level measurement

Questions: Level instruments, advanced c 2002-2003 Tony R. Kuphaldt Copyright ° • Learning Objectives: • How to calculate differential pressure transmitter range values for level measurement scenarios with elevated or suppressed transmitters.

• How to develop calibration tables for any level measurement scenario, given an allowable percentage of span error.

• How to calculate differential pressure transmitter range values for level measurement scenarios with wet legs.

• Identify the potential problem associated with locating a liquid pressure transmitter above the process connection.

• How to calculate differential pressure transmitter range values for level measurement scenarios with remote seals.

• How to calculate differential pressure transmitter range values for level measurement scenarios with gas and liquid purge.

• Develop calibration tables for displacer-based level transmitters.

• How to calculate differential pressure transmitter range values for level measurement scenarios with interfaces of two different liquids.

• Identify volume measurement nonlinearities caused by vessel shape.

Question 1 Determine the lower and upper range-values for the differential pressure transmitter being used here to measure water level, in pressure units of inches water column (”W.C.). Assume a measured variable span of 40 feet:

100%

Measurement span = 40 ft Water H

L

0%

Question 2 Determine the LRV and URV points for a transmitter measuring water level in the same vessel, but this time located 10 feet beneath the vessel: 1

100%

Measurement span = 40 ft Water

0% 10 ft H

L

Question 3

Determine the LRV and URV points for a transmitter measuring water level in the same vessel, but this time located 10 feet above the bottom of the vessel:

100%

Measurement span = 40 ft Water

H

L

10 ft 0%

Question 4

What do the terms elevation and suppression refer to in regard to level measurement by head pressure?

Question 5

A pneumatic dp cell (3-15 PSI output range) is used to measure the level of water in this vessel: 2

100%

Measurement span = 35 ft Water

DP cell with 3-15 PSI output

0% 7 ft

H

L

Determine the LRV and URV points for the transmitter’s calibration, and also the output signal pressure if the water level happens to be 10.7 feet.

Question 6

Determine a basic 5-point (0%, 25%, 50%, 75%, and 100%) calibration table for the transmitter in this level measurement scenario. In other words, what applied pressures correspond to the ideal transmitter signal output values for 5 points along the 0% to 100% scale?

100% Process liquid S.G. = 0.85 Measurement span = 14 ft DP cell with 3-15 PSI output 0% 5 ft

H

L

Question 7

In one calculation, determine the span of this transmitter (in inches of water column). Then, calculate a basic 5-point (0%, 25%, 50%, 75%, and 100%) calibration table for the transmitter.

3

100% Process liquid S.G. = 0.81 Measurement span = 110 in

DP cell with 4-20 mA output 2 ft H

L

0%

Question 8 In one calculation, determine the span of this transmitter (in inches of water column). Then, calculate a basic 5-point (0%, 25%, 50%, 75%, and 100%) calibration table for the transmitter. The tubing connecting the ”low” side of the transmitter to the top of the pressurized vessel is dry: that is, there is no liquid in it to generate any head pressure.

100%

pressure Process liquid S.G. = 0.85

Measurement span = 30 ft

"dry" leg DP cell with 10-50 mA output

H

L

0%

Question 8.5 What will happen if the ”dry” leg tubing connecting the ”low” side of the differential pressure transmitter to the top of the vessel in question #8 were to fill with liquid from condensing vapors inside the vessel? If this ”dry” leg were to become ”wet,” what effect would it have on the transmitter’s ability to measure vessel liquid level?

Question 9 In one calculation, determine the span of this transmitter (in inches of water column). Then, calculate a basic 5-point (0%, 25%, 50%, 75%, and 100%) calibration table for the transmitter. The tubing connecting the ”low” side of the transmitter to the top of the pressurized vessel is wet: it is filled with a liquid of specific gravity = 1.1. Note the height of this ”wet” leg: 35 feet!

4

pressure

100%

Process liquid S.G. = 0.85 Measurement span = 30 ft

"wet" leg S.G. = 1.1 DP cell with 35 ft 10-50 mA output

H

L

0%

Question 10 Determine a basic 5-point (0%, 25%, 50%, 75%, and 100%) calibration table for the transmitter in this level measurement scenario.

