Exercise
2-1
Pressure Measurement
EXERCISE OBJECTIVE
In this exercise, you will become familiar with classic pressure measurement
devices. You will also measure pressure using a pressure gauge, a pressure
transmitter, and a liquid manometer.
DISCUSSION OUTLINE
The Discussion of this exercise covers the following points:
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DISCUSSION
Classic pressure measurement devices
Strain-gauge pressure sensing devices
How to install a pressure-sensing device to measure a pressure
What is bleeding?
Classic pressure measurement devices
Most pressure measurement devices belong to the manometers or to the elastic
pressure sensors category. Manometers use a column of liquid to measure
pressure. The manometer mechanism applies the measured pressure to a
column of liquid and the variation in the liquid level is measured. The use of a
column of liquid limits the use of manometers to small near-atmospheric
pressure. Piezometer tubes, U-tube manometers, and inclined-tube manometers
are examples of simple manometers.
Elastic pressure sensors use an elastic element to measure pressure. The
measured pressure pushes on an elastic element and the resulting deformation
enables production of a signal proportional to the pressure. Most of the time,
primary sensing elements in local indicators or in electronic transmitters are
elastic pressure sensors. Bourdon tubes, strain gauges, diaphragms, and
bellows meters are elastic pressure sensors. The sections below present the
working principles of liquid manometers and Bourdon tube pressure gauges.
These devices are classic pressure measurement devices and understanding
how they work will help your comprehension of the physics behind pressure
measurement devices.
U-tube manometers
U-tube manometers are one of the oldest and simplest pressure measurement
devices. The main element of U-tube manometers is a U-shaped glass or plastic
tube that contains a liquid such as water or mercury. The liquid is selected so
that it does not react when in contact with the process fluid (Figure 2-12). One
end of the tube is open to the atmosphere and the process fluid exerts a
pressure at the other end of the tube. This pressure pushes the manometric
liquid and causes it to rise in the tube proportionally. The height, or head, to
which the manometric liquid rises above the point of contact with the process
fluid is proportional to the process fluid pressure (when the density of the
manometric liquid is significantly higher than that of the process fluid).
© Festo Didactic 87996-00
39
Ex. 2-1 – Pressure Measurement  Discussion
You can convert the height of liquid to a gauge pressure using Equation (2-5).
where
ܲ௚
ߩ
݃
݄
ܲ௚ ൌ ߩ݄݃
(2-5)
is the gauge pressure
is the fluid density
is the acceleration due to gravity
is the head
Open to atmosphere
Process fluid
Manometric liquid
Figure 2-12. U-tube manometer.
U-tube manometer manufacturers must select the manometric liquid with care so
that it provides the desired measurement accuracy. The use of water
manometers is limited to the measurement of pressure close to atmospheric
pressure because a small variation in pressure causes a relatively large
displacement of water. For example, to measure a gauge pressure
of 7 kPa (1 psig) with a water manometer, the manometer column has to be
over 71 cm (28 in) high. The manufacturer can significantly increase the
measurement range of a manometer by using mercury instead of water. The
density of mercury is 13.6 times the density of water. Thus, for a given pressure,
the mercury displacement is 13.6 times less than the water displacement.
Liquid manometers are sufficiently accurate to serve as standards for checking
the calibration of other pressure measurement devices. However, liquid
manometers are fragile and bulky, which restricts their use to laboratories or as
local indicators.
Bourdon tube pressure gauges
Bourdon tube pressure gauges provide a direct reading of the pressure. They
use a primary sensing element called a Bourdon tube to sense pressure.
Figure 2-13 shows a typical Bourdon tube pressure gauge. The pressure gauge
consists of a needle pointer attached through a gear linkage to a Bourdon tube,
which is a C-shaped flexible coiled tube. The Bourdon tube is hollow and it
connects directly into the process fluid line. As the pressure increases, the
bourdon tube straightens. This moves the gear linkage and causes the needle
pointer to move on the dial. The Bourdon tube is made of material with elastic
properties so that it deforms under pressure and returns to its original shape
when it is no longer subject to pressure.
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© Festo Didactic 87996-00
Ex. 2-1 – Pressure Measurement  Discussion
Bourdon tube
(expanding)
Dial face
Needle pointer
Figure 2-13. Bourdon tube pressure gauges.
