Pressure Measurement

TTC                    PRESSURE MEASUREMENT                   LAB #X

                                                        

                  

OBJECTIVES

1.         Examine a strain gauge pressure transducer.

2.         Calibrate a pressure switch and determine its dead band.

3.         Calibrate a pressure transmitter and test its operation.

THEORY

Pressure is a physical property which can be represented in units of force per unit area (kPag, psig, etc.), or as head of a column of known liquid (mm of mercury, inches of water, etc.).  In order to properly calibrate pressure transducers and pressure transmitters, pressure sources or pressure reference instruments must be at least 5 times more accurate than the instruments being calibrated.

The most fundamental pressure reference source is ob­tained from dead weight testers and manometers.  Accurate mechanical master gauges and electronic pressure calibrators can also be used as secondary pressure standards for calibration pur­poses.

EQUIPMENT

- Strain Gauge Pressure Transducer (Viatran Model 108 )

- Pressure Switch (UE 555)

- Differential Pressure Switch (UE 455)

- Pressure Transmitter (Foxboro 841GM)

- Decade Box

- Fluke 8060A DMM

PRE-LAB

Strain Gauge Pressure Transducer  (GP-50)

A strain gauge undergoes a change in resistance when it is strained (stretched or compressed). This small change in resistance can be detected and changed into a proportional mV signal by use of a bridge circuit. The Viatran Model 108 Pressure Transducer uses four active strain gauges mounted on a diaphragm which is deflected when exposed to a pressure.

1.         Sketch the hook-up for testing the Viatran Model 108 given the following equipment:

·                     5 VDC Power Supply

·                     DMM

·                     Pressure gauge and regulator

Pressure Switch Calibration  (UE 555 and UE 455)

Pressure switches open and close a set of contacts which may trigger alarms or operate equipment to control the pressure.  These contacts allow connection in one of two ways: “Common” connected to “Normally Open”, or “Common” connected to “Normally Closed”.  The method of connection depends on the required use.

Fig A shows the pressure switch when it is in it’s “off the shelf” state.

Fig B shows the pressure switch when it is in it’s operating state (i.e. pressure alarm state).

2.         Sketch the hook-up necessary for testing the UE 555 pressure switch given the following equipment:

·                     Pressure gauge and regulator

·                     DMM to test switch state (measuring resistance:  low resistance shows that switch is closed, high resistance shows that switch is open)

3.         Explain how to adjust the trip point of the UE J402k pressure switch.

Pressure Transmitter  (Foxboro 841GM)

This transmitter is designed to provide a 4 to 20 mA output signal for a 0 to 25 psig input pressure signal.

4.         Sketch the hook-up necessary to calibrate the Foxboro 841GM pressure transmitter using the following equipment:

·                     The bench regulator and test gauge to produce the input pressure

·                     The bench 24 Volt power supply to power the PT

·                     A DMM to measure the current output

5.         Explain how to calibrate this transmitter for a range of 0 to 160 kPag.

6.         What is the maximum loop resistance for the pressure trans­mitter if powered by a 24V power supply?

7.         How can we use tp1 & tp2 to calibrate the pressure trans­mitter?

PROCEDURE

Strain Gauge Pressure Transducer  (Viatran Model 108)

1.         Connect up the Viatran Model 108 Pressure Transducer pressure transducer to the bench regulated pressure and apply the pressures required to fill in table 1.  Note, use the bench 5 V power supply to power the transducer and the Fluke 8060A to measure the mV output.  Note, the pressure gauge is not connected to the regulated pressure, therefore connect one of the regulated outlets to this gauge.

2.         From the mV measured at pressure inputs of 0 kPag and 120 kPag, use the input/output formula to calculate the mV that you would expect to see at the other pressures in the table.  Show your calculation for an input of 60 kPag.

3.         The calculated output values from step 2 assume that this strain gauge is a perfectly linear device.  Calculate the error between the measured and calculated output. Record your results in table 1.

