11); // read state of i/o pin if(debug & 1) // report status errors? { if(inDHT11) // high?
(continued)
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(Listing 2 continued) { Serial.println(“DHT-11 low condition illegal”); pinMode(pinDHT11, OUTPUT); // sets the digital pin as output digitalWrite(pinDHT11, HIGH); // sets the digital output high return 0;
} for (byte i=0; i<5; i++) // read 40 bits (5 bytes) { byte result=0; for(byte j=0; j< 8; j++) { while(!digitalRead(pinDHT11)); // wait for 50us delayMicroseconds(30); if(digitalRead(pinDHT11)) { result |=(1<<(7-j)); } while(digitalRead(pinDHT11)); // wait ‘1’ finish } dataDHT11[i]=result; } // 7. force i/o high as output pinMode(pinDHT11, OUTPUT); // sets the digital pin as output digitalWrite(pinDHT11, HIGH); // sets the digital output high if(debug & 1) { byte checksumDHT11 = dataDHT11[0]+dataDHT11[1]+dataDHT11[2]+dataDHT11[3]; if(dataDHT11[4]!= checksumDHT11) { Serial.println(“DHT-11 checksum error”); } } pinMode(pinDHT11, OUTPUT); // sets the digital pin as output digitalWrite(pinDHT11, HIGH); // sets the digital output high return 1; } //***************************************** // DHT11 Support End //*****************************************
to convert the sensor’s data. Environmental monitoring is being used to establish air pollutant concentrations. Air monitors are operated by citizens, regulatory agencies and researchers to investigate air quality and the effects of air pollution on us and our world. Our fragile weather system is entangled with what we put in the environment. On January 23, 1978, Sweden announced it would ban aerosol sprays containing chlorofluorocarbons (CFCs) as the propelling
agent. Scientific evidence had mounted that CFCs were damaging to Earth’s ozone layer. Virtually every country on Earth ultimately followed Sweden in banning CFCs, via an international treaty known as the Montreal Protocol by January 1, 1989. We are still evaluating the results today, but monitoring suggests there is evidence of a reversal. What other pollutants could be affecting how the atmosphere protects our world? Can we have any significant impact on Mother Nature?
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} delayMicroseconds(80); inDHT11 = digitalRead(pinDHT11); if(!inDHT11) // low? { Serial.println(“DHT-11 high condition illegal”); pinMode(pinDHT11, OUTPUT); // sets the digital pin as output digitalWrite(pinDHT11, HIGH); // sets the digital output high return 0; } delayMicroseconds(80);
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From the Bench
Gas Monitoring and Sensing (Part 2)
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Putting the Sensor to Work
Jeff continu continues es his exploration of gas monitoring and sensing. Thi Thiss ttime he discusses some of the inexpensive sensors available avaiilab that can be applied to this application. Jeff then tac tackles ckle the factors to consider when calibrating these sensor sensors rs and a how to use them effectively. By Jeff Bachi Bachiochi ioc
L
ast month’s article left you with the question: “Can we have any significant impact on Mother Nature?” According to Simon Lewis, an ecologist at University College London, and geologist Mark Maslin of Leeds University, a massive dip in carbon dioxide levels can be seen in Antarctic ice cores dating back to 1620, when the Mayflower arrived in the New World. They suggest this is the result of as many as 50 million Native Americans dying due to infectious diseases such as smallpox brought over from Europe. As their numbers dwindled, the resultant loss in agriculture allowed forests to re-grow throughout the Americas. These expanded forests scrubbed the atmosphere of carbon dioxide. Whether this makes sense to you or not, you can’t help but see the effect we have on water, air and soil. It’s hard to separate any one of these from the others as they are so entwined with the global weather of our planet. The 2011 Tōhoku earthquake and tsunami damaged nuclear facilities in Fukushima. Dust particles contaminated with radioactive cesium were found more than 100 miles from the site, and in April of that same year, particles could be detected on the West Coast of the U.S. We are all connected
caretakers of this planet and it's foolish to think: “It doesn’t affect me.” Fortunately for our planet, our health is also being affected by these same pollutants. While the planet can survive without us, we can’t survive without the environment. But it’s not all doom and gloom. Our civilization is making small changes that protect us. The weatherman warns us of unhealthy conditions, such as the UV index and air quality levels. Building codes now require carbon monoxide detectors in addition to smoke alarms in our homes. There are global discussions on CO2 reduction. Meanwhile, the recent withdrawal from the Paris climate accord by the US is unfortunate as it revokes our reduction level promises. And in turn, meeting global reduction goals is now in jeopardy. It’s now more important than ever to be able to identify and monitor those components that jeopardize humankind. Air pollutants can be blown either to a new location or cleansed from low earth atmosphere by weather conditions. While the atmospheric cleansing process helps us breathe easier, it really only changes the habitat of pollutants from the air to the soil, where they can affect crop growth and our food chain. Besides their pollutant side effects, technology has given us
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the ability to measure those harmful/valuable elements around us. The oxygen content makes up only around 20% of our breathable atmosphere. Yet its concentration is critical to life and other functions, such as combustion, that we take for granted every day.