Strain-gauge pressure sensing devices
The differential-pressure transmitter of the training system uses diaphragms to
detect changes in pressure. A diaphragm for a pressure measurement device is
usually made from a thin sheet of metal. The diaphragm may be flat or may have
concentric corrugations. Figure 2-14 shows how two corrugated discs can be put
together to form a capsule diaphragm.
Figure 2-14. Top-view
and side-view of a
capsule diaphragm.
When a diaphragm is under pressure, it deforms proportionally to the magnitude
of the pressure and a strain gauge measures the deformation of the diaphragm.
Whatever the type of strain gauge(s) in the pressure-sensing device, the
deformation of the diaphragm(s) due to the pressure is converted into a change
of electrical resistance. An electrical circuit called a Wheatstone bridge measures
this change in electrical resistance. Figure 2-15 shows a quarter-bridge strain
gauge circuit. In industrial measurement devices, this circuit is usually modified to
compensate for the wire’s resistance and for the effect of temperature on the
strain gauge.
Figure 2-15. Wheatstone bridge coupled
with a strain gauge.
© Festo Didactic 87996-00
Figure 2-16 shows two types of strain gauges: a wire-type strain
gauge and a semiconductor strain gauge. The wire-type strain
gauge is the older of the two types. It consists of a length of
conductor glued onto a flexible membrane using an epoxy resin.
When the membrane deforms, the conductor length changes and
the resistance of the conductor varies proportionally.
Semiconductor strain gauges use the piezoresistive properties of
semiconductors to measure a deformation. The electrical
resistance of the semiconductor changes when it is subject to a
mechanical stress (piezoresistive effect).
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Ex. 2-1 – Pressure Measurement  Discussion
Strain-sensitive foil
pattern (grid)
Terminals
Active grid
length
(a) Wire-type strain gauge
n+ contacts
Terminal
Separating
diaphragms
Terminal
n-well
p-substrate
Sensing
diaphragm
(b) Semiconductor gauge
Figure 2-16. Two types of strain gauges.
Figure 2-17 shows a typical arrangement for the primary element of a pressure
measurement device. In such a transmitter, the sensing element converts the
pressure into a change in electrical resistance and the secondary element (the
conditioning circuit) converts this change in electrical resistance into a signal
suitable for transmission to a controller. This signal can be either a voltage,
current, or pressure of normalized range.
Filling oil
Sensing
element
Figure 2-17. Primary
element of a pressure
transmitter.
How to install a pressure-sensing device to measure a pressure
To ensure accurate pressure measurement, you must take some precautions
when you install a pressure-sensing device. The list below enumerates these
precautions.
x
x
x
x
x
x
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For pressure measurement in liquids, mount the pressure-sensing device
below the measurement point.
For pressure measurement in gases, mount the pressure-sensing device
above the measurement point. This prevents accumulation of liquid in
the impulse line due to condensation.
Make sure the impulse lines are not bent, restricting the fluid flow.
Liquid in the impulse line creates a pressure on the sensing element of
the pressure-sensing device. You must adjust the zero of the device to
compensate for the pressure due to liquid in the impulse line.
Attach the impulse lines securely to something solid. If the impulse lines
vibrate or if you accidentally move an impulse line, the zero of the device
may shift.
Try to keep the temperature of the impulse line as close as possible to
the process temperature.
© Festo Didactic 87996-00
Ex. 2-1 – Pressure Measurement  Discussion
x
x
On a differential pressure-sensing device, make sure both impulse lines
have the same length.
You must bleed the pressure-sensing device to fill both the device and
the impulse lines with the process fluid.
What is bleeding?
When you connect a pressure-sensing device to a pressure port, you must fill the
impulse line linking the instrument to the pressure port with the process fluid.
This is especially important if you are measuring the pressure of a process using
a liquid, such as measuring the pressure in a pipe filled with water. Filling the
impulse line with the process fluid helps to avoid inaccurate pressure
measurements due to the compression of air that may be trapped in the impulse
line or in the pressure-sensing device. The procedure for purging air from both
the impulse line and the instrument is called bleeding.