100% wet leg S.G. = 1 70 in

50 in S.G. = 1

DP cell with 6-30 PSI output

0%

H

L

6 in Question 11 Most pneumatic differential pressure transmitters are not able to measure ”negative” pressures of the kind encountered in question #10 (where the ”high” side pressure is actually less than the ”low” side pressure), at least not without special ”elevation” springs installed to introduce a bias force in the forcebalance mechanism. If a normal, unaltered pneumatic transmitter with an output signal range of 6-30 PSI were used to measure liquid level in the vessel shown in question #10, how would it have to be connected to the process, and what would its 5-point calibration table look like?

Question 12 Determine a 5-point (0%, 25%, 50%, 75%, and 100%) calibration table for the transmitter in this level measurement scenario, with a calibration tolerance of +/- 1%. Also, specify which side (high or low) of the transmitter that the calibration pressure must be applied for each calibration point (in other words, assume that the calibrator has no capacity for producing precision vacuums, only precision pressures).

5

DP cell with 4-20 mA output

100% Process liquid D = 75 lb/ft3 Measurement span = 15 ft

H

L

7 ft 0%

Question 13

Determine a 5-point (0%, 25%, 50%, 75%, and 100%) calibration table for the transmitter in this level measurement scenario, with a calibration tolerance of +/- 2%, specifying which side (high or low) of the transmitter that the calibration pressure must be applied for each calibration point.

100% Process liquid S.G. = 2

DP cell with 10-50 mA output

Measurement span = 19 ft H

L

5 ft 0%

11 ft

Question 14

Generally speaking, it is not a good practice to locate a liquid pressure transmitter above the process connection when head pressures or other low pressure ranges are being measured. Why is this?

Question 15

A pressure transmitter is used to measure pressure (not level!) inside of a large pipe. Its measurement range is 0 to 500 PSI, and it is connected to the pipe by a vertical stretch of tubing 12 inches high: 6

Pressure transmitter

H

12 in

L

Range = 0-500 PSI

Pipe Why is the mounting position of the transmitter (above the process connection to the pipe) not a problem here, although it would almost certainly be a problem if liquid head or some other low pressure range were being measured?

Question 16 One solution to the problem highlighted in question #14 is to use a remote seal isolating the transmitter from the process liquid. What is a pressure transmitter remote seal (sometimes called a chemical seal), and why would it address the problem described in question #14?

Question 17 Determine a basic 5-point (0%, 25%, 50%, 75%, and 100%) calibration table for the transmitter in this level measurement scenario.

DP cell with 4-20 mA output

100% Process liquid D = 60 lb/ft3 Measurement span = 11 ft

H

L

5 ft 0% Remote seal Seal fill fluid S.G. = 1.9

Question 18 Determine a basic 5-point (0%, 25%, 50%, 75%, and 100%) calibration table for the transmitter in this level measurement scenario.

7

DP cell with 10-50 mA output

100% Process liquid D = 88 lb/ft3

15 ft

Measurement span = 24 ft H

L

9 ft 0% Remote seal Seal fill fluid S.G. = 1.75

Question 19 What would happen to the level measurement system shown in question #18 if the vapor pressure within the vessel were to suddenly increase (assuming an unchanging liquid level)?

100%

Measurement span = 24 ft

Pressure INCREASE

DP cell with 10-50 mA output

Process liquid D = 88 lb/ft3

15 ft H

L

9 ft 0% Remote seal Seal fill fluid S.G. = 1.75

Question 19.5 Draw the P&ID (”flowsheet”) symbol for an electronic differential pressure transmitter with remote seals measuring liquid level, as shown in questions #18 and #19.

Question 20 A liquid storage vessel holding a very corrosive liquid has its level measured by a bubbler system, whereby a transmitter measures the backpressure of air inside a ”dip tube” inserted into the vessel: 8

Compressed air supply dip tube 100%

Measurement span = 18 ft

needle valve Pressure regulator DP cell with 4-20 mA output

Process liquid D = 94 lb/ft3

H

L

bubbles

0% Determine a 5-point (0%, 25%, 50%, 75%, and 100%) calibration table for the transmitter in this level measurement scenario, with a calibration tolerance of +/- 0.5%, specifying which side (high or low) of the transmitter that the calibration pressure must be applied for each calibration point. Assume that the lower range-value of the process (0% level) is exactly the same height as the bottom of the dip tube.