LOOKING BACK
Sensor
Sensing Resistance
Heater
Resistance in Gas = Resistance in Air
MQ-2
2 kΩ - 20 kΩ
5V
(1,000 ppm H2) hydrogen
LPG, methane, carbon monoxide, alcohol, propane
MQ-3
2 kΩ - 20 kΩ
5V
(0.4 mg/l OH) alcohol
hydrogen, methane, carbon monoxide
MQ-4
2 kΩ - 20 kΩ
5V
(5,000 ppm CH4) methane
hydrogen, LPG, carbon monoxide, alcohol
MQ-6
2 kΩ - 20 kΩ
5V
(2,000 ppm C3H8) LPG/propane
hydrogen, methane, carbon monoxide, alcohol
MQ-7
2 kΩ - 20 kΩ
5 V / 1.5 V
(100 ppm CO) carbon monoxide
hydrogen, methane
MQ-8
10 kΩ - 60 kΩ
5V
(1,000 ppm H2) hydrogen
LPG, methane, carbon monoxide, alcohol LPG, methane hydrogen, sulfide, benzene
MQ-9
2 kΩ - 20 kΩ
5 V / 1.5 V
(600 ppm CO) carbon monoxide
MQ-135
2 kΩ - 20 kΩ
5V
(100 ppm NH3) ammonia
Suggested Other Gases
MQ-137
2 kΩ - 15 kΩ
5V
(50 ppm NH3) ammonia
hydrogen, ethanol
MQ-138
2 kΩ - 20 kΩ
5V
(50 ppm toluene)
methanol, acetone, ethanol, hydrogen
MQ-216
30 Ω – 200 Ω
6V
(1,000 ppm) isobutane
LPG, methane, alcohol, propane butane, hydrogen, ethanol
MQ303A
4 kΩ - 400 kΩ
0.9 V
(1,000 ppm) alcohol RS/RO=0.1
MQ306A
2 kΩ - 200 kΩ
0.9 V
(1,000 ppm) butane RS/RO=0.1
methane, hydrogen, ethanol
MQ309A
2 kΩ - 20 KΩ
0.9 V / 0.2 V
(1,000 ppm CH4) methane
hydrogen, ethanol, carbon monoxide
TABLE 1 Here are a few gas sensors you can find for sale on the internet. Carrier PCBs are also available that run on 3.3 VDC to 5 VDC. Most have both analog and digital outputs. The digital output switching point can be set with an on-board pot.
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Last month we looked at the principal ways in which a gas might be measured. One of those methods is by measuring the heat produced through catalytic combustion. This type of sensor is constructed of a catalytic bead impregnated with a special catalyst that promotes oxidation and a fine platinum wire embedded in the bead. Current is passed through a heating element causing the bead to reach a temperature at which oxidation of a gas readily occurs (about 500°C). The combusted gas raises the temperature further which increases the resistance of the platinum coil in the catalyzed bead. This change can be measured, and is linear for most gases. Note that a minimum oxygen content is required for oxidation to take place. It only made sense to begin this project by measuring oxygen content. This was accomplished by using a ME2-02 sensor. Its output, like most gas sensors, have been tailored to present an analog voltage output that is related to
gas concentration. I began the project using an Arduino to provide access with at least 8 analog inputs capable of measuring input voltage with a precision of 10 bits or 4.9 mV per bit (5V / 1024 bits = .0049 V). Another advantage of using an Arduino is its friendly math functions including floating point arithmetic that makes analog conversions much easier—and more precise. Before concluding the last article, I added another sensor that also has a bearing on how gases react. Humidity and temperature can affect the concentration of gas in the atmosphere, so I added a DHT-11 combination sensor that communicates digitally via a 1-bit bus. This sensor is factory calibrated to output humidity and temperature whenever it recognizes that the normally high idle bus state is forced low for at least 18 ms. It outputs the integer value and decimal value of the humidity followed by the integer value and decimal value of the temperature along with checksum (LSB of the sum of the previous 4 bytes). A simple application was presented to demonstrate acquiring data for these sensors. Now it’s time to discuss some of the available catalytic pellistor type sensors that can be used in this project. Table 1 is a list of the inexpensive sensors I picked up to
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FIGURE 1 This is the sensitivity graph from the MQ-2 hydrogen gas sensor data sheet. Note the range of the sensor is from 200 ppm to 10,000 ppm.
