Work5450 Short

Questions: Level instruments, advanced

Copyright c© 2002-2003 Tony R. Kuphaldt

• Learning Objectives:

• How to calculate differential pressure transmitter range values for level measurement scenarios withelevated or suppressed transmitters.

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

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

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

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

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

• Develop calibration tables for displacer-based level transmitters.

• How to calculate differential pressure transmitter range values for level measurement scenarios withinterfaces 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 tomeasure water level, in pressure units of inches water column (”W.C.). Assume a measured variable span of40 feet:

H L

Measurementspan = 40 ft

Water

0%

100%

Question 2

Determine the LRV and URV points for a transmitter measuring water level in the same vessel, but thistime located 10 feet beneath the vessel:

1

H L

Measurementspan = 40 ft

Water

0%

100%

10 ft

Question 3

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

H L

Measurementspan = 40 ft

Water

0%

100%

10 ft

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

Measurement

Water

0%

100%

H L

span = 35 ft

7 ft

3-15 PSI outputDP cell with

Determine the LRV and URV points for the transmitter’s calibration, and also the output signal pressureif 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 thislevel measurement scenario. In other words, what applied pressures correspond to the ideal transmittersignal output values for 5 points along the 0% to 100% scale?

Measurement

0%

100%

H L

3-15 PSI outputDP cell with

5 ft

span = 14 ft

Process liquidS.G. = 0.85

Question 7

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

3

Measurement

0%

100%

DP cell withProcess liquid

4-20 mA output

H L

2 ft

S.G. = 0.81

span = 110 in

Question 8

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

Measurement

0%

100%

DP cell with

Process liquid

H L

S.G. = 0.85

10-50 mA outputspan = 30 ft

pressure

"dry" leg

Question 8.5

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

Question 9

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

4

Measurement

0%

100%

DP cell with

Process liquid

H L

S.G. = 0.85

10-50 mA outputspan = 30 ft

pressure"wet" leg

S.G. = 1.1

35 ft

Question 10

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

H L

DP cell with6-30 PSI outputS.G. = 1

wet legS.G. = 1

0%

100%

50 in

6 in

70 in

Question 11

Most pneumatic differential pressure transmitters are not able to measure ”negative” pressures of thekind encountered in question #10 (where the ”high” side pressure is actually less than the ”low” sidepressure), at least not without special ”elevation” springs installed to introduce a bias force in the force-balance mechanism. If a normal, unaltered pneumatic transmitter with an output signal range of 6-30 PSIwere used to measure liquid level in the vessel shown in question #10, how would it have to be connectedto 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 levelmeasurement scenario, with a calibration tolerance of +/- 1%. Also, specify which side (high or low) of thetransmitter that the calibration pressure must be applied for each calibration point (in other words, assumethat the calibrator has no capacity for producing precision vacuums, only precision pressures).

5

Measurement

0%

100%

DP cell with

Process liquid

4-20 mA output

H L

7 ft

span = 15 ft

D = 75 lb/ft3

Question 13

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

Measurement

0%

100%DP cell with

Process liquid

H L

10-50 mA output

5 ft

11 ft

S.G. = 2

span = 19 ft

Question 14

Generally speaking, it is not a good practice to locate a liquid pressure transmitter above the processconnection 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 measurementrange is 0 to 500 PSI, and it is connected to the pipe by a vertical stretch of tubing 12 inches high:

6

H L

Pipe

12 in Range = 0-500 PSI

Pressure transmitter

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

Question 16

One solution to the problem highlighted in question #14 is to use a remote seal isolating the transmitterfrom the process liquid. What is a pressure transmitter remote seal (sometimes called a chemical seal), andwhy 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 thislevel measurement scenario.

Measurement

0%

100%

DP cell with

Process liquid

4-20 mA output

H L

Remote sealSeal fill fluid S.G. = 1.9

span = 11 ft

5 ft

D = 60 lb/ft3

Question 18

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

7

Measurement

0%

100%

DP cell with

Process liquid

H L

Remote seal

10-50 mA output

span = 24 ft

Seal fill fluid S.G. = 1.75

9 ft

15 ftD = 88 lb/ft3

Question 19

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

Measurement

0%

100%

DP cell with

Process liquid

H L

Remote seal

10-50 mA output

span = 24 ft

Seal fill fluid S.G. = 1.75

9 ft

15 ft

PressureINCREASE

D = 88 lb/ft3

Question 19.5

Draw the P&ID (”flowsheet”) symbol for an electronic differential pressure transmitter with remoteseals 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, wherebya transmitter measures the backpressure of air inside a ”dip tube” inserted into the vessel:

