Pressure gauge

1. SUMMARY
Calibration is the term applied to checking the accuracy or the
working condition of the concerned device. So, the calibration of
Bourdon Pressure Gauge refers to the checking of its accuracy or
reliability in taking a reading.
The piston was taken out and cylinder was filled with water until it
overflowed, the piston is replaced and allowed to settle and then
loaded with 10 loads of 0.5kg increments starting from 1kg. The true
pressure for each load is calculated by multiplying the mass of the
load by constant ‘g’, then dividing it by the area of the piston.
The graph drawn gives a positive slope meaning the gauge pressure
acquired corresponds to the true pressure, finding the real pressure
in fluids.
This exercise being a simple one, Nene Addico and Ms Ahiable read
and recorded the values, while Adjei Kofi and Prince Abrokwah
loaded and unloaded the setup.

2. INTRODUCTION
Typically, pressure gauges are devices used for measuring the pressure of a gas
or liquid. The bourdon gauge was in 1849 named after its French inventor
Eugène Bourdon.
It is still one of the most widely used instruments for measuring the pressure of
liquids and gases of all kinds, including steam, water, and air up to pressures of
100,000 pounds per square inch.
It consists of a hollow metal tube with an oval cross section, bent in the shape
of a hook. One end of the tube is closed, the other open and connected to the
measurement region. If pressure (above local atmospheric pressure) is applied,
the oval cross section will become circular, and at the same time the tube will
straighten out slightly. The resulting motion of the closed end, proportional to
the pressure, can then be measured via a pointer or needle connected to the
end through a suitable linkage.
OBJECTIVE
To calibrate a pressure gauge
APPARATUS
1. The bourdon Pressure gauge
2. Dead weights (in kilograms)

3. DESCRIPTION OF EXPERIMENTAL SETUP
The bourdon pressure gauge was placed on stable surface and the piston carrying the
weights was inserted into the cylinder which had initially been filled to its maximum
capacity with water (lubricant).

4. THEORY
Pressure is defined as force per unit area. It is usually more convenient to use pressure
rather than force to describe the influences upon fluid behaviour. The S.I. unit for pressure
is the pascal (Pa), equal to one newton per square meter (N/m2
or kg·m−1
·s−2
).
Mathematically:
Where:
P is the pressure,
F is the normal force,
A is the area.
Pressure is a scalar quantity. It relates the vector surface element (a vector normal to the
surface) with the normal force acting on it. The pressure is the scalar proportionality
constant that relates the two normal vectors:
The minus sign comes from the fact that the force is considered towards the surface
element, while the normal vector points outward.
It is incorrect (although rather usual) to say "the pressure is directed in such or such
direction". The pressure, as a scalar, has no direction. It is the force given by the previous
relationship to the quantity that has a direction, not the pressure. If we change the
orientation of the surface element, the direction of the normal force, changes accordingly,
but the pressure remains the same.
The S.I. unit for pressure is the pascal (Pa), equal to one newton per square meter (N/m2
or
kg·m−1
·s−2
).

5. PROCEDURE
1. The weight of the piston and its cross-sectional area were recorded.
2. The piston was removed, and the cylinder was filled with water (acting
as a lubricant) till it overflowed.
3. By tilting and gently tapping the apparatus, trapped air in the tube was
cleared.
4. Ensuring the cylinder was vertical; the piston was replaced and allowed
to settle.
5. The dead weighs were loaded onto the platform on the piston in 0.5
increments starting from 1kg, for 10 increments.
6. For each weight increment the gauge reading on the dial was read.
7. The process was repeated in both loading and unloading the weights.

6. RESULTS
Piston (plunger + platform) = 1kg
Piston Area (A) = 315mm2
= 0.315m2
Total mass (KG) × 31.14 = KN/m2
TRUE PRESSURE (PT) =
Where; g = 9.81m/s2
LOAD
m (KG)
GAUGE PRESSURE (KN/m2) WEIGHT
(mg)(N)
TRUE PRESSURE
(PT)(N/m2)INCREASING DECREASING
1.0 56 58 9.810 31.14
1.5 72 73 14.715 46.71
2.0 89 91 19.620 62.29
2.5 103 108 24.525 77.86
3.0 120 123 29.430 93.43
3.5 135 138 34.335 109.00
4.0 150 153 38.240 124.57
4.5 165 168 44.145 140.14
5.0 181 182 49.050 155.71
5.5 194 194 53.955 171.29

7. COMMENTS
The following deductions can be made by the test performed on the Bourdon
Pressure Gauge;
 The gauge has a zero error, because with no applied pressure, the gauge
still gave a reading.
 Neglecting the zero error the gauge always gave a result which was less
than the applied pressure. The degree of discrepancy increased with the
increasing pressure.
Following points can be raised to account for the test results.
 May be the fluid used was compressible, so it does not transmit pressure
equally in all directions, so recorded values were less. Also since
compression increases with greater applied force, so does the error in
recoded values.
 The gauge’s Bourdon tube could have been misshaped.
 The gauge’s gear system might be faulty.
From the graph, the gauge pressure is seen to be directly proportional to the
true pressure, as the graph is positive.
This implies that the true pressure against the gauge pressure will help read
the correct bourdon gauge pressure, since the gauge pressure acquired will
have a corresponding true pressure reading from the graph.