10
Air Hydrogen Lpg Alcohol Carbon monoxide Methane Propane
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Ratio 1
0.1 100
1,000
10,000
ppm
experiment with. Many portable instruments use these sensors in their products. Most handheld instruments are designed for one particular gas of interest, but you will find that most are sensitive to multiple gases. Lets use the first one on the list—MQ-2 (hydrogen) sensor—to illustrate how they are to be used.
LOOKING FORWARD The MQ-2 sensor is optimized for measuring hydrogen. This means the sensor materials have been selected to have optimum response to hydrogen. The MQ-2’s data sheet presents a graph (Figure 1) of the sensor’s sensitivity to specific gas as its resistance ratio to parts per million (ppm) gas concentration. It’s resistance ratio is unity at
(100) 2 Slope = (0.3–[− 0 .5]) ⁄ (2.3 –4) = − 0.8 ⁄ 1.7 = –0.47 (31.6) (10) 1 H2 (2.05) (1) 0 (0.31)
(200) (0.1) –1
(0.1)–1
(1) 0
(10)1
(100)2
(1,000)
(10,000) (100,000)
FIGURE 2 The log-log graph from Figure 1 has been labeled with base 10 in parenthesis. The values from Figure 1 are in the yellow oval. Note the slope is extended to the estimated point of intersection for 1 ppm. The slope of the H2 sensor can be calculated at -0.47. Since we know the reference point at 1,000 ppm is (0,3) and the slope of the line, we can calculate the point on the x axis (x,0) as 0 - 3= -3. Here’s the math: -3 x -0.47 = 1.41 (1.41, 0). This point references the resistance of the sensor with no hydrogen.
1000 ppm of hydrogen. This resistance ratio is the resistance of the sensor at different concentrations of gas over its reference resistance at 1,000 ppm. The graph also shows a minimum and maximum ppm that can be measured using the sensor. The span of concentration can be considered linear. It might help to see this graph presented with its base 10 representation (Figure 2). You’ll note that with two points on the graph’s H2 (hydrogen) line we can determine its slope. This slope represents the resistance of the sensor as it changes due to gas concentration. Using one reference point (at 1,000 ppm) and the line’s slope, we can project where the sensor might see say 1 ppm hydrogen. It is a point about 1.5 times greater than that of the reference point (at 1,000 ppm). In reality, there is some amount of hydrogen in ‘fresh’ air and the graph in Figure 1 has this as the horizontal line labeled ‘air’. Why is this important? We don’t know the actual resistance of the sensor. If we could apply exactly 1,000 ppm of hydrogen to the sensor and measure its resistance, we would have a calibrated resistance for this reference point and could base all measurements from this calibration point. If you want to truly calibrate the sensor, you will need to have access to a supply of gas in this concentration. For this project, I will use fresh air as the calibration point. Calculating this reference point (as explained earlier) gives us a place to start, because we can measure the sensor‘s resistance while exposed to fresh air.