8

H L

bubbles

diptube

Pressure regulator

needle valve

Compressedair supply

4-20 mA outputDP cell with

Measurementspan = 18 ft

Process liquid

0%

100%

D = 94 lb/ft3

Determine a 5-point (0%, 25%, 50%, 75%, and 100%) calibration table for the transmitter in this levelmeasurement scenario, with a calibration tolerance of +/- 0.5%, specifying which side (high or low) of thetransmitter that the calibration pressure must be applied for each calibration point. Assume that the lowerrange-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. Forexample, in this level measurement system, compressed air is used as a purge medium directly into the vesselwhere the transmitter tubing connects:

H L

Pressure regulator

needle valve

Compressedair supply

Measurement

Process liquid

0%

100%

bubbles

needle valve

span = 22 ft

air

air

D = 73 lb/ft3

9

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

H L

Pressure regulator

needle valve

Compressedair supply

Measurement

Process liquid

0%

100%

needle valve

span = 22 ft

air

Blockage

D = 73 lb/ft3

Question 22

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

H L

Pressure regulator

needle valve

Compressedair supply

Measurement

Process liquid

0%

100%

bubbles

needle valve

span = 22 ft

air

Blockage

D = 73 lb/ft3

Question 23

In purged (bubbler) instrument systems, simple flow-indicating devices are usually installed in line withthe purge tubing to indicate purge fluid flow:

10

H L

bubbles

diptube

Flow indicator

FISupply

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

H L

bubbles

diptube

Supply

Such a device is called a sight feed bubbler.

Another popular flow-indicating device is called a rotameter: a vertical, conical tube made of transparentmaterial, a ”plummet” inside the tube supported against the force of gravity by the force of the moving purgefluid:

11

H L

bubbles

diptube

Supply

Rotameter

Rotameters can withstand greater static pressures than sight feed bubblers, and are able to indicate theflow 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 wellwithout purge fluid flow indication?

Question 24

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

12

H L

Measurement

Process liquid

0%

100%

span = 22 ft

Pressurizedwater supply

water

water

24 ft

10 ft

Static pressure= 50 PSIG

100 PSI

10-50mA

D = 73 lb/ft3

Question 25

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

13

Vessel(dry)

Scale

2 in

iron5 ft

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

Scale

iron

Water3 ft

Question 26

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

14

Processliquid

Weight-measuringmechanism

Vessel

Displacer"cage"

Blockvalves

disp

lace

rOften, 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:

disp

lace

r

Processliquid

Weight-measuringmechanism

Vessel

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

15

Weight-measuringmechanism

Vessel

Scale

Pull up on stringuntil scale registersthe desired force

"Dry" calibration

Processliquid

valvesclosed

Liquid drainedout of cage

disp

lace

r

But suppose you had no scale to use for such a ”dry” calibration. Can you think of another way tosimulate 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, scalyresidue accumulates and adheres to the displacer surface over time. What effect will such a residue have onthe transmitter’s calibration, as it alters both the dry weight and the effective volume of the displacer? Willthere 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 atransmitter with no residue accumulation:

Process liquid level

Transmitteroutput

100%

0%

0% 100%

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

16

Question 28

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

Blockvalves

disp

lace

r

0%

100%

Measurementspan = 24 in

Water0%

100%

Measurementspan = 24 in

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

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

Blockvalves

disp

lace

r

0%

100%

Measurement

0%

100%

Measurementspan = 30 in span = 30 in

Processliquid

D = 55 lb/ft3

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

Overflow

(water)

Head pressure= ???

Liquidin

11 ft

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

Overflow

Head pressure= ???

Liquidin

11 ft(gasoline)

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)?

Overflow

Head pressure= ???

Liquidin

11 ft

(gasoline)

(water)

Question 31

18

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

Overflow

Liquidin

11 ft

(gasoline)

(water)

H L

DP cell with4-20 mA output

D = 42 lb/ft3

D = 62.428 lb/ft3

Question 32

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

DP cell with

S.G. = 0.8

S.G. = 1.0

Interfacemeasurement

span = 6 ft

H L

3-15 PSI output

100%

0%

"wet" legS.G. = 0.8

6 foot

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 levelmeasurement scenario, with a calibration tolerance of +/- 0.75%. Also, specify which side (high or low) ofthe transmitter that the calibration pressure must be applied for each calibration point.

19

DP cell with

Interfacemeasurement

100%

0%

4-20 mA output

H L

Remote sealSeal fill fluid S.G. = 1.1

span = 30 in

S.G. = 0.75

S.G. = 1.2

30 in

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 (abovethe upper transmitter remote seal height).

