This paper is a lab report for an experiment on the calibration of a differential pressure transducer. Through this experiment, we will be able to measure the accuracy of the differential pressure transducer. Ideally, this equipment works in accordance with the principle of static differential pressure measurement. The pressure transducer comprises a sensor which is a cylinder consisting of a dual chamber where one chamber is for negative pressure and the other for positive one. The dual chamber is separated by a diaphragm. Whenever the pressure in both chambers is even, the diaphragm stays in the middle and lacks deformation. However, when the pressure deviates, there is deformation of the diaphragm towards the chamber consisting of negative pressure. The differential pressure is measured by measuring the degree of the deformity of the diaphragm. This explains the reason behind the differential pressure being proportional to the voltage signal. You will find more of this in the background theory section. In the experimental procedure and equipment section, you will see a detailed procedure of the experiment that will allow you to duplicate the same experiment. The steps are carefully put down and at the end of the section you will see the objective of the experiment. In the results and discussion section, you get to see the report of the experiment results. A detailed analysis of the results includes a graph with a line of best fit. The equation of the line has been used to further discuss the results. The equation has assisted in determining the maximum error allowable when making a reading of equipment. This is through the figure of standard deviation. The conclusion of this paper summarizes some key points of the lab report.
Introduction
This report sets to solve the problem of accuracy in the measurement of a differential pressure transducer. Sometimes the differential pressure transducer can reveal figures like 54.5 degrees Celsius. Calibrating the equipment will prove the accuracy of this figure and what room for error is allowable. The objective of this experiment is to find out the relationship between pressure which is the desired input and the scale reading which is the output.
Background/Theory
The differential pressure transducer works according to the principle of static differential pressure measurement (Anastasia 1982). The pressure transducer comprises a sensor which is a cylinder consisting of a dual chamber where one chamber is for negative pressure and the other for positive pressure. The dual chamber is separated by a diaphragm. Whenever the pressure in both chambers is even, the diaphragm stays in the middle and lacks deformation (Schmidt 1983). However, when the pressure deviates, there is deformation of the diaphragm towards the chamber consisting of negative pressure. The differential pressure is measured by measuring the degree of the deformity of the diaphragm. This explains the reason behind the differential pressure being proportional to the voltage signal.
Applications of a Differential Pressure Transducer
- It is used for control system of a laboratory. When a room controller is connected to a transducer, the extract air or supply air flow is regulated on the basis of the differential pressure.
- In conjunction with TCU-LON-II or with EASYLAB controllers TCU3 it is used for duct or room pressure control.
- When combined with TPM monitoring systems it is used to monitor pressure in ducts and rooms.
- It is also used in offices where control requirements are demanded, intensive care units, operating theatres, clean rooms in semiconductor and pharmaceutical industries, and laboratories.
- In water plants, water transmission between two locations heavily depends on pressure. Suction pressure is important for pumps whereas discharge pressure is important in a discharge line. Surge and differential pressure are important for surge vessels.
- In gas and oil plants, transmissions happen via the use of pneumatic actuators. Since gas and oil are very dangerous and burn easily, values of pressure during transmission are keenly monitored.
- A differential pressure transducer is used to measure absolute pressure where absolute pressure is the difference between a perfect vacuum and a give fluid pressure. The side that is low and senses’ pressure connects to a chamber of vacuum, whereas the side that is high connects to a process vessel. Therefore, when pressure is greater than a perfect vacuum, it will record as a pressure difference which is positive.
- A differential pressure transducer is also used to measure a vacuum. The high side is vented to the atmosphere while the low side is connected to the vacuum process. When pressure in the process vessel is more than the atmospheric pressure a positive pressure difference will register to the differential pressure transducer (Panagotopulos and Enrichsen 1998). Hence, the strength of the signal output in the differential pressure transducer is determined by the strength of the vacuum in the process vessel.
- Differential pressure transducer is used to measure liquid level. Differential pressure transducers infer many process variables due to their great versatility. An example of such process variable is process measurement. Due to their weight, liquids generate pressure that is proportional to their depth because of their weight, according to elementary physics. A vertical column of liquid generates pressure that is directly proportional to column height, the mass density of the liquid and the gravity acceleration. Mathematically this can be expressed as h=pgh (Valentin and Maitre 1989).
- The above simple relation allows a differential pressure transducer to be used as a liquid level sensing device when the liquid’s density is fairly constant. When the level of liquid in the vessel increases, the hydrostatic pressure on the differential pressure transducer’s high port goes up in direct proportion (Kazahaya and Porter 1988). Hence, the increasing signal of the differential pressure transducer stands for liquid’s height in the vessel because h=P/pg
Measuring Liquid and Gas Flow
The measurement of flow of fluid through a pipe is another common inferential measurement using a differential pressure transducer (Akeley 1972). Pressure went down across a pipe constriction and changed relative to fluid density and flow rate. When the density of the fluid is constant, drop in pressure across a pipe constriction can be measured and the measurement used to infer the rate of flow in the pipe. The common most constriction form used to infer flow in many industrial applications is the orifice plate. This is a metal plate comprising a machined hole in the middle. When the fluid goes through the middle, the velocity adjusts leading to a drop in pressure. This drop in pressure across the orifice plate is the used to infer the rate of flow in the pipe. When the orifice plate is used to measure flow, one port of the differential pressure transducer connects to the pipe’s upstream side whereas the other port connects to the pipe’s downstream side. Variation in pressure between downstream and upstream constriction sides of the orifice plate leads to flow registration by the differential pressure transducer (Orlowski and Liehr 1985). Therefore, the flow of liquid and gas can be reliably measured.
