Pressure Transducer Calibration - MyWeb at WIT

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```					Pressure Transducer Calibration

Objectives:
1. Configure and acquire 1 channel of analog voltage data using DAQ assistant
2. Utilize specification sheet for a pressure transducer to determine proper set up and
interface with a data acquisition system
3. Develop a simple VI to acquire and display voltage measurement
4. Acquire test signal data to ensure proper VI operation
5. Modify VI to acquire and display both voltage and pressure from a pressure
transducer with manufacturer’s supplied calibration curve fit data
6. Acquire a statistically significant number of calibration data samples and
determine perform a linear regression analysis for the voltage vs. theoretically
applied pressure curve.
7. Evaluate the data and discuss the error in the set up relative to bias, precision and
hysteresis of the setup.

PRESSURE MEASUREMENT PRINCIPLES:
The most accurate instrument to measure moderate to high pressures is a dead
weight tester. This type of tester operates on the principle of balancing a known mass
against the force exerted by an unknown pressure on a piston of known area. When an
exact balance is achieved, the unknown pressure P is equal to the weight of the weights
divided by the area of the piston, according to the formula P=F/A.

The dead weight tester can be used to calibrate pressure gauges and pressure transducers.
It is also used to determine unknown pressures. A precisely measured piston is used to
apply pressure to a test fluid connected to a pressure gauge or unknown pressure source.

As the weight is increased on the table, the pressure increases in the system based on the
formula P=W/A. Where W is the weight of the table and weights loaded on it and A is
the area of the piston under the table.

PRESSURE GAGE:
A mechanical pressure gage reads directly in Lbf/in2 or Kpa. In order to change the
readings between the two unit systems we must multiply the reading by the correct
conversion factor.

1 psi = 6.8947 kPa

PRESSURE TRANSDUCER:
The pressure transducer does not indicate pressure directly, but as the pressure changes
the voltage output changes. Therefore the pressure transducer voltage output is used as an
indicator of pressure. To find out what the pressure is for a particular voltage, the
transducer has to be calibrated. As long as the relationship between voltage output and
pressure is linear, the relationship can be expressed as a linear equation of the form:
Y= M*X+b

WHERE:          Y = Pressure (P)
X = Voltage (V)
M = slope of the graph

Y P2  P
M               1

X V2  V1
b = the Y intercept of the curve

b is found by substituting M, P1 and V1 into the equation above and solving for b, or it
can be found by substituting M, P2 and V2 into the equation and solving for b.

Omega PX139-005D4V model pressure transducer will be utilized for the laboratory and
the spec sheet can be found at www.omega.com. Download the specification sheet PDF
and utilize this information in determining the electrical signal interfaces and above
critical parameters to convert the voltage into a pressure indication on the front panel of
the VI. The pressure transducer requires an excitation voltage and it must be supplied
from the ELVIS system, determine where it will be derived from on the circuit board pin
outs.

Procedure:

1. Read the lab report again carefully and develop necessary tables to document all
your results. Developing tables in Excel to transpose your data will greatly
simplify data reduction time. Ensure you set up the tables to handle eventual
statistical analysis for each of the 5 measured pressure levels.
2. Develop a simple VI and use ELVIS DMM to verify correct operation utilizing
appendix A, Acquiring a Signal with DAQ
3. Simulate signal with ELVIS and ensure proper VI operation
a. Generate a voltage with ELVIS and measure on the ELVIS DMM to
confirm level
b. Input the voltage into channel 0 in differential measurement mode.
c. Compare the readings to verify VI operation
4. Field wire the pressure transducer with the supplied shielded 4 strand wire and
connector
5. Utilize the 5V ELVIS output power supply to provide power to the pressure
transducer.
6. Check output from terminals with ELVIS DMM or volt meter to verify level prior
to attaching to pressure transducer.
7. Input leads to VI input terminals and check VI operation for voltage with readings
obtained from the DMM
8. Modify VI with calibration parameters and connect simulated voltage of 2, 2.5, 3
and 3.5 V to compare to pressure calibration curve manual output.
9. Use the PASCO heat engine as a dead weight tester along with the weights
supplied to generate an input pressure to the pressure transducer.
10. Determine the pressure applied from the following relationship: P=F/A obtaining
the piston area from the PASCO write up and F=mg for the applied load.
11. Measure air pressure or atmospheric pressure without the pressure transducer
connected to the PASCO unit and then with it attached, record voltage.
12. Add weights to the platform slowly and gently in succession to obtain 4 distinct
pressures and record the voltages in a table marked upward. Do not apply more
than a total of 4 kg to the system.
13. Remove weights slowly in succession to measure pressure going downward until
all weights are removed record downward voltage plateaus in a separate table.
14. Repeat the process including the atmospheric pressure measurement until 30
values are obtained for each plateau in both tables.

Data analysis
1. Develop a table of the statistical value for the true mean with 95% confidence
interval for each plateau both upward and downward. (2 Tables)
2. Develop a ±3σ range of expected measurements for each plateau up and down. (2
Tables)
3. Develop a statistical value for the true mean atmospheric pressure with 95%
confidence interval. Compare this value to the barometer reading in the thermo
lab.
4. Develop a statistical value for the true mean of the pressure induced by the weight
of the piston.
5. Develop a linear regression analysis of the entire data population for voltage (y-
axis) vs. applied pressure (x-axis) (linear curve fit to data)
6. Develop upper and lower limit lines based on a 95% confidence interval for both
slope and offset.
7. Using our calibration device and linear regression curve, compare the values of
pressure obtained with theoretically applying loads of .5-5 kg in .5 kg increments
to the calibration curve obtained from Omega.
8. Calculate the %error of the derived curve to the Omega curve as follows:

Graphs
1. Plot all the raw data both upward and downward on a zero error plot for measured
voltage vs. applied pressure (data points only).
2. Show the upper and lower range of expected value curves based on the linear
regression analysis. (plot upper and lower offset curves)
3. Plot the expected voltage from both the linear regression curve develop in the lab
with the theoretical curve from Omega using the data from #7 above.
4. Plot the % error obtained from the comparison of experimental to Omega data.
Concluding Questions
1. Did you observe differences in the error depending on the direction the weights
were applied? If so why?
2. How well did your calibration curve compare to the one supplied by Omega?
3. Characterize the errors in comparing the two curves such as bias, accuracy,
precision etc.
4. What was the difference in measured pressure due to the weight of the piston?
5. What type of error does this introduce into the pressure measurement.
Appendix A: Acquiring a Signal with DAQ

Acquiring a Signal with DAQ

Note: Before beginning this exercise, copy the Exercises and Solutions Folders to the
Complete the following steps to create a VI that acquires data continuously from your
simulated DAQ device.
1. Launch LabVIEW.
2. In the Getting Started window, click the New or VI from Template link to display
the New dialog box.
3. Open a data acquisition template. From the Create New list, select VI»From
Template»DAQ»Data Acquisition with NI-DAQmx.vi and click “OK”.
4. Display the block diagram by clicking it or by selecting Window»Show Block
Diagram. Read the instructions written there about how to complete the program.
5. Double-click the DAQ Assistant to launch the configuration wizard.
b. Choose Dev1 (PCI-6014)»ai0 to acquire data on analog input channel 0
and click “Finish.”
On the task timing tab, choose “Continuous” for the acquisition mode,
enter 1000 for samples to read, and 10000 for the rate. Leave all other
choices set to their default values. Click “OK” to exit the wizard.
7. On the block diagram, right-click the black arrow to the right of where it says “data.”
Choose Create»Graph Indicator from the right-click menu.
8. Return to the front panel by selecting Window»Show Front Panel or by pressing
<Ctrl+E>.
9. Run your program by clicking the run button. Observe the simulated sine wave on the
graph.
10. Click stop once you are finished.
11. Save the VI as “Exercise 2 – Acquire.vi” in the Exercises folder. Close the VI.

Notes:
• The solution to this exercise is printed in the back of this manual.
• You can place the DAQ Assistant on your block diagram from the Functions
Palette. Right-click the block diagram to open the Functions Palette and go to
Express»Input to find it. When you bring up the functions palette, press the
small push pin in the upper left hand corner of the palette. This will tack down the
palette so that it doesn’t disappear. This step will be omitted in the following
exercises, but should be repeated.

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