62
CIRCUIT CELLAR • OCTOBER 2017 #327
From the Bench
Gas Monitoring and Sensing (Part 1)
COLUMNS
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.