4.         From your results, produce an error graph.  Comment on the linearity of this pressure transducer.  List possible sources of error in your results.

Pressure Switch Calibration  (UE 555 and UE 455)

5.         Calibrate the UE 555 dual pressure switch.  Have one switch trip at 10 psig rising, and the other switch trip at 20 psig rising.  Use the bench regulator and Druck 145 DPI to apply/indicate the pressure and the DMM to determine when the contacts open/close.  Use the Common and Normally Open contacts.  Note the state of the contacts before and after the pressure trip.  Record the actual trip pressure in table 2.

6.         Slowly decrease the pressure until the contacts change back to their original state. Record this pressure and calculate the dead band in psig.  Record your results in table 2.

7.         Calibrate the UE 455 differential pressure switch.  Have one switch trip at 30 “W.C. differential falling, and the other switch trip at 60 “W.C. differential falling..  Use the bench regulator and Druck 145 DPI to apply/indicate the pressure and the DMM to determine when the contacts open/close.  Use the Common and Normally Open contacts.  Note the state of the contacts before and after the pressure trip.  Record the actual trip pressure in table 3.

8.         Slowly decrease the pressure until the contacts change back to their original state. Record this pressure and calculate the dead band in psig.  Record your results in table 3.

9.         Comment on your observations and explain the need for some dead band with pressure switches.

Pressure Transmitter  (Foxboro 841GM)

10.       Calibrate this pressure transmitter for a 0 to 160 kPag input as per pre-lab question #4.

11        Check the calibration of this transmitter, and calculate error values.  Record your results in table 4.

12.       Calculate the calibrated accuracy of this transmitter in % of span.  Show your accuracy calculation.

13.       Compare the accuracy of this transmitter to the manufacturers stated accuracy.  Explain possible sources of error in the calibration procedure.

14.       Connect a decade box in series with the output current loop.  Initially, set the decade box resistance to zero.  Adjust the pressure input until the loop current is exactly 20 mA.  Next, slowly increase the decade box resistance until the transmitter can no longer maintain it’s 20 mA output (i.e. increase resistance until the current just starts to drop below 20 mA.).  Record the decade box setting.   Compare this resistance to the expected maximum loop resistance and explain any discrepancies.

TTC                                       RESULTS                                      LAB #X

NAME:          ____________________________                          DATE: _______________

LAB PARTNER:     _____________________________      CLASS: _________________

Pre-Lab

Signature:    ________________________

Strain Gauge Pressure Transducer  (GP-50)

Test set-up (pre-lab Q#1):

Results:

Input Pressure (kPag)	0	20	40	60	80	100	120
GP-50 Output (mV)
Calculated Output (mV)
Linearity Error (mV)

                                                                          Table 1

Sample calculation of output at 60 kPag (step 2):

Error Graph (step 4):

Discussion and Observations (step 4):

Pressure Switch Calibration  (UE J402k)

Test set-up (pre-lab Q#2):

Trip adjustment (pre-lab Q#3):

Results:

	Switch 1	Switch 2
Trip Pressure (psig)
Reset Pressure (psig)
Dead Band (psig)

                                                                        Table 2

Results:

	Switch 1	Switch 2
Trip Pressure (“W.C.)
Reset Pressure (“W.C.)
Dead Band (“W.C.)

                                                                        Table 3

Discussion and observations and need for dead band (step 9):

Pressure Transmitter  (Foxboro 841GM)

Test set-up (pre-lab Q#4):

Calibration procedure (pre-lab Q#5):

Maximum loop resistance (pre-lab Q#6):

Use of tp1 and tp2 (pre-lab Q#7):

Results:

Input Pressure (kPag)	Expected Output (mA)	Measured Output (mA)	Error (mA)
0
20
40
60
80
100
120
140
160

                                                            Table 3

Accuracy calculation (% of span)(step 12):

Discussion and observations (step 11):

Maximum loop resistance calculation and discussion (step 13):

Reference : NAIT Edmonton CA