When the process fluid is a gas, such as when you measure the air pressure at
the top of a column, it is a good habit to purge any liquid from the impulse line
and the instrument. The liquid in the impulse line does not significantly influence
the pressure readings (liquids are relatively incompressible); but this helps
protect the process from contamination. In any industry, liquid trapped in the
impulse line or in the pressure-sensing device can contaminate the process and
ruin a whole batch of product. To avoid such circumstances, always fill the
impulse line and device with the fluid of which you are measuring the pressure.
Figure 2-18 illustrates this principle with a column partially filled with water. If you
measure water pressure (left), you must fill the impulse line (and the instrument)
with water. If you measure air pressure (right), you must fill the impulse line with
air.
Impulse line filled
with air
Air
Water
Impulse line filled
with water
Figure 2-18. Filling the impulse line with water or air.
© Festo Didactic 87996-00
43
Ex. 2-1 – Pressure Measurement  Procedure Outline
PROCEDURE OUTLINE
The Procedure is divided into the following sections:
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PROCEDURE
Pressure gauge measuring
Measuring pressure with a differential-pressure transmitter
Verifying the accuracy of a pressure gauge with a liquid manometer
End of the exercise
Pressure gauge measuring
In this first circuit, hand valve HV4 will be used to vary the circuit resistance to
flow. Pressure gauge PI1 will be used to measure the pressure upstream of that
valve.
1. Set up the circuit depicted in Figure 2-19 and Figure 2-20.
x
x
Make sure the expanding work surface is mounted vertically on the main
work surface.
Use a clear, plastic tube with a male quick-connect fitting on both ends to
connect pressure gauge PI1 to valve HV4.
Hand valve HV4
Pressure gauge PI1
Return hose
Discharge hose
Clear plastic tube
Figure 2-19. Measuring pressure with a pressure gauge.
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© Festo Didactic 87996-00
Ex. 2-1 – Pressure Measurement  Procedure
2. Make sure the reservoir of the pumping unit is filled with about 12 liters
(3.2 gallons) of water. Make sure the baffle plate is properly installed at the
bottom of the reservoir.
Figure 2-20. Equivalent ISA diagram.
3. On the pumping unit, adjust valves HV1 to HV3 as follows:
x
x
x
Open HV1 completely.
Close HV2 completely.
Set HV3 for directing the full reservoir flow to the pump inlet.
4. Open hand valve HV4 completely.
5. Turn on the pumping unit and put the drive in the manual mode.
6. Make the pump rotate at maximum speed.
With valves HV4 and HV1 in the fully open position, the pumped flow is
allowed to return to the reservoir through these valves with little restriction.
Yet you should read a certain amount of pressure on pressure gauge PI1.
This pressure is created by internal frictional resistance of the hoses, fittings,
and valves.
7. Record the gauge pressure reading.
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Ex. 2-1 – Pressure Measurement  Procedure
8. While observing the gauge pressure reading, slowly turn the handle of HV4
until it is fully closed. Observe that as the valve is closed, it creates a greater
resistance to flow, causing the gauge pressure reading to increase. Is this
your observation?
‰ Yes
‰ No
9. With hand valve HV4 fully closed, the pumped flow is now blocked and the
pressure gauge reads the maximum pressure that can build upstream of
valve HV4. Record this pressure below.
Do not let the pump rotate for prolonged periods with the pumped flow blocked to avoid
pump overheating.
10. Open hand valve HV4 completely.
11. Stop pump. Leave your circuit as it is and proceed with the exercise.
Measuring pressure with a differential-pressure transmitter
a
This subsection can also be accomplished using the optional industrial
differential-pressure transmitter (Model 46929). Should you choose this piece
of equipment, refer to Appendix I for instructions on how to install and use the
transmitter for pressure measurements. Perform steps 12 to 14, calibrate the
transmitter for pressure measurements between 0 kPa and 100 kPa (0 psi
and 14.5 psi), and carry out steps 23 to 27.
12. Get the differential-pressure transmitter and the 24 V dc power supply from
your storage area. Mount these components on the expanding work surface
next to the hand valve and the pressure gauge.
a
The DP transmitter must be mounted vertically.
13. Referring to Figure 2-21, connect the DP transmitter upstream of valve HV4:
x
x
x
Get a clear plastic tube with a male quick-connect fitting on one end.
Connect the bare end of the tube directly into the high-pressure port of
the DP transmitter.
