Post on 22-Oct-2014
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
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Capacitance:
LI
CA
Answer 38
The instrument in question is a level gauge, otherwise known as a sightglass.
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