Your goal in this experiment is to obtain a functional form for a pressure versus voltage plot for data collected from a pressure transducer. The results will be used in later experiments to calculate pressures from transducer voltages
Note that the Omega Engineering pressure transducers used in Ch 444 are gage-type transducers which operate over a pressure range of 0-15 psi. The transducers have two ports, one of which is connected to the measurand and the other of which is left open to atmospheric pressure. Thus, the voltage produced by the device is proportional to the difference between the measured pressure and atmospheric pressure. For purposes of this experiment, you may plot the pressure difference (as measured by reading a manometer) vs voltage, but later on in the term you will need to know how to calculate the actual pressure in a system from barometric pressure and the transducer voltage. Further, the Omega transducers utilize an amplifier circuit which not only neccesitates the use of an external 8 V DC power supply (see below), but also provides very linear output over the entire working range of the device. Note that, because the device is amplified, it will output an 'offset' voltage of 1.000V DC (with 8.00 VDC input at 25oC) when there is no pressure difference applied to the measuring ports.
Read the barometric pressure using the barometer at the rear of Sc 270. If you do not know how to read a barometer, consult your instructor.
We will utilize the vacuum rack along the side wall of Sc 270 for this experiment. Note that safety goggles are essential when working with vacuum systems. The vacuum rack is connected to a mechanical vacuum pump, and is manipulated by a series of valves. Your instructor will explain the operation of the vacuum rack, but you are urged to study the valve system by starting at the vacuum pump and following the glass tubing, noting the various valves along the way.
The vacuum line has been adapted to use a 90 cm open-arm Hg manometer and a pressure transducer; locate these devices. A trap has been placed in the line in the event that sufficient vacuum is supplied to pump the mercury from the manometer; even though the mercury cannot enter the vacuum system, please pay attention to what you are doing and do not pump the mercury from the manometer.
Connect the power leads for the transducer to a 8 V DC power supply, and connect the transducer output leads to the multiplexer in slot 100 on the 34970A. Record the channel address of the transducer. Configure this channel for a DC voltage measurement. Use 6 1/2 digit resolution for the measurments. Set the scale to autorange. Configure the 34970A to scan this channel 25 times. Monitor this channel throughout the course of the experiment. The display on the 34970A will be the 'live' reading of the transducer output.
Note that the Omega pressure transducers are calibrated at 8 +/- 0.01 V DC, 25oC. Therefore, for purposes of this experiment, you must supply 8 V DC to the device. The power supply used for this experiment can be set to provide 8 V via a switch on its front panel. Check the output voltage with a multimeter prior to beginning the experiment, and monitor the voltage throughout. You may monitor the power supply voltage with the 34970A if you wish. Obtain a pair of wires with alligator clips on one end. Connect the alligator clips to the +/- terminals of the power supply, and the other ends to an unused channel in the Agilent multiplexer, and configure whatever channel you chose for a DC voltage measurement (autorange,6 1/2 digit resolution.)
You should plug and switch on the power supply at least 2 hours before you start your experiment, in order to let it warm up and stabilize. You must keep the voltage from the power supply as close to 8V as possible. Be sure to note any large voltage fluctuations which occur during your experiment.
Before switching on the vacuum pump or otherwise changing the pressure in the vacuum system, scan the channel to which the transducer is connected 25 times and record the average voltage obtained. This is the offset voltage at the time of the experiment. It should be very close to 1.0000 V DC. Why won't it be exactly 1.000 V? What should you measure to verify this?
Open the appropriate valves on the vacuum line (primarily, the large valve which couples the system to the vacuum pump and the valve which connects the two sections of the vacuum line). Be sure that the vent to atmosphere is closed. Switch the vacuum pump on. Watch the manometer during this process. When the mercury levels between the two arms of the manometer are about 50 cm apart, shut the valve which isolates the right hand side of the vacuum line. When you have isolated the right side of the rack, you may turn the vacuum pump off.
Have the 34970A scan the voltage produced by the transducer. You will want to average 50 voltages at a given pressure and record these values in your lab notebook.
After you have collected your first pressure-voltage pair, carefully vent the system to atmosphere and raise the pressure in the ballast tank by about 2 cm Hg. Record another pressure-voltage pair, and repeat this process until the ballast tank is at atmospheric pressure.
When you are finished, the right hand side of the vacuum system should be at atmospheric pressure (i.e., the Hg levels in the manometer arms will be the same. However, the side of the system hooked to the vacuum pump will be under a considerable vacuum. Slowly and carefully use the valve which isolated the right hand side of the system to bring the left hand side to atmospheric pressure.
Carefully disconnect any leads (transducer, power supply, etc) from the multiplexer on the 34970A and leave the work area cleaner than you foud it.
Import your pressure-voltage data into a Mathcad sheet, and plot the pressure difference between the manometer arms versus voltage. The pressure-voltage data should be linear. Perform matrix linear regression on the data, and include a complete statistical analysis of the data. The slope of your plot (in units of volts/torr) is referred to as the sensitivity of the device; convert to appropriate units and compare your value to the manufacturer's value of 0.333 volts/psi (see ref (1) below.) You will utilize the inverse of this quantity (in units of torr/volt) in future experiments to convert transducer voltages to pressures.
Report, in tabular form, your calculated regression parameters and their relative standard deviations. Be sure to record this information in your laboratory notebook.
Follow the report format outlined in the course syllabus. Be sure to include the Mathcad analysis, along with a table containing your results. In your report, comment upon the behavior of the y-intercept of the pressure-voltage plot, i.e., does the y-intercept pass through the origin? Why or why not? Calculate the sensitivity of the transducer in (torr/volt) based on your data, and compare your results to those from the manufacturer (reference given below). Report your calculated sensitivity at the 95% confidence level.
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