Question 21 Purge systems may be used to detect head pressure in a vessel even when there is no dip tube. For example, in this level measurement system, compressed air is used as a purge medium directly into the vessel where the transmitter tubing connects:

needle valve Compressed air supply

air

100%

Pressure regulator

Process liquid D = 73 lb/ft3 Measurement span = 22 ft

air bubbles H

L

0% needle valve 9

What would happen to the transmitter’s output if the lower process connection were to become plugged by debris (despite the cleaning action of the compressed air flowing through it)?

needle valve Compressed air supply

air

100%

Pressure regulator

Process liquid D = 73 lb/ft3 Measurement span = 22 ft

0%

H

L

Blockage needle valve

Question 22 Given the level measurement system shown in question #21, what would happen to the transmitter’s output if the upper process connection were to become plugged by debris?

needle valve Compressed air supply

Blockage 100%

Pressure regulator

Process liquid D = 73 lb/ft3 Measurement span = 22 ft

air bubbles H

L

0% needle valve Question 23 In purged (bubbler) instrument systems, simple flow-indicating devices are usually installed in line with the purge tubing to indicate purge fluid flow: 10

Flow indicator Supply FI

dip tube H

L

bubbles

If the purge fluid is a gas (such as compressed air), the flow indicator may be as simple as a glass jar partially filled with oil, with a dip tube indicating gas flow by a series of bubbles coming out the end:

Supply

dip tube H

L

bubbles

Such a device is called a sight feed bubbler.

Another popular flow-indicating device is called a rotameter: a vertical, conical tube made of transparent material, a �plummet� inside the tube supported against the force of gravity by the force of the moving purge fluid: 11

Rotameter Supply

dip tube H

L

bubbles

Rotameters can withstand greater static pressures than sight feed bubblers, and are able to indicate the flow of purge liquids as well as purge gases.

Why is a flow indicator desirable to have in a purge system, when the system will function quite well without purge fluid flow indication?

Question 24

Determine a 5-point (0%, 25%, 50%, 75%, and 100%) calibration table for the transmitter in this waterpurged level measurement system, with a calibration tolerance of +/- 0.3%, specifying which side (high or low) of the transmitter that the calibration pressure must be applied for each calibration point.

12

Static pressure = 50 PSIG water

100% Process liquid D = 73 lb/ft3 Measurement span = 22 ft

0%

24 ft

water

Pressurized 100 PSI water supply

10-50 mA

10 ft H

L

Question 25

What will be the weight of an iron rod (D = 490.68 pounds per cubic foot), 5 feet long and 2 inches in diameter, as it hangs inside a dry vessel?

13

5 ft

iron

Scale

Vessel (dry)

2 in What will the scale indicate when the vessel fills with water until 3 feet of the rod is submerged?

iron

Scale

Water

3 ft

Question 26

The principle of liquid displacement may be used to create a level transmitter instrument, generating an output signal proportional to the change in weight of a �displacer� rod suspended in a liquid: 14

Block valves Process liquid

displacer

Vessel

Weightmeasuring mechanism

Displacer "cage"

Often, the displacer is housed inside its own ”cage” for easy removal from the process, as shown, or it may be inserted directly into the process vessel like this:

Vessel

displacer

Weightmeasuring mechanism

Process liquid

A common means of ”dry-calibrating” a displacer-type level instrument is to close both block valves and drain the displacer cage of all liquid to simulate 0% process level (LRV), then use a string and mechanical scale to apply a measured amount of upward force on the displacer to simulate the buoyant force generated by submersion in the process liquid at 100% of measurement range (URV): 15

Pull up on string until scale registers the desired force "Dry" calibration Scale

valves closed Process liquid

Weightmeasuring mechanism

displacer

Vessel

Liquid drained out of cage But suppose you had no scale to use for such a ”dry” calibration. Can you think of another way to simulate a 100% level (URV) condition without actually filling the process vessel level up to that level?

Question 27 Suppose that a displacer-type level transmitter is used in a liquid process service where a hard, scaly residue accumulates and adheres to the displacer surface over time. What effect will such a residue have on the transmitter’s calibration, as it alters both the dry weight and the effective volume of the displacer? Will there be a zero shift, a span shift, or both? In which direction(s) will the shift(s) be?

The following graph is a transfer function depicting process liquid level versus transmitter output for a transmitter with no residue accumulation:

100%

Transmitter output

0% 0%

Process liquid level

100%

What will this graph look like after a substantial amount of residue has accumulated on the displacer?

16

Question 28

100%

Block valves

Measurement span = 24 in

displacer

Determine a basic 5-point (0%, 25%, 50%, 75%, and 100%) calibration table (upward force on the displacer vs. percentage of measurement range) for the displacer level transmitter in this scenario:

100% Measurement span = 24 in

Water 0%

0%

The displacer weighs 10 pounds (dry) and has a diameter of 3 inches. The 0% process liquid level (LRV) is even with the bottom of the displacer.

Question 29

100% Measurement span = 30 in

Process liquid D = 55 lb/ft3

Block valves

displacer

Determine a basic 5-point (0%, 25%, 50%, 75%, and 100%) calibration table (upward force on the displacer vs. percentage of measurement range) for the displacer level transmitter in this scenario:

0%

100% Measurement span = 30 in 0%

The displacer weighs 15 pounds (dry) and has a diameter of 3.5 inches. The 0% process liquid level (LRV) is even with the bottom of the displacer.

Question 30 How much head pressure (in PSI) will there be at the bottom of this vessel when filled with water?

17

Liquid in

Overflow

11 ft

(water)

Head pressure = ???

How much head pressure (in PSI) will there be at the bottom of this vessel when filled with gasoline (42 lb/ft3 )?

Liquid in

Overflow

11 ft

(gasoline)

Head pressure = ???

How much head pressure (in PSI) will there be at the bottom of this vessel when half filled with gasoline (42 lb/ft3 ) and half filled with water (a water-gasoline �interface� at the 50% level mark)?

Liquid in

Overflow (gasoline) 11 ft (water) Head pressure = ???

Question 31 18

Determine a basic 5-point (0%, 25%, 50%, 75%, and 100%) calibration table for the transmitter in this gasoline/water interface level measurement scenario.

Liquid in

Overflow (gasoline) D = 42 lb/ft3 11 ft (water) D = 62.428 lb/ft3

DP cell with 4-20 mA output H

L

Question 32

Determine a 5-point (0%, 25%, 50%, 75%, and 100%) calibration table for the transmitter in this level measurement scenario, with a calibration tolerance of +/- 0.25%. Also, specify which side (high or low) of the transmitter that the calibration pressure must be applied for each calibration point.

6 foot "wet" leg S.G. = 0.8

100% S.G. = 0.8 Interface measurement span = 6 ft

DP cell with 3-15 PSI output S.G. = 1.0 H

L

0% The lighter liquid has a specific gravity of 0.8, while the heavier liquid has a specific gravity of 1.0.

Assume that the liquid’s total level always remains above the 100% level for interface measurement.

Question 33

Determine a 5-point (0%, 25%, 50%, 75%, and 100%) calibration table for the transmitter in this level measurement scenario, with a calibration tolerance of +/- 0.75%. Also, specify which side (high or low) of the transmitter that the calibration pressure must be applied for each calibration point.

19

DP cell with 4-20 mA output 100% S.G. = 0.75 Interface measurement span = 30 in

30 in H

L

S.G. = 1.2

0% Remote seal Seal fill fluid S.G. = 1.1 The lighter liquid has a specific gravity of 0.75, while the heavier liquid has a specific gravity of 1.2.

Assume that the liquid’s total level always remains above the 100% level for interface measurement (above the upper transmitter remote seal height).

Question 34 Identify and describe at least three technologies for measuring liquid level other than head-pressure (or displacement) transmitters.

Question 34.5 Suppose the level of an oil sump is to be measured by a displacer-type level instrument. Of course, the displacer must be immersed in the oil bath in order for it to function. The problem is this: the oil in this sump is very turbulent, owing to large flow rates entering and exiting the sump. This turbulence will cause the displacer instrument to exhibit errors, as the displacer gets pushed laterally by the turbulent oil. Can you think of a solution to this problem (and don’t say, ”Use a different type of level instrument”!)?

LT

"choppy" liquid surface

cer

displa

Flow

Flow will exert lateral force on displacer, causing measurement errors

Flow

Question 35 Most liquid level measurement technologies work by sensing the height of the liquid in a storage vessel.

However, what is often desired to be known about a storage vessel is how much volume or mass of liquid that it holds. Depending on the shape of the vessel, height may or may not directly correlate to volume or mass.

In which of these vessels will the relationship between liquid level and liquid volume be linear (directly and constantly proportional throughout the entire measurement range)? Assume the use of a dp cell (head pressure) level transmitter in all cases: 20

Cylindrical (vertical)

Cylindrical (horizontal)

Spherical

Rectangular

Question 36 Plot an approximate transfer function graph (volume versus level) for this conical liquid storage vessel:

Cylindrical (vertical)

Level Volume

Show how the vessel level will change as the volume changes, with the volume as the independent variable (horizontal axis), and the level as the dependent variable (vertical axis):

100%

Liquid level

???

0% 0%

Liquid volume

100%

Question 37 Draw the symbols for the following types of liquid level indicating instruments, each one mounted to the top of a process vessel: • • • • • •

Float Radar gauge Ultrasonic (sound) gauge Laser (light) gauge Resistive tape Capacitance probe

21

Question 38 Examine this P&ID drawing, and determine what type of instrument is attached to the process vessel:

LG

22

Short answers

Answer 1 LRV = 0 ”W.C. URV = 480 ”W.C. Answer 2 LRV = 120 ”W.C. URV = 600 ”W.C. Answer 3 LRV = -120 ”W.C. (120 ”W.C. of vacuum applied to the ”high” side, or 120 ”W.C. of pressure applied to the ”low” side) URV = 360 ”W.C. Answer 4 The terms elevation and suppression (or depression) describe situations where the pressure transmitter is not located at the 0% process level height. Believe it or not, these terms are often interchanged when speaking of the same scenario (the transmitter mounted either above or below the vessel’s 0% level)!

The Instrument Engineer’s Handbook, however, attempts to clarify(?) the issue by distinguishing elevated zero and suppressed zero from elevated span and suppressed span. One refers to the perspective of the transmitter while the other refers to the perspective of the process.

Answer 5 LRV = 84 ”W.C. URV = 504 ”W.C. Transmitter output at 10.7 feet of water level = 6.669 PSI Answer 6

Input (" W.C.)

%

Output (ideal)

51

0

3 PSI

86.7

25

6 PSI

122.4

50

9 PSI

158.1

75

12 PSI

193.8

100

15 PSI 23

Answer 7 Span = 89.1 ”W.C.

A vacuum applied to the "high" side, or a pressure of 19.44 "W.C. applied to the "low" side.

Input (" W.C.)

%

Output (ideal)

-19.44

0

4 mA

2.835

25

8 mA

25.11

50

12 mA

47.385

75

16 mA

69.66

100

20 mA

Answer 8 Span = 306 ”W.C.

Input (" W.C.)

%

Output (ideal)

0

0

10 mA

76.5

25

20 mA

153

50

30 mA

229.5

75

40 mA

306

100

50 mA

Answer 8.5 If the formerly ”dry” leg were to become ”wet,” there will be a zero shift in the transmitter’s response.

More specifically, the transmitter will register a falsely low liquid level in the vessel.

Answer 9 Span = 306 ”W.C. 24

Either calibrate the transmitter with vacuums of these magnitudes, or with pressures applied to the "low" side

Input (" W.C.)

%

Output (ideal)

-462

0

10 mA

-385.5

25

20 mA

-309

50

30 mA

-232.5

75

40 mA

-156

100

50 mA

Answer 10

Calibrate transmitter using vacuums of these magnitudes applied to "high" side or pressures of these magnitudes applied to "low" side

Input (" W.C.)

%

Output (ideal)

-64

0

6 PSI

-51.5

25

12 PSI

-39

50

18 PSI

-26.5

75

24 PSI

-14

100

30 PSI

Answer 11 Connecting the transmitter to the process vessel:

wet leg S.G. = 1

S.G. = 1

DP cell with 6-30 PSI output

L

H

Note "low" and "high" port orientations!

25

Calibration table (all positive pressures):

Process level, in percent Input (" W.C.)

%

Output (ideal)

14

100

6 PSI

26.5

75

12 PSI

39

50

18 PSI

51.5

25

24 PSI

64

0

30 PSI

When calibrating this transmitter, the pressure values shown in the table will all be applied to the ”high” side, with the ”low” side vented to atmosphere. Note how lower transmitter signal pressures correspond to higher process level percentages – the transmitter is indicating in reverse!

Answer 12

Input (" W.C.)

%

Output (ideal)

Output Output (minimum) (maximum)

100.92 (L)

0

4 mA

3.84 mA

4.16 mA

46.85 (L)

25

8 mA

7.84 mA

8.16 mA

7.21 (H)

50

12 mA

11.84 mA

12.16 mA

61.27 (H)

75

16 mA

15.84 mA

16.16 mA

115.33 (H)

100

20 mA

19.84 mA

20.16 mA

Answer 13 26

Input (" W.C.)

%

Output (ideal)

Output Output (minimum) (maximum)

120 (L)

0

10 mA

9.2 mA

10.8 mA

6 (L)

25

20 mA

19.2 mA

20.8 mA

108 (H)

50

30 mA

29.2 mA

30.8 mA

222 (H)

75

40 mA

39.2 mA

40.8 mA

336 (H)

100

50 mA

49.2 mA

50.8 mA

Answer 14 If a pressure transmitter is elevated above the connection point to a vessel or pipe containing a liquid, there is always a possibility that the liquid will run out of the tubing if the vessel or pipe ever goes dry:

(empty vessel) H

L

...

Fill liquid dribbling out of vertical piping between transmitter and vessel If this were to happen, the amount of head ”suction” normally created by the liquid height in the connecting tubing would be reduced, shifting the zero of the measurement system.

Answer 15 The change in head pressure (or suction) caused by a liquid column in the 12 inch length of connecting tubing is inconsequential compared to the pressure range being measured (500 PSI). There would be so little applied pressure difference between a ”filled” tube versus an ”empty” tube that it would hardly be noticed.

Answer 16 A remote seal, or chemical seal, is one or more diaphragm units that convey pressure to a remotelylocated pressure transmitter by means of fluid-filled capillary tubing. Here is a diagram of remote seals used on a pneumatic force-balance differential pressure transmitter: 27

Pneumatic differential pressure transmitter orifice

Regulated compressed air supply

nozzle flapper bellows

diaphragm

fulcrum and seal

force bar

Air pressure signal out

(transmitter filled with oil as well)

oil

oil

Chemical seal

diaphragm

diaphragm

capillary tubing

Chemical seal

A remote seal would solve the problem of question #14 because the diaphragm between the process liquid and the capillary liquid prevents the capillary liquid from ever �dribbling� out into the process vessel. In other words, the head pressure (or suction) caused by the vertical column of capillary fill fluid is unchanging: 28

Transmitter

H

L

capillary tube

Process vessel

Fill fluid head

remote diaphragm

Answer 17

Input (" W.C.)

%

Output (ideal)

-114

0

4 mA

-82.28

25

8 mA

-50.57

50

12 mA

-18.85

75

16 mA

12.87 100

20 mA

Note how all but one of the calibration points involve negative pressures. If the ”low” side of the transmitter is vented (without a remote seal attached), these negative pressures may be easily simulated by applying positive pressure directly to that port. If the ”low” side has a remote seal, the transmitter must be calibrated by bolting a pressure flange on to the seal and applying pressure there.

In any case, it is imperative that all remote seal diaphragms are maintained level with the transmitter while being calibrated, so that there are no head pressure effects while on the calibration bench!

Answer 18 29

Input (" W.C.)

%

Output (ideal)

-504

0

10 mA

-402.51

25

20 mA

-301.01

50

30 mA

-199.52

75

40 mA

-98.03

100

50 mA

Answer 19 Ideally, there would be no change in the transmitter’s response if the vessel’s static pressure were to increase or decrease. This is because the transmitter only measures the difference in pressure between the top and bottom seals. This differential pressure is entirely the result of liquid head, not static pressure.

Answer 19.5

LT 10-50 mA output

Answer 20

Input (" W.C.)

%

Output (ideal)

0

4 mA

3.92 mA

4.08 mA

81.31 (H)

25

8 mA

7.92 mA

8.08 mA

162.62 (H)

50

12 mA

11.92 mA

12.08 mA

243.93 (H)

75

16 mA

15.92 mA

16.08 mA

325.24 (H)

100

20 mA

19.92 mA

20.08 mA

0

Output Output (minimum) (maximum)

30

Answer 21 If the lower process connection were to become blocked by debris, the transmitter’s output signal would increase, quite possibly to a magnitude greater than 100%.

Answer 22 If the upper process connection were to become blocked by debris, the transmitter’s output signal would decrease, quite possibly to a magnitude less than 0%.

Answer 23 Having a purge fluid indicator in place is an excellent troubleshooting tool, for problems such as those mentioned in questions #21 and #22.