ANALOG The datasheet suggests using a load resistor of from 5 kΩ to 47 kΩ for this sensor. I’ve found my carrier boards to have a 1 kΩ resistor in this location. This simplified circuit shows a 1 kΩ resistor from the analog input
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to ground, with the sensor between +5 V and the analog input: +5V-----RX-----A0-----1K-----GND
VL =
' raw ADC value ' × Vref 1023
(V L is the voltage calculated across R L from the ADC value)
float VL = analogRead(chanel) ×
Vref 1023.0
VS = Vref − V:L (voltage across the sensor = Voltage Reference - V L)
IL =
VL 1000
(current through R L = voltage across the load resistor/its value)
RX
A0 Voltage Tap (with Vref= 5 V)
1 kΩ
100 Ω (1/10 of R1)
4.54 V
1 kΩ
1 kΩ (=R1)
2.5 V
1 kΩ
10 kΩ (10 times R1)
0.45 V
RS =
VS IL
(resistance of the sensor = voltage across the sensor/current through it) Putting those together:
⎛ Vref − VL ⎞ ⎟⎠ ⎜⎝ V (V − VL ) L = 1000 × ref RS = 1000 VL R S [channel] = 1000 ×
(Vref − VL [channel]) VL [channel]
Note: The actual code uses arrays for some values. This will allow the same routine to be used for additional sensors. The sensor resistance (in fresh air) is just less than 10 times (from the graph) that of the resistance at 1,000 ppm. So, we can set RO = 14032/10 or 1403 Ω. This is our calibration reference. From here on out we’ll use this with actual measurements to determine the sensitivity RS/RO and to determine gas concentration along with the slope. Ideally, we like to have our R L equal to that. The carrier board has a 1 kΩ resistor on it so we’ll work with that. We determine the slope using points taken from the graph. Here’s the math to determine the slope: MQ2[] = {2.10, 200, 0.31, 10000}; where data is X and Y values for 2 points on the graph: {point 1X, point 1Y, point2X, point 2Y} X1=pow(10, MQ2[0]); X2=pow(10, MQ2[2]); Y1=pow(10, MQ2[1]); Y2=pow(10, MQ2[3]); slope = Y1-Y2 / X1-X2; slope = 0.32 - (-0.51) / 2.3 -4 = 0.83/1.7 = -0.49 1,000 ppm is equal to 0.1% concentration. So, this sensor (when R L = R S at 1,000 ppm) will most accurately measure 0.02% to 1% concentration. You can see that selecting a fixed-load resistor is important to the how the measured voltage will relate to the gas concentration.
SENSOR MEASUREMENT VS CONCENTRATION We now have the basics to determine the sensor’s resistance and the data required to extrapolate how a new sensor resistance relates gas concentration based on the
TABLE 2 This shows the relationship between the load resistor across the analog input and a change in sensor resistance due to gas concentration. The most accurate readings will be when the two are equal. This will diminish greatly as the sensor’s factor surpasses 10 in either direction— either 1/10th or 10x.
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If the resistance of the sensor RX = 1 kΩ, then the +5 V is equally divided and +2.5 V is applied to the analog input. As the RX increases, the analog voltage goes down. As RX decreases, the analog voltage goes up. The change is not linear however. The best resolution occurs around the point where the load resistor equals the sensor resistance at the 1,000-ppm reference point. Table 2 shows the relationship between the load resistor across the analog input and a change in sensor resistance due to gas concentration. The most accurate readings will be when the two are equal. This will diminish greatly as the sensor’s factor surpasses 10 either way (either 1/10 or 10x). Note that with R1 = 1 kΩ, if the sensor’s resistance is either less than 1/10th of R1 or greater than 10 X R1, then the ADC voltages quickly approach values that can no longer be differentiated. Hold that thought for now and look more closely at the resistance of the sensor in fresh air. Using the application previously written for the humidity/temperature and oxygen sensors, we can add an additional sensor to analog input 1 and print out the voltage measurement from the stock carrier board. Assuming we have fresh air conditions, the gas concentration should be about nil. With the Arduino powered from USB, the Vref = 4.67 V. The MQ-2 sensor reads 0.31 V. From this we know two things: the voltage across the sensor Vx = 4.67 (Vref ) - 0.31 (ADC) or 4.35 V, and the current through R1 =0.31 V / 1,000 Ω or 0.31 mA. This current also runs through the sensor, so its resistance must be 4.35V/0.00031A or 14,032 Ω in fresh air.
R1
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calibration resistance of 0 ppm. Again, we can start with the slope. We know point2 X2 and Y2 from MQ2[] and the slope (-0.47). When we use log(RS/RO) for Y1, we can solve for X1 like this:
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slope slope x (X1-X2) X1-X2 X1 Next, un-log the
= (log(RS/RO)-Y2) / (X1-X2) = (log(RS/RO)-Y2)) = (log(RS/RO)-Y2) / slope = ((log(RS/RO)-Y2) / slope)+X2 result to get the ppm.