Connect the male fitting of the tube into the unused pressure port on
pressure gauge PI1.
a
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Since the low-pressure port of the DP transmitter is left open to atmosphere,
the DP transmitter generates a signal proportional to the gauge pressure at its
high-pressure port.
© Festo Didactic 87996-00
Ex. 2-1 – Pressure Measurement  Procedure
Open to atmosphere
Figure 2-21. Measuring pressure with a differential-pressure transmitter.
14. Power up the DP transmitter, using the following steps:
x
x
Connect the + and – terminals of the power supply to the corresponding
power terminals of the DP transmitter.
Turn on the power supply.
Transmitter calibration
Throughout the manual, we
use the current output of the
DP transmitter. However,
you can use the 0-5 V output in a similar manner.
In steps 15 through 22, you will adjust the ZERO and SPAN knobs of the DP
transmitter so that its output current varies between 4 mA and 20 mA when the
gauge pressure at its high-pressure port varies between 0 kPa
and 100 kPa (0 psi and 14.5 psi).
15. Make the following settings on the DP transmitter:
x
x
x
ZERO adjustment knob: MAX.
SPAN adjustment knob: MAX.
LOW PASS FILTER switch: I (ON)
16. Connect a multimeter to the 0-20 mA output of the DP transmitter.
17. Since the pump does not rotate, a gauge pressure of 0 kPa, gauge (0 psig) is
present at the high-pressure port of the DP transmitter.
While monitoring the current at the 4-20 mA output with a multimeter, turn the
ZERO adjustment knob counterclockwise until you read 4.00 mA.
© Festo Didactic 87996-00
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Ex. 2-1 – Pressure Measurement  Procedure
a
If you continue to turn the ZERO adjustment knob after the current has
reached 4.00 mA, a dead band occurs at low pressure levels. A dead band
causes the output signal of the DP transmitter to remain null (4.00 mA) even
when the gauge pressure changes at the high-pressure port of the DP
transmitter.
18. Make the pump rotate at maximum speed.
19. On the pumping unit, close valve HV1 completely. Observe that the
multimeter reading has increased because a gauge pressure of
about 100 kPa (14.5 psi) is now present at the high-pressure port of the
DP transmitter.
20. Adjust the SPAN knob in order to obtain a current of 20.0 mA at the 4-20 mA
output.
21. Open valve HV1 of the pumping unit completely.
22. Due to interaction between the ZERO and SPAN adjustments, repeat
steps 17 through 21 until the output of the DP transmitter actually varies
between 4.00 mA and 20.0 mA when the gauge pressure at the highpressure port is varied between 0 kPa and 100 kPa (0 psi and 14.5 psi).
Comparison between PI and PT
23. Now that the DP transmitter is calibrated, make the pump rotate at maximum
speed.
24. With valves HV1 and HV4 open, what is the analog output of the DP
transmitter? Does this value correspond to the pressure indicated by
pressure gauge PI1? Explain.
25. Close hand valve HV4 and observe what happens to the analog output of the
DP transmitter. Record your observations below.
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© Festo Didactic 87996-00
Ex. 2-1 – Pressure Measurement  Procedure
26. On the DP transmitter, switch off the LOW PASS FILTER and observe what
happens to the analog output of the transmitter. Does this value fluctuate?
Explain.
a
Set DAMPING VALUE to 0 second if you are using the industrial DP
transmitter.
27. Stop the pump.
Verifying the accuracy of a pressure gauge with a liquid manometer
The column will be used as a liquid manometer. The manometer will serve as a
reference to check the accuracy of pressure gauge PI1.
28. Set up the circuit shown in Figure 2-22 and Figure 2-23.
x
x
x
© Festo Didactic 87996-00
Mount the column upright so that its bottom is four rows of perforations
higher than the pressure ports on pressure gauge PI1 and hand
valve HV4.
Use an extra-long hose to connect the top right port of the column to
either of the auxiliary return ports of the pumping unit. This hose is used
as an overflow to discharge excess water to the reservoir when the
column becomes full.
On the column, block the unused hose ports using the provided plugs
and firmly tighten the top cap.
49
Ex. 2-1 – Pressure Measurement  Procedure
Column opening
Plug
Cap
Column
Plug
Hand valve HV4
20 cm (8 in) /
4 rows of perforation
Pressure gauge PI1
Clear plastic tube
To auxiliary
return port
Discharge hose
Return hose
(leave unconnected)
Figure 2-22. Verifying the accuracy of a pressure gauge with a liquid manometer.
