Measuring Pressure: Comparison Of Bourdon Gauge And Semiconductor Pressure Sensor

Pressure measurement devices

Pressure given conventionally obtained by the formula P = F / A, is defined as the force that acts normal to an object’s surface area per unit area. The experiment involved measuring the pressure variable by utilizing two devices- “TH2 Apparatus”. The apparatus contain two dead weights and a calibrator that is connected to the pressure measuring devices- “Bourdon Pressure Gauge” and the “Electronic Pressure Sensor Device”( Maul, 2005, pp. 781-782). For use of the equipment to be effective, it is necessary for priming of the same equipment (Agrawal, 2008). When the equipment are primed before use, their accuracy and linearity of measurement of the experiment improves greatly. The equipment is cleaned using a special equipment in the laboratory followed by their calibration so as to ensure that the measurement during the experiment are taken correctly (Avison, & Barham, 2014, pp. 09007).

The figure below is an illustration of experiment setup. The TH2 equipment is used for calibration of pressure. It compares the theoretic value of pressure to the experimental value. The accuracy ratio should not be more than 3:1 between the equipment and the theoretic value. The equipment utilizes weights that are put on the equipment to create pressure since pressure=force/area. The area is the place where the weight are placed then the force is obtained from; Force=m*a where a=gravitational acceleration. The different pressures that are known and fixed from the weights placed on the equipment (deadweight) are generated. The characteristics of the unit including the linearity and accuracy are then determined by use of a Bourdon pressure gauge and an electronic pressure sensor.

The Bourdon gauge together with the pressure sensor is mounted on the manifold block. A separate reservoir is used to store the hydraulic fluid. The valves are used to enable priming of the equipment. Hey enable restricted flow of the water for damping and also enable the connection of other devices for easy calibration of the equipment. The electronic pressure sensor on the other hand uses a semiconductor diaphragm that deflects when pressure is applied by the fluid. A voltage output is generated that is proportional to the applied pressure.

The problem presented herein is that of measuring pressure by use of two methods that use two different tools. It is therefore a requirement that a laboratory experiment be conducted so as to calibrate and measure pressure by use of these devices and compare the results with the theoretical results. 

1. To measure and compare pressure using two devices.
2. To analyse the linearity and accuracy of the two set of devices that were used in performing the experiment.

The TH2 Apparatus

TH2 Pressure equipment:  

Parts and Functions of the TH2

Parts	Function (s)
1  Base plate	Supporting the equipment as a stand so that it is in an upright operational position.
2  Manifold block	Offering support for the pressure gauge
3  The Storage	Storage of data that is obtained during the calibration process.
4  Priming vessel	Minimizes errors by clearing the system off any blockage
5 The Bourdon gauge	Used to take the readings for measured pressure
6 The  Pressure sensor	Sensors and indicates the pressure as read by the equipment
7  The Priming valve	releases all the blockage in the system
8  The Damping valve	Used to let in air into the system
9  An additional isolating valve	Isolation of air for purifying
10  The Precision ground piston	Used to hold weights
11  Matching cylinder	Used for comparing
12 Weights	Create force/pressure
13 Pressure sensor cable	sensing and transmitting of pressure
14 support	Offers support for the stand of the equipment
15 Electrical console	Isolation of air for purifying
16 Mains on/off switch	Switching the equipment on/off
17 Digital meter	used to give the output reading
18 Selector switch	used to witch between Pressure and voltage readings
19 I/O port	For connecting to other outside sources
20 Socket	For power connection for the equipment
21 Zero control	Adjusting to zero before the readings are taken
22 Span control	Adjusting for the span of measurements to be taken
23 Electrical output (OUTPUT)	Used  to give output
24 Power input	Used to provide power for the equipment
25 CONT	Used to protect the equipment from excess power supply
26 O/P	Used to protect the electrical output
27 Mains lead	Supply power to the equipment
28 Quick	Used to quickly release the valve coupling

Table 1: List of parts and functions (Selvik, et al., 1983, pp. 343-352)       

A= pd2/4

Figure 3.1 : mechanism of dead weight calibrator (Balling Jr,  & Cerveny) 

The procedure for the experiment involved the use of two equipment. The process followed is thus outlined as below:

1. A)For the Bourdon gauge sensor : 

• Calibrate the Bourdon gauge
• Decrease friction loss achieved  by spinning the piston (Bonnar, 1956, pp. 351-359)
• Take the readings and or measurements  of angle in degrees and pressure for piston I Newtons
• Slowly add 0.5 kg weights to the piston, afterwards  take readings
• Calculate the theoretical force and pressure for each of the reading. Compare with the experimental results
• Take off all the weights, repeat experiment for accuracy (Edmund, 1968). 