ppm[channel] = ⎛ ⎛ RS ⎞⎞ − Y2 log ⎜ ⎜ ⎟⎟ RO + X 2⎟ ⎟ pow ⎜10, ⎜ slope ⎜ ⎜ ⎟⎠ ⎟ ⎝ ⎝ ⎠
So, if the sensor measurement is RS= 1403 then we have:
ppm[channel] = ⎛ ⎛ 1403 ⎞⎞ − ( −0.51) log ⎜ ⎜ ⎟⎟ 1403 + 4⎟ ⎟ pow ⎜10, ⎜ −0.49 ⎜ ⎜ ⎟⎠ ⎟ ⎝ ⎝ ⎠ ppm[channel] = ⎛ ⎛ log (1) − ( −0.51) ⎞ ⎞ + 4⎟ ⎟ pow ⎜10, ⎜ −0.49 ⎠⎠ ⎝ ⎝ ⎛ ⎛ 0 − ( −0.51) ⎞ ⎞ + 4⎟ ⎟ ppm[channel] = pow ⎜10, ⎜ ⎠⎠ ⎝ ⎝ −0.49
⎛ ⎛ 0.51 ⎞⎞ + 4⎟ ⎟ ppm[channel] = pow ⎜10, ⎜ ⎝ ⎝ −0.49 ⎠ ⎠ ppm[channel] = pow (10, ( −1.04 + 4))
ppm[channel] = pow (10, ( 2.96)) ppm[channel] = 912
WebPlotDigitizer app scanned the plot above and output the following: 9.292391310855573, 200.5202666126466 9.464455149705113, 9999.99999999996 5.261572243931346, 200.5202666126466 1.4041398293327734, 9999.99999999998 3.0343921987338427, 200.5202666126468 0.712179783743607, 9999.99999999998 2.8196759701242295, 200.5202666126468 0.6497540631502284, 10000 2.0641489636309776, 200.5202666126468 0.335657545405408, 10000 1.6869089152540644, 200.5202666126468 0.27939275455712637, 10000 1.5675417746311666, 200.5202666126468 0.25026860594701816, 10000 FIGURE 3 Using the WebPlotDigitizer app, I prepped by scanning this graph and downloading it into the application on line. After picking two X and Y axis points along with the data of interest, the app listed all the points with X and Y coordinates. I captured the data and entered it into my application.
With the data points estimated from the data sheet graphs, the calculations are within about 9% of the actual reference point (at 1,000 ppm). Perhaps we could have better results with more accurate data? Actually, there are applications that enable you to extract data from graphs. I found WebPlotDigitizer online at arohatgi.info/ WebPlotDigitizer/app. This browser based app lets you import a picture of a graph, calibrate the axis, pick data points and present a list of the points. I copied the MQ-2 graph and picked off the end points of each element. Figure 3 shows this graph and the data. So far, we’ve dealt with only H2, but I took extracted data for all the graph elements. If I use the data for H2 from this list for the calculations above, the error between calculated and reference gets cut in half. This shows the importance of the data GIGO (Garbage In, Garbage Out) concept. While a sensor is optimized for a specific element, it will also be sensitive to other elements. This means that a particular sensor will be affected by other elements and you may be seeing concentrations of those. It is important therefore, to know what gasses you are expecting in your environment.
ADDING ADDITIONAL SENSORS With the basics down for calculating gas concentration, I added not only the other
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the setup with a full complement of sensors and truncated application output.
SENSOR INCOMPATIBILITY You’ll note that I used only 5-V compatible sensors with the circuit presented last month. Of the list of sensors in Table 1, a few sensors require heater voltages other than 5 V. In fact, for proper operation some sensors require two heater voltages. A change in heater voltage (temperature of the pellistor) can be used to adjust combustion temperatures for specific gases. Or it might be used to burn off contaminants that would affect the reading of a gas. You may need to design a special circuit for using one of these sensors. The //***************************************** // MQ-2 Declare Start //***************************************** const int pinMQ2ADC = A1; // pin 55 is the second analog pin const String SignOnMQ2=”MQ-2 6/12/2017”; const int MQ2Elements = 7; const float MQ2[(MQ2Elements*5)+1] = { 0, 9.108990, 200.0, 9.420527, 10000.0, 5, 5.261572, 200.0, 1.404139, 10000.0, 8, 3.034392, 200.0, 0.712179, 10000.0, 1, 2.819675, 200.0, 0.649754, 10000.0, 6, 2.064148, 200.0, 0.335657, 10000.0, 9, 1.686908, 200.0, 0.279392, 10000.0, 7, 1.567541, 200.0, 0.250268, 10000.0}; // Sensitivity from graph [gasNames[],Y1,X1,Y2,X2] float MQ2Slope[MQ2Elements+1]; float MQ2PPM[MQ2Elements+1]; //***************************************** // MQ-2 Declare End //*****************************************
Where the gas list is: String gasNames[]={ “Air”,”Alcohol (OH)”, “Ammonia (NH3)”, “Benzine “, “Butane (C4H10)”, “Carbon Monoxide (CO)”, “Hydrogen (H2)”, “Liquid Propane Gas (Propane/Butane)”, “Methane (CH4)”, “Propane (C3H8)”, “Hexane (C6H14)”, “smoke “, “Carbon Dioxide (CO2)”, “Toluene (CH3)”, “Acetone ([CH3]2CO)”}; LISTING 1 Shown here is the data for a typical complete sensor, the MQ-2 in this case.
