exercise pressure measurement - lab-volt · most pressure measurement devices belong to the...

© Festo Didactic 87996-00 39

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.

The Discussion of this exercise covers the following points:

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).

Pressure Measurement

Exercise 2-1

EXERCISE OBJECTIVE

DISCUSSION OUTLINE

DISCUSSION

Ex. 2-1 – Pressure Measurement Discussion

40 © Festo Didactic 87996-00

You can convert the height of liquid to a gauge pressure using Equation (2-5).

(2-5)

where is the gauge pressure

is the fluid density is the acceleration due to gravity

is the head

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.

Process fluid

Open to atmosphere

Manometric liquid



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.

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 2-16 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).

Figure 2-14. Top-view and side-view of a

capsule diaphragm.

Figure 2-15. Wheatstone bridge coupled with a strain gauge.

Dial face

Needle pointer

Bourdon tube (expanding)



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.

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.

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.

Figure 2-17. Primary element of a pressure

transmitter.

Sensing diaphragm

Separatingdiaphragms

Sensing element

Active grid length

(a) Wire-type strain gauge

(b) Semiconductor gauge

Strain-sensitive foil pattern (grid)

Terminals

Terminal Terminal

n-well p-substrate

n+ contacts

Filling oil



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.

Figure 2-18. Filling the impulse line with water or air.

Air

Water

Impulse line filled with air

Impulse line filledwith water

Ex. 2-1 – Pressure Measurement Procedure Outline


The Procedure is divided into the following sections:

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.

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.

Figure 2-19. Measuring pressure with a pressure gauge.

PROCEDURE OUTLINE

PROCEDURE

Hand valve HV4

Pressure gauge PI1

Clear plastic tube

Return hose

Discharge hose

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:

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.



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:

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 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.



Figure 2-21. Measuring pressure with a differential-pressure transmitter.

14. Power up the DP transmitter, using the following steps:

Connect the + and – terminals of the power supply to the corresponding

power terminals of the DP transmitter.

Turn on the power supply.

Transmitter calibration

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:

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.

Throughout the manual, we

use the current output of the

DP transmitter. However,

you can use the 0-5 V out-

put in a similar manner.

Open to atmosphere



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 high-pressure 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.



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.

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.



Figure 2-22. Verifying the accuracy of a pressure gauge with a liquid manometer.

Hand valve HV4

Pressure gauge PI1

Clear plastic tube

Return hose(leave unconnected)

Discharge hose

To auxiliary return port

Cap

Column opening

Plug

Column

Plug

20 cm (8 in) / 4 rows of perforation



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:

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).

Overflow hose

Return hose (not connected)



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:

or

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.

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.

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.

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?

CONCLUSION

REVIEW QUESTIONS

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?

exercise pressure measurement - lab-volt · most pressure measurement devices belong to the...

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