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© Festo Didactic 87996-00
Ex. 2-1 – Pressure Measurement  Procedure
Overflow hose
Return hose
(not connected)
Figure 2-23. Equivalent ISA diagram.
29. On the pumping unit, make sure valve HV1 is open, valve HV2 is closed, and
valve HV3 is set as per Figure 2-23. Close hand valve HV4 completely.
30. Make the pump rotate at maximum speed. With valves HV2 and HV4 closed,
the pumped flow is blocked and pressure gauge PI1 should read the
maximum pressure.
31. Bleed air from the system. Use a plastic tube with a male quick-connect
fitting at one end to complete the following:
x
x
While directing the bare end of the tube into the reservoir of the pumping
unit, connect the tube male fitting into the unused port of the pressure
gauge. This causes water from the system to flow to the reservoir and
forces air out of the system.
When a constant stream of water is flowing out of the tube and no more
air bubbles are present, disconnect the tube from the pressure gauge
and return it to the storage location.
32. Reduce the pump speed until pressure gauge PI1 reads 21 kPa, gauge
(3 psig).
© Festo Didactic 87996-00
51
Ex. 2-1 – Pressure Measurement  Procedure
33. Open hand valve HV4 completely, which causes the water level to increase
in the column.
a
Observe that the reading of pressure gauge PI1 has dropped because the
pumped water is now allowed to flow into the column with very little restriction.
This demonstrates that the amount of pressure created in a system is only as
high as required to counteract the resistance to flow.
34. Wait until the column becomes full and the excess water returns to the
reservoir through the overflow hose.
35. Slowly reduce the pump speed to decrease the water level in the column
until the water level is stable at 51 cm (20 in). This may require you to make
several adjustments.
Now that the water level is stable in the column, the system is in a state of
equilibrium (steady state) where the weight of the column of water is supported
by the pressure applied at pressure gauge PI1. Consequently, the pressure
indicated by pressure gauge PI1 corresponds to the head of the water in the
column. This head, ݄, is the height of water above the gauge level, that is:
݄ ൌ ʹͲܿ݉ ൅ ͷͳܿ݉ ൌ ͹ͳܿ݉
or
݄ ൌ ͺ‹ ൅ ʹͲ‹ ൌ ʹͺ‹
36. Using Equation (2-5) convert the head, ݄, into a gauge pressure reading.
Assume that water density is 1000 kg/m3 (62.4 lbm/ft3) and that the
gravitational acceleration is 9.8 m/s2 (32.2 ft/s2).
a
If you are using US customary units, assume ratio g/gc to be equal to 1 lbf/lbm.
37. Record the current reading of pressure gauge PI1 in the space provided
below. This reading should correspond to the gauge pressure you obtained
in step 36. Does this reading fall inside the 3% of the full-scale margin
specified by the manufacturer? Explain.
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© Festo Didactic 87996-00
Ex. 2-1 – Pressure Measurement  Conclusion
End of the exercise
38. Stop the pump and turn off the pumping unit. Wait until all the water flows out
of the column.
39. Disconnect the circuit. Return the components and hoses to their storage
location.
40. Wipe off any water from the floor and the training system.
CONCLUSION
In this exercise, you learned that a wide variety of pressure measurement
devices exist. Selecting the proper device for an application is a matter of
matching the operational specifications of the device to the requirements of the
application.
Since pressure gauges and liquid manometers provide a direct visual reading of
the pressure, they are normally limited in their use as local indicators. To perform
closed-loop control of the pressure, a pressure transmitter must be used to
provide the controller with a normalized voltage, current, or air pressure signal
proportional to the measured pressure.
REVIEW QUESTIONS
1. How is pressure created in a flow system?
2. What is the difference between a gauge pressure reading and an absolute
pressure reading?
3. What are the two basic types of pressure measurement devices? How do
they operate?
© Festo Didactic 87996-00
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Ex. 2-1 – Pressure Measurement  Review Questions
4. What is a Bourdon tube pressure gauge of the C type? How does it operate?
5. What is a strain gauge pressure transmitter? How does it operate?
6. What is the purpose of bleeding ports on differential-pressure transmitters?
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© Festo Didactic 87996-00