Mechanical pressure gauges

Mechanical pressure gauges

1. Mechanical Pressure Gauges
Pressure gauges or Mechanical gauges these are the devices used for measuring the
pressure by balancing the column of fluid by spring or dead weight.
Types of Pressure Gauges
Following are the commonly used mechanical pressure gauges:
1. Diaphragm pressure gauge
2. Bourdon tube pressure gauge
3. Bellows pressure gauge
4. Dead-weight pressure gauge
1. Diaphragm Pressure Gauge
A mechanical pressure-measuring instrument used to measure pressure above or below the
atmospheric pressure. It is usually employed to measure relatively low pressures. A
diaphragm pressure gauge, in its simplest form, consists of a corrugated diaphragm as
shown in the figure.

The gauge is connected to the fluid which is under pressure causes some deformation to the
diaphragm. With the help of pinion system, elastic deformation of the diaphragm rotates the
pointer. This pointer moves over a calibrated scale, which directly gives the pressure.
Advantages of Diaphragm Pressure Gauge
1. This is good for low pressure.
2. It is inexpensive.
3. It has a wide range
4. Used to measure gauge, atmospheric and differential.
5. it is very reliable.
Disadvantages of Diaphragm pressure gauge
1. Impact resistance is not good.
2. Difficulty in maintenance.
3. Lower measurement pressure.
2. Bourdon’s Tube Pressure Gauge
A mechanical pressure-measuring instrument employing as its sensing element a curved or
twisted metal tube flattened in cross-section and closed known as Bourdon tube.

2. It is a device for measuring the pressure of gases or liquids. It consisting of a semicircular or
coiled, flexible metal tube connected to a gauge, which records the degree to which the tube
is straightened by the pressure of the gas or liquid inside. Generally, it is used for measuring
high pressures.
The instrument is connected to the fluid which is under pressure flows into the Bourdon’s
tube. As a result of the increased pressