Department of Mechanical Engineering
University of Bahrain
MEG 381 Instrumentation:
Lab: Calibration of Pressure Gauge
Objective:
To calibrate a Bourdon tube pressure gauge using Dead weights.
Introduction:
To calibrate a measurement system you have to compare its output with a standard
known input. Consider, for example, a pressure gauge. This device could be calibrated by
mounting the transducer in a closed vessel with a piston at one end. (Such a vessel is
called a dead-weight tester). Placing a weight on the piston produces a known pressure in
the vessel equal to the weight divided by the piston area. After applying a series of
weights and measuring the corresponding output voltages of the pressure gauge you can
plot the relationship between the gauge reading and added weights. Now, if you want to
measure the pressure in a pipe you can use the gauge and the calibration curve to convert
mass weights into pressures at the pipe. Having a repeatable and accurate calibration is
obviously a prerequisite for an accurate measurement. Therefore, repeating a calibration
at regular intervals is a good idea since it may drift with time or ambient conditions.
Apparatus:
1- Accurate Weights.
2- Pressure gauge (Bourdon Tube)
3- Dead Weight Tester.
Theory:
Bourdon pressure gauge is a thin-walled tube that is
bent into a quarter-circular arc. The tube is rigidly
fixed at the end where the pressure is applied, and
free to move at the end. The exterior of the tube is
usually open to the atmosphere. When the fixed end
is subjected to a pressure, the arc tends to
straighten, thus causing the tube end to move
upward (or downward for vacuum measurements).
Dr. Mostafa S. Habib
MEG 381 Instrumentation Prerssure Gauge Calibration
1
The deflection of the end is transmitted mechanically to rotate the pressure gauge needle.
The magnitude of the deflection is proportional to the applied pressure.
The Bourdon tube pressure gauge is loaded with known
pressures using a Dead weight tester. This Known pressure
will be used for calibration. A diagram of Bourdon pressure
gauge with a dead weight tester is shown in the figure.
Figure 2: Dead Weight tester for calibration of pressure gauge.
• A piston of known area (A) is loaded by a known Weights, therefore the
actual pressure in the cylinder is
P
F mg

*10 3
A
A
( KN / m 2 )
• For the piston with d  20 mm, the piston area can be calculated
A

4
(20 *103 ) 2
 314.16 *106 m 2
• For each mass, calculate the actual pressure, P and record the gauge reading.
Dr. Mostafa S. Habib
MEG 381 Instrumentation Prerssure Gauge Calibration
2
Procedure:
1- Observe the Pressure gauge unit setup. Be sure all hoses are attached securely to
the Bourdon gauge.
2
2- Record the gauge reading ( KN / m ) for the unloaded piston, which corresponds
to a total mass of 1 Kg.
3- Place the given dead weight one by one and record the reading of the
gauge.
4- To prevent the piston from sticking, rotate the piston gently as each mass is
added, so that the friction in the piston can be reduced.
( under static conditions the friction is significant, therefore its better to change
the static friction to sliding friction which is considerably less)
5- Remove mass from the piston in the same order added, recording the gauge
readings.
6. Plot the calibration curve and determine the best fit, and the bias error.
Results:
Total
Mass
(Kg)
Actual
Pressure
Gauge
Reading
PC (bar )
Pg (bar )
(increasing)
Gauge
Reading
Pg (bar )
(decreasing)
1
2
3
4
5
6
7
8
9
Dr. Mostafa S. Habib
MEG 381 Instrumentation Prerssure Gauge Calibration
3
Results:
Total
Mass
(Kg)
Actual
Pressure
Gauge
Reading
PC (bar )
Pg (bar )
Gauge
Reading
Pg (bar )
(increasing)
(decreasing)
Average
Pg (bar )
Absolute
Error
PC  Pg
% Error
For Gauge
Reading
1
0.31
0.15
0.25
0.20
0.11
35.5
2
0.62
0.35
0.45
0.40
0.22
35.5
3
0.94
0.64
0.75
0.70
0.24
25.5
4
1.25
0.93
1.18
1.10
0.15
12.0
5
1.56
1.23
1.48
1.40
0.16
10.3
6
1.87
1.54
1.80
1.67
0.20
10.7
7
2.19
1.85
2.13
1.99
0.20
9.1
8
2.50
2.14
2.38
2.26
0.24
9.6
9
2.81
2.44
2.44
2.44
0.37
13.2
Note: first reading is due to weight of the piston m  1 kg .
Dr. Mostafa S. Habib
MEG 381 Instrumentation Prerssure Gauge Calibration
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%LAB#3_calibration curve
%Prepared by AMMAR AHMED
%------------------------------------------M=[1:9];
P=[0.20 0.40 0.70 1.10 1.40 1.67 1.99 2.26 2.44];
plot(M,P,'x'); grid
xlabel('Standard Mass (Kg)');
ylabel('Gauge pressure (bar) ');
n=length(M);
X=[M' ones(n,1)];
A=inv(X'*X)*X'*P';
y=A(1,1)*M+A(2,1);
S_square=sum((P-y).^2);
num=sum((y-mean(P)).^2);
r_square=num/(S_square+num);
r_square
>> r_square =
0.9952
Dr. Mostafa S. Habib
MEG 381 Instrumentation Prerssure Gauge Calibration
5
Calibration Curve:
Dr. Mostafa S. Habib
MEG 381 Instrumentation Prerssure Gauge Calibration
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Discussion:
Possible sources of error in this experiment:
1- Wear and backlash in the gauge linkage can increase or decrease the
pressure.
2- Air bubbles inside the tester unit may result in inaccurate reading.
3- Static friction between the piston and cylinder, yield smaller gauge
reading.
4- Human error.
Conclusion:
In this experiment, our task was to calibrate a Bourdon tube gauge. The
result obtained shows the calibration curve of the measurement system.
The curve represent relation between a known input (mass) and a
measuring instrument (pressure). The relation between the applied mass
and the gauge pressure is determined using linear regression. The fitted
curve is linear. The sensitivity of the pressure gauge is the slope of the
calibration curve and equal to (0.29 Kg/bar).
Dr. Mostafa S. Habib
MEG 381 Instrumentation Prerssure Gauge Calibration
7