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gasses listed on a sensor’s data sheet, but also seven other sensors for a total of eight— including oxygen. It was easy to add these using the WebPlotDigitizer application mentioned earlier. The data for a typical complete sensor, like the MQ-2, is seen in Listing 1. In most cases you won’t have a need to use eight sensors. This project assigned particular sensors to the first eight analog inputs on the Arduino Mega board. One of the effects of using a full complement of eight sensors is the total current needed power them— especially from a USB port. Each sensor adds up to 200 mA to the circuit current. A USB hub will limit current to 100 mA unless it is a powered hub, which limits current to 500 mA. Therefore, a full complement of sensors will cause a shut down USB—putting the hub in self-preservation mode. Applying external power through the on-board Arduino regulator has problems as well. The regulator on my Mega is the NCP1117ST50T3G. This is a surface mount SOT-223 part that can handle 1 A at 12 V. You couldn’t heatsink enough heat away from this device at those levels. The 5.0 V fixed regulation would drop 7 V across it and at 1 A that would result in 7 W. This is a lowdrop out regulator that requires at least 1.2 V for regulation. With a 7 V input and a full complement of sensors, that would be approximately 640 mA which is greater than 1.3 W (2V x 0.64A). Still quite warm for such a tiny device. If all eight sensors were required, you could power the board from an externally regulated wall wart power supply. For many this won’t be an issue, but it was of concern to me in this project. With an analog input left unconnected (no sensor plugged in) the analog reading just floats around. That’s because there is no real load of any kind on the input. I added a high resistance load across each input that would not interfere with a sensor when plugged in. This causes the input to get pulled to ground when no sensor is connected. In the calibration routine that runs every time the application is started, I calculate RO[channel] to determine whether or not a sensor is attached and give an appropriate message when necessary. All the sensor datasheets suggest each sensor be preheated or burned in for 48 hours prior to trusting any output. This must be repeated if the sensor is left unpowered for any great length of time. This can easily be seen with the oxygen sensor. When powered up in fresh air after a period of nonuse, I found concentration levels that could not possibly sustain human life. We know the normal oxygen concentration should be around 20% to 23%. It began producing a reasonable output in less than an hour. Photo 1 shows
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CIRCUIT CELLAR • NOVEMBER 2017 #328
based on the voltage between the ground terminal and the output terminal. You’ll note the LM317 labels this pin as “adjust.” It works the same way. It regulates the voltage on its output pin to 1.25 V, based on a 240 Ω resistor from adjust to Vout . With 1.25 V across the 240 Ω resistor we get a current through this resistor of 1.25V/240Ω or 5.2 mA. If a second resistor is placed from the adjust pin to ground, then the 5.2 mA will also flow through this second resistor. If this resistor is also 240 Ω then we’ll have another 1.25 V drop across it, for a total drop of 2.5 V from Vout to ground. And, voila: a 2.5 V regulator. There is a formula for figuring out what that resistor value must be to produce a required output voltage. It can be calculated by rearranging the formula as follows:
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PHOTO 1 As shown here, my application for this project will dump the measured concentrations for all connected sensors in one big list. Also shown is the sensor PCB I’ve wired to the Arduino’s analog port.
For detailed article references and additional resources go to: www.circuitcellar.com/ article-materials
RESOURCES Arduino | www.arduino.cc
carrier boards that came with most of my sensors have one leg of the heater and the measurement electrode tied together. This means whatever voltage you supply is applied to both the heater and the measurement electrode. That particular arrangement doesn’t allow for separate voltages for the heater and measurement electrode. This is interesting since the MQ-7 and MQ-9 require high and low heater voltages, and the carrier board does not allow separating the heater from the electrode. Sensors like the MQ-7 require an upper heater voltage of 5 V and a lower voltage of 1.5 V. There are a number of ways to do this. Using a PWM produced from the Arduino, you can charge a cap to various levels. The sensors require up to 200 mA and the Arduino cannot source that amount of current. As a result, the PWM must drive a transistor that would obtain current from an external source. While the Arduino Mega can produce separate PWMs, you may want to use extra analog inputs to monitor that the PWM produced voltages are correct. This requires coding closed loop regulation for each channel in order to see whether the voltages remain within specs. Hmm...that might make a good project in itself for a future article! I suggest using an adjustable regulator instead. The LM317 has been around for a long while and is used to produce any voltage from 1.2 V to 37 V, with sufficient input. Two additional resistors are required to set the regulation output level. Figure 4 shows what this circuit would look like—using one circuit for each sensor for individual control. When you use a regulator like the LM7805, it has a GND terminal and regulates its output
((Vout
Vout = Vout / 1.25V = (Vout / 1.25) -1 = / 1.25) -1) x R1 =
((5V / 1.25) – 1) x 240 = (4 – 1) x 240 = 3 x 240 = 720 =
1.25V x (1+(R2/R1)) 1 + (R2/R1) R2/R1 R2 R2 R2 R2 R2
By using a 720 Ω resistor as R2, we get 5 V at Vout . From previous discussions we saw that the MQ-7 requires 1.5 V as well as 5 V, so we also need a 1.5 V regulator. Using the above formula, we would find that replacing the 720 Ω resistor with a 48 Ω resistor would produce a Vout of 1.5 V. In the schematic that a second resistor R3 is in parallel with R2, with a transistor that can both connect it and disconnect it from the circuit. There are two things that come into play here. First, the value of 48 Ω in parallel with 720 Ω has an effective resistance of 34 Ω— which would produce a Vout = 1.48 V (a bit low). Second, with the transistor OFF (Control = 0 V) it will look like an almost infinite resistor and will have little effect on R2. When Control is brought high, the transistor turns ON and its resistance goes way down. How far depends on the base current and transistor gain. However, the low current (5.2 mA) through the transistor keeps the transistor’s CE drop to a minimum. This will also affect the total series resistance of the transistor and R3—and ultimately the series equivalent. A digital output signal from the Arduino can then be used to switch between the two output voltage levels. I’ll leave the coding issue up to the you for this. You’ll find that the multivoltage sensors have timing associated with each voltage level and a proper point in time in which to measure them. Enough said here.
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GAS SENSOR CALIBRATION
Volume of Chemical = Cppm Ă&#x2014; Volume of Air Ă&#x2014; Molecular Weight of Chemical 24.5 Ă&#x2014; 109 Ă&#x2014; Density of Chemical
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FIGURE 4 The LM317 adjustable regulator can be used to provide a programmable regulated voltage. The voltage is programmed via a resistor divider between ground, the regulator â&#x20AC;&#x2DC;adjâ&#x20AC;&#x2122; terminal and output terminal. The resistor-to-ground is calculated for any specific regulated output between 1.2 V and 37 V (assuming a Vin at least 5 V > Vout). Here a parallel resistor can be switched in and out to change the regulation output between two voltages.
or direct contact with water (check humidity). A catalytic sensor must be installed in a vertical position, with the sensing elements pointed at the ďŹ&#x201A;oor. This prevents the sensing elements from collecting moisture, and the ďŹ&#x201A;ame arrestor from becoming clogged. Our homes are protected from ďŹ re by particle monitoring smoke detectors and from dangerous levels of uncombusted gases by carbon monoxide detectors. It is suggested that paints, thinners, gasoline and other VOCs (volatile organic compounds) be stored outside of the home to protect inhabitants from the buildup of potentially hazardous gasses. When this is not possible or in an industrial environment, it only makes sense to protect people and property from these issues. Low-cost gas concentration sensors can be used to monitor the environment and indicate when an unsafe condition exists.
COMMON SENSE Be aware that the gas of interest may be heavier or lighter than fresh air, so when indoors you will want to locate the sensor appropriatelyâ&#x20AC;&#x201D;either at ceiling or ďŹ&#x201A;oor level. When there is forced air movement (HVAC) remember that air currents will move the gas and you will want to locate the sensor downwind. Sensors should be located as close to any potential leak source as possible. Dispersion will reduce readings if the sensors are any distance from the potential spill or leak. Sensors should be protected from immersion
ABOUT THE AUTHOR Jeff Bachiochi (pronounced BAH-key-AHkey) has been writing for Circuit Cellar s i n c e 1 9 8 8 . H i s b a c kg ro u n d i n c l u d e s product design and manufacturing. You can reach him at: jeff.bachiochi@imaginethatnow.com or at: www.imaginethatnow.com.