Question 34

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

Question 34.5

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

displacer

LT

Flow Flow

Flow will exert lateralforce on displacer, causing

measurement errors

"choppy" liquid surface

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 liquidthat it holds. Depending on the shape of the vessel, height may or may not directly correlate to volume ormass.

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

20

Cylindrical Rectangular(vertical)

Cylindrical(horizontal)

Spherical

Question 36

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

Cylindrical(vertical)

LevelVolume

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

100%

0%

0% 100%Liquid volume

Liquid level ???

Question 37

Draw the symbols for the following types of liquid level indicating instruments, each one mounted tothe 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 appliedto the ”low” side)

URV = 360 ”W.C.

Answer 4

The terms elevation and suppression (or depression) describe situations where the pressure transmitteris not located at the 0% process level height. Believe it or not, these terms are often interchanged whenspeaking 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 ele-vated zero and suppressed zero from elevated span and suppressed span. One refers to the perspective of thetransmitter 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 Output(ideal)%

0

25

50

75

100

(" W.C.)

3 PSI

15 PSI

9 PSI

6 PSI

12 PSI

51

193.8

122.4

86.7

158.1

23

Answer 7

Span = 89.1 ”W.C.

Input Output(ideal)%

0

25

50

75

100

(" W.C.)

4 mA

20 mA

12 mA

8 mA

16 mA

2.835

-19.44

A vacuum appliedto the "high" side,or a pressure of19.44 "W.C. appliedto the "low" side.

25.11

47.385

69.66

Answer 8

Span = 306 ”W.C.

Input Output(ideal)%

0

25

50

75

100

(" W.C.)

10 mA

50 mA

30 mA

20 mA

40 mA

0

306

153

76.5

229.5

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

Input Output(ideal)%

0

25

50

75

100

(" W.C.)

10 mA

50 mA

30 mA

20 mA

40 mA

-385.5

-462

-309

-232.5

-156

Either calibratethe transmitter

with vacuums ofthese magnitudes,or with pressures

applied to the"low" side

Answer 10

Input Output(ideal)%

0

25

50

75

100

(" W.C.)

6 PSI

30 PSI

18 PSI

12 PSI

24 PSI

-51.5

-64

-39

-26.5

-14

Calibrate transmitterusing vacuums of thesemagnitudes applied to

"high" side or pressuresof these magnitudesapplied to "low" side

Answer 11

Connecting the transmitter to the process vessel:

HL

DP cell with6-30 PSI outputS.G. = 1

wet legS.G. = 1

port orientations!Note "low" and "high"

25

Calibration table (all positive pressures):

Input Output(ideal)%

0

25

50

75

100

(" W.C.)

6 PSI

30 PSI

18 PSI

12 PSI

24 PSI

64

14

39

26.5

51.5

Process level,in percent

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 tohigher process level percentages – the transmitter is indicating in reverse!

Answer 12

Input Output(ideal)%

0

25

50

75

100

(" W.C.)

100.92 (L)

46.85 (L)

7.21 (H)

61.27 (H)

115.33 (H)

4 mA

8 mA

12 mA

16 mA

20 mA

Output(minimum) (maximum)

Output

4.16 mA3.84 mA

7.84 mA 8.16 mA

12.16 mA

16.16 mA

20.16 mA

11.84 mA

15.84 mA

19.84 mA

Answer 13

26

Input Output(ideal)%

0

25

50

75

100

(" W.C.)Output

(minimum) (maximum)Output

10 mA

20 mA

30 mA

40 mA

50 mA

10.8 mA

20.8 mA

30.8 mA

40.8 mA

50.8 mA

9.2 mA

19.2 mA

29.2 mA

39.2 mA

49.2 mA

120 (L)

6 (L)

108 (H)

222 (H)

336 (H)

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:

H L

...

(empty vessel)

Fill liquid dribbling out ofvertical piping between transmitter and vessel

If this were to happen, the amount of head ”suction” normally created by the liquid height in theconnecting 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 connectingtubing is inconsequential compared to the pressure range being measured (500 PSI). There would be so littleapplied 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 remotely-located pressure transmitter by means of fluid-filled capillary tubing. Here is a diagram of remote seals usedon a pneumatic force-balance differential pressure transmitter:

27

diap

hrag

m

bellows

nozzle

flapper

Air pressuresignal out

Regulatedcompressedair supply

orifice

fulcrum andseal fo

rce

bar

capillary tubing

(transmitterfilled with oil

as well)

diap

hrag

m

oil

diap

hrag

m

oil

Chemical seal Chemical seal

Pneumatic differential pressure transmitter

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

28

Processvessel

H L

capillarytube

remotediaphragm

Transmitter

Fill fluidhead

Answer 17

Input Output(ideal)%

0

25

50

75

100

(" W.C.)