Answer 24

Output Output (minimum) (maximum)

Input (" W.C.)

%

Output (ideal)

288 (L)

0

10 mA

9.88 mA

10.12 mA

210.82 (L)

25

20 mA

19.88 mA

20.12 mA

133.65 (L)

50

30 mA

29.88 mA

30.12 mA

56.47 (L)

75

40 mA

39.88 mA

40.12 mA

20.71 (H)

100

50 mA

49.88 mA

50.12 mA

Answer 25 Dry weight = 53.52 pounds Submerged (by 3 feet) weight = 49.44 pounds

Answer 26 Vent the displacer cage to atmosphere by opening it up at the top (the same hole through which you would normally access the displacer to attach a string and scale), then slowly open the lower block valve to let process liquid into the cage. If the vessel is pressurized significantly above atmospheric pressure, the liquid will rise up and fill the chamber as far as you let it: 31

(vented) valve closed

Process liquid

valve open

Weightmeasuring mechanism

displacer

Vessel

Be careful, though! If the process liquid is hazardous and/or under high pressure inside the process vessel, this technique would not be recommended!

Answer 27

The zero will be shifted down, and the span shifted up, by accumulated residue on the displacer:

With residue

100%

Transmitter output

No residue

0% 0%

Process liquid level

100%

On the graph, the downward zero shift is represented by the lower left-hand end of the line, while the upward span shift is represented by a steeper slope.

Answer 28 32

Input (lb)

%

0

0

1.532

25

3.064

50

4.597

75

6.129

100

Answer 29

Input (lb)

%

0

0

2.297

25

4.593

50

6.890

75

9.187

100

Answer 30 Head pressure when completely full of water = 4.769 PSI Head pressure when completely full of gasoline = 3.208 PSI Head pressure when water-gasoline interface is at 50% level = 3.988 PSI Answer 31

Input (" W.C.)

%

Output (ideal)

88.81

0

4 mA

99.6

25

8 mA

110.4

50

12 mA

121.2

75

16 mA

132

100

20 mA 33

Answer 32

Input (" W.C.)

%

Output (ideal)

0

3 PSI

2.97 PSI

3.03 PSI

3.6 (H)

25

6 PSI

5.97 PSI

6.03 PSI

7.2 (H)

50

9 PSI

8.97 PSI

9.03 PSI

10.8 (H)

75

12 PSI

11.97 PSI

12.03 PSI

14.4 (H)

100

15 PSI

14.97 PSI

15.03 PSI

Input (" W.C.)

%

Output (ideal)

10.5 (L)

0

4 mA

3.88 mA

4.12 mA

7.125 (L)

25

8 mA

7.88 mA

8.12 mA

3.75 (L)

50

12 mA

11.88 mA

12.12 mA

0.375 (L)

75

16 mA

15.88 mA

16.12 mA

3.00 (H)

100

20 mA

19.88 mA

20.12 mA

0

Output Output (minimum) (maximum)

Answer 33

Output Output (minimum) (maximum)

Answer 34 Liquid level measurement technologies other than head pressure or displacement: • • • • • • •

Vessel weight measurement Radar gauge Ultrasonic (sound) gauge Laser (light) gauge Resistive tape Capacitance probe Float

Answer 34.5 The simplest solution would be to mount a vertical pipe inside the sump, both ends open, with the bottom end fully submerged and the top end above the highest oil level, to act as a stilling well for the 34

displacer to rest in. This �stilling well� duplicates the same liquid level inside of it as there is throughout the rest of the sump, without all the turbulence to drag the displacer laterally:

LT

support

Stilling well (vertical pipe)

Flow

Flow support

Answer 35

The relationship between level and volume will be linear for the (vertical) cylindrical and rectangular vessel shapes. It will be nonlinear for the others (horizontal cylindrical, and spherical).

Answer 36

Imagine liquid filling this vessel at a constant flow rate. Level in this vessel will rise slowly at first, then more rapidly as the cross-sectional area decreases. The result is a transfer function that looks like this:

100%

Liquid level

0% 0%

Liquid volume

100%

Answer 37

Float: 35

LI

Radar:

LI RADAR

Ultrasonic: 36

LI

Laser:

LI LASER

Resistive tape:

LI TAPE

37

Capacitance:

LI CA

Answer 38 The instrument in question is a level gauge, otherwise known as a sightglass.

38