COLUMNS
Gas sensors need to be calibrated and periodically checked to ensure sensor accuracy and system integrity. Normally, a monthly calibration is adequate to ensure the eďŹ&#x20AC;ectiveness of each sensor and to maintain the systemâ&#x20AC;&#x2122;s accuracy. Calibration of the gas sensor involves two steps. The â&#x20AC;&#x153;zeroâ&#x20AC;? must be set and then the â&#x20AC;&#x153;spanâ&#x20AC;? must be calibrated. In this project, we used fresh air as the zero point to establish the sensorâ&#x20AC;&#x2DC;s base resistance and the slope was taken from the datasheetâ&#x20AC;&#x2122;s graph. A point and slope determined the concentration. The span in the calibration sense is taking a second point to determine slope. Each point comes from using a test gas of a known concentration, both fresh air and concentrated. The sensor readings with the test gas become the calibration points. The slope can then be calculated using those two points, reading 1 at 0% concentration and reading 2 at ?% concentration. Test gases are bottled in pressurized cylinders of many sizes and pressures. Essentially, the gas of interest is mixed with fresh air to a speciďŹ c concentration. When you expose the ďŹ&#x201A;ow from the tank to the sensor, it will provide a reading of that concentration. You might want to put the sensor into an empty zip lock bag and add gas to the bag. This keeps the gas from dissipating into the environment before you had a chance to get a stable reading. When using liquids, it is much more diďŹ&#x192;cult as you need to know how much liquid to add to a container of known volumeâ&#x20AC;&#x201D;a zip lock for instance. You might want to use a micro syringe to measure the chemical and add it to the zip lock bag. Once the liquid has vaporized in the container the concentration can be made. The volume can be calculated with this equation:
Handson Technology Data Specs
MQ-3 Alcohol Sensor Module MQ-3 gas sensor has high sensitivity to alcohol, and has good resistance to disturbances of gasoline, smoke and vapor. The sensor could be used to detect alcohol with different concentration; it is with low cost and suitable for different application. Sensitive material of MQ-3 gas sensor is SnO2, which with lower conductivity in clean air. When the target alcohol gas exists, sensor’s conductivity is proportional to the gas concentration.
SKU: SSR-1000
Description:
Model No: MQ-3. Heater voltage: 5±0.2V. Loop voltage: ≤24V (DC) Load resistance: Adjustable. Heating Resistance: 31Ω±3Ω (Room temperature). Heating Power: ≤ 900mW. Surface thermal resistance: 2Kohm-20Kohm (0.4mg/L alcohol). Sensitivity: Rs (in air)/Rs (0.4mg/L alcohol) ≥ 5.
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Schematic Diagram:
Mechanical Dimension:
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Application Example with Arduino Uno: Connect the MQ-3 alcohol sensor module to Arduino Uno board as shown below:
Upload the below sketch to Arduino Uno board: /* MQ-3 Alcohol Sensor Circuit with Arduino */ const int AOUTpin=A0; //the AOUT pin of the alcohol sensor goes into analog pin A0 of the arduino const int DOUTpin=8; //the DOUT pin of the alcohol sensor goes into digital pin D8 of the arduino const int ledPin=13; //the anode of the LED connects to digital pin D13 of the arduino int limit; int value; void setup() { Serial.begin(115200); pinMode(DOUTpin, INPUT); pinMode(ledPin, OUTPUT);
//sets the baud rate //sets the pin as an input to the arduino //sets the pin as an output of the arduino
} void loop() { value= analogRead(AOUTpin); //reads the analaog value from the alcohol sensor's AOUT pin limit= digitalRead(DOUTpin); //reads the digital value from the alcohol sensor's DOUT pin Serial.print(" Alcohol value: "); Serial.println(value); //prints the alcohol value Serial.print("Limit: "); Serial.print(limit); //prints the limit reached as either LOW or HIGH (above or underneath) delay(100); if (limit == HIGH){ digitalWrite(ledPin, HIGH); } else{ digitalWrite(ledPin, LOW); }
//if limit has been reached, LED turns on as status indicator //if threshold not reached, LED remains off
}
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Open up the Serial Monitor with Baud rate of 115200, the alcohol level detected will be shown as analog value. The alcohol limit value can be set with sensitivity potentiometer: if the alcohol level detected is below the set limit, the D0 green indicator will be off. If detected alcohol level is beyond the set limit, the DO LED will light up.
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Handson Technology Sensors Selection Guide
MQ-3 Alcohol Sensor
HX711 Load Cell Sensor ADC Module
10KG Load Cell Weigh Sensor
MQ-2 Gas Sensor
PIR501 Motion Detector
Capacitive Touch Sensor
DS18B20+ Digital Temperature Sensor
ACS712 Hall Current Sensor
MPU6050 Accelerometer/ Gyro Sensor
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HC-SR04P Ultrasonic Sensor
HT1209 Digital Thermostat
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