4 mA

20 mA

12 mA

8 mA

16 mA

-114

-82.28

-50.57

-18.85

12.87

Note how all but one of the calibration points involve negative pressures. If the ”low” side of thetransmitter is vented (without a remote seal attached), these negative pressures may be easily simulated byapplying positive pressure directly to that port. If the ”low” side has a remote seal, the transmitter mustbe 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 transmitterwhile being calibrated, so that there are no head pressure effects while on the calibration bench!

Answer 18

29

Input Output(ideal)%

0

25

50

75

100

(" W.C.)

10 mA

50 mA

30 mA

20 mA

40 mA

-504

-402.51

-301.01

-199.52

-98.03

Answer 19

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

Answer 19.5

LT

10-50 mAoutput

Answer 20

Input Output(ideal)%

0

25

50

75

100

(" W.C.)

4 mA

8 mA

12 mA

16 mA

20 mA

Output(minimum) (maximum)

Output

4.08 mA

8.08 mA

12.08 mA

16.08 mA

20.08 mA

3.92 mA

7.92 mA

11.92 mA

15.92 mA

19.92 mA

81.31 (H)

0

162.62 (H)

243.93 (H)

325.24 (H)

30

Answer 21

If the lower process connection were to become blocked by debris, the transmitter’s output signal wouldincrease, 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 woulddecrease, 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 thosementioned in questions #21 and #22.

Answer 24

Input Output(ideal)%

0

25

50

75

100

(" W.C.)Output

(minimum) (maximum)Output

10 mA

20 mA

30 mA

40 mA

50 mA

10.12 mA

20.12 mA

30.12 mA

40.12 mA

50.12 mA

39.88 mA

49.88 mA

29.88 mA

19.88 mA

9.88 mA

210.82 (L)

133.65 (L)

288 (L)

56.47 (L)

20.71 (H)

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 youwould normally access the displacer to attach a string and scale), then slowly open the lower block valveto let process liquid into the cage. If the vessel is pressurized significantly above atmospheric pressure, theliquid will rise up and fill the chamber as far as you let it:

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Weight-measuringmechanism

Vessel

Processliquid

closedvalve

valveopen di

spla

cer

(vented)

Be careful, though! If the process liquid is hazardous and/or under high pressure inside the processvessel, 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:

Process liquid level

Transmitteroutput

100%

0%

0% 100%

No residue

With residue

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

Answer 28

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Input%

0

25

50

75

100

(lb)

0

6.129

3.064

1.532

4.597

Answer 29

Input%

0

25

50

75

100

(lb)

0

9.187

6.890

4.593

2.297

Answer 30

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

Answer 31

Input Output(ideal)%

0

25

50

75

100

(" W.C.)

4 mA

20 mA

12 mA

8 mA

16 mA

88.81

99.6

110.4

121.2

132

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Answer 32

Input Output(ideal)%

0

25

50

75

100

(" W.C.)Output

(minimum) (maximum)Output

3 PSI

15 PSI

12 PSI

6 PSI

9 PSI

3.03 PSI

6.03 PSI

9.03 PSI

12.03 PSI

15.03 PSI

2.97 PSI

5.97 PSI

8.97 PSI

11.97 PSI

14.97 PSI

0

14.4 (H)

7.2 (H)

3.6 (H)

10.8 (H)

Answer 33

Input Output(ideal)%

0

25

50

75

100

(" W.C.)

4 mA

8 mA

12 mA

16 mA

20 mA

Output(minimum) (maximum)

Output

4.12 mA

8.12 mA

12.12 mA

16.12 mA

20.12 mA

3.88 mA

7.88 mA

11.88 mA

15.88 mA

19.88 mA

10.5 (L)

7.125 (L)

3.75 (L)

0.375 (L)

3.00 (H)

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 thebottom end fully submerged and the top end above the highest oil level, to act as a stilling well for the

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

LT

Flow Flow

Stilling well(vertical pipe)

support

support

Answer 35

The relationship between level and volume will be linear for the (vertical) cylindrical and rectangularvessel 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, thenmore rapidly as the cross-sectional area decreases. The result is a transfer function that looks like this:

100%

0%

0% 100%Liquid volume

Liquid level

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.

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