Installation and Commissioning of a Differential Pressure Transducer
- It requires that you select suitable location such as a room consisting of stable pressure.
- Make sure that the cross section is sufficient and that the measuring tubes are placed carefully.
- The monitoring system or the controller must be connected to the differential pressure transducer.
- The controller’s supply voltage must be same as the differential pressure transducer’s supply voltage.
- The sensor’s time must be put into consideration.
- The installation of the sensor must be away from interference sources such as heat sources, motors, senders and transformers.
- The installation location must be stable to prevent distortion of output signal by vibrations or shocks.
- Connections must face downwards and the installation vertical in order to allow for calibration like the factory set up. This vertical position also prevents the ingress of condensate from pressure tubes.
- A correction of zero point is needed.
Static Calibration
When a single input of the instrument is adjusted at a time, while all other inputs remain unchanged, input-output relations which are obtained are referred to Static calibration. To every input which is constant and measured with another instrument, a sensor output is associated. The specific input’s sensor static characteristic is the plot for these points.
Importance of Static Calibration
Static calibration assists to reduce uncertainty in measurement via making sure that the test equipment is accurate. It controls and quantifies uncertainties or errors to an acceptable level within processes of measurement (Frick 1971).
For instance, if a specific food product requires keeping above 68 degrees Celsius and the instrument system being used shows 68.8 degrees Celsius, then as long as the system has been calibrated to have accuracy within 0.5 degrees Celsius, you can have confidence in the safety of food at 68 degrees Celsius.
Experimental Procedure and Equipment
Calibration Steps
- Locate all the instrument’s possible inputs
- Make a decision on which inputs will be important in your experiment.
- Make a determination of the apparatus together with methods to curb (maintain constant or vary) all important inputs throughout the required range.
- Through adjusting a single input and keeping all other inputs constant, come up with input-output relations of the sensor.
The objective of this experiment is to find out the relationship between the output (scale reading) and the desired input (pressure).
Results and Discussion
Graph 1-plot of true pressure again indicated pressure
The average calibration curve in Graph 1 is assumed to be a straight line of equation (Bryan 1991)
y = mx + b
m and b can be acquired with the criterion of least-squares.
(3,3.9)
m=
5.2-3.9=
1.3=
1.3
(4,5.2)
4-3=
1
y=
mx+
b
5.2=
1.3*4+
b
b=
0
y=
1.3x-0
The reading y allows us to get an estimate of the true pressure as
x=y/1.3
However, this figure attained from the least squares line in Graph 1 should have a minus or plus error attained from the standard deviation Sx,
Sx^2=(1/N)Sum[(y-b)/m-x]^2
Sx=0.28kPa
Hence, in case the gage’s reading is 4.32, the true pressure estimate will be
X+ or- 3Sx=4.32/1.3+ or –(3*0.28)
=3.23+ or – 0.84kPa with a + or – 3 Sx limits
By using an error of thrice the standard deviation, we make sure that the true pressure lies in the defined range of 3.23-0.84; 3.23+0.84 with a probability of 99.7%.
Conclusion
As you can see from the calibration exercise, the accuracy of the differential pressure transducer can be measured (Bell 1987). Rapidly changing quantities have been measured through the study of input-output relations of the instrument and making use of the differential equation. The data resulting from the calibration exercise has been used to come up with a graph and a line of best fit. The equation from this line has assisted in coming up with the standard deviation and the maximum error allowable when a reading is made.
References
Akeley, L.T., Beckman Coulter Inc, 1972. Differential pressure transducer. U.S. Patent 3,691, p. 842.
Anastasia, H.G., Bendix Corp, 1982. Differential pressure transducer. U.S. Patent 4,336,567.
Bell, R.L., Bell Microsensors Inc, 1987. Differential pressure transducer. U.S. Patent 4,670, pp. 733
Bryan, J.C., Imo Industries Inc, 1991. Differential pressure transducers. U.S. Patent 5,029,479.
Frick, R.L., Rosemount Engineering Co Ltd, 1971. Differential pressure transducer. U.S. Patent 3,618,390.
Kazahaya, M., Fischer and Porter Co, 1988. Differential pressure transducer. U.S. Patent 4,754,365.
Orlowski, R.U., Kobs, R.U. and Liehr, M.R., North American Philips Lighting Corp, 1985. Differential Pressure Transducer. U.S. Patent 4,531,415.
Panagotopulos, L.J. and Erichsen, H.W., DATA INSTRUMENTS Inc, 1998. Differential pressure transducer. U.S. Patent 5,796, 7.
Schmidt, C., Kernforschungszentrum Karlsruhe GmbH, 1983. Differential Pressure Sensor. U.S. Patent 4,393,714.
Valentin, J.P. and Maitre, P., Fischer and Porter Co, 1989. Differential-Pressure Transducer. U.S. Patent 4, 829, 826.