1. B)For the Semiconductor pressure sensor:

• Remove all the weights from the piston
• Calibrate the semiconductor, do this until the optimum zero pressure can be read.
• Repeat steps 1 and 2 taking readings of voltage and pressure at an increment of 0.5 kg.
• Repeat the experiment to obtain average readings (Parr, 2011). 

The results of the first method, the Bourdon gauge sensor as a device to measuring pressure are indicated as below and the graph shown. From the results, it is evident that there is a linear increase in both the pressure at is applied on the device and the pressure indicated by the device. On average, the rate of increase is 40 by increase in 0.5 kg of applied pressure. Comparing the results with theoretic results, the percentage difference is 0.003% (Hay, 1999).

Table 2: pressure results to applied masses

From the table below, the results indicate an increase in the equipment pressure with increase in the pressure applied. The semiconductor pressure Ps however deviates at a certain point and the linearity is withdrawn as indicated by the graph 2. As more mass is applied the deviation from linearity of the results and the graph increases. The percentage difference for this equipment in terms of the comparison of applied pressure and equipment pressure Ps is 0.05%. The semiconductor pressure on average becomes more than the applied pressure. To cater for this difference, a formula is applied (Mizuno, 1992): 

                 %difference = 100% x theoretical – experimental / theoretical value

1. Priming of the equipment is necessary to ensure that any blockage in the system is removed for effective functioning of the equipment. It should be done before commencing of the experiment so as to avoid recording errors.
2. The pressure gauge and pressure sensor need to calibrate in order to determine the accuracy of the equipment. Essentially calibration enables the comparison of the measured quantity which is pressure in this case to the theoretical quantity so as to get the accuracy and functionality of the equipment. 

1. The accuracy of the equipment could be improved through priming of the equipment. Measurements during the experiment need to be taken during the specified times following the procedure. The equipment should be operated following the manual, it should be given time to settle after a reading has been taken. Readjust the equipment after a reading so as to start on a new level and give time to the equipment to readjust too.
2. Sources of errors for the equipment could have resulted from parallax during taking of the readings from the gauge. Another source of error could be failure of enough priming of the equipment before the beginning of the experiment. Human error could have also caused error in the experiment due to forgetting to record the right measured value.
3. Other than use of experimentation as this report suggest as a method of calibrating pressure gauges, use of pressure gauge calibrators can also be used as an alternative for calibrating pressure gauges. They come as bench or workshop models and pressure measurement is done with an internal or external pressure module

Conclusion

In conclusion, measurement of pressure by the two methods, Bourdon gauge and Semiconductor pressure sensor were evaluated and compared. Measurement by the Bourdon gauge proved to be a better method with a deviation of 0.003% as compared to the theoretic value. The semiconductor presented a good method but the average deviation was big as compared to the Bourdon gauge, 0.05%. This result indicates that the Semiconductor might have had some issues relating to resistance or due to external dumping.

References 

Agrawal, B., 2008. Basic Mechanical Engineering. John Wiley & Sons. 

Avison, J. and Barham, R., 2014. Final rport on key comparison CCAUV. A-K5: pressure calibration of laboratory standard microphones in the frequency range 2 Hz to 10 kHz. Metrologia, 51(1A), p.090

Balling Jr, R.C. and Cerveny, R.S., Pressure gradient force. 

Bonnar, W.B., 1956. Boyle’s Law and gravitational instability. Monthly Notices of the Royal Astronomical Society, 116(3), pp.351-359. 

Edmund, N.T., Van Kuyk, H.J. and John, S., Trane US Inc, 1968. Bourdon tube pressure sensor having improved mounting. U.S. Patent 3,407,665. 

Hay, A.D., Maron, R.J., Dunphy, J.R. and Pruett, P.E., CiDRA Corp, 1999. Bourdon tube pressure gauge with integral optical strain sensors for measuring tension or compressive strain. U.S. Patent 5,877,426. 

Kamen, D.L., Kamen Dean L, 1988. Flow control system using boyle’s law. U.S. Patent 4,778,451. 

Maul, G.A., 2005. Pressure Gradient Force. In Encyclopedia of Coastal Science (pp. 780-781). Springer Netherlands. 

Mizuno, M., Mitsubishi Electric Corp, 1992. Semiconductor pressure sensor. U.S. Patent 5,101,665.  

Parr, A., 2011. Hydraulics and pneumatics: a technician’s and engineer’s guide. Elsevier. 

Selvik, G., Alberius, P. and Aronson, A.S., 1983. A roentgen stereophotogrammetric system: construction, calibration and technical accuracy. Acta Radiologica. Diagnosis, 24(4), pp.343-352.

Webster, J.G., 1998. The measurement, instrumentation and sensors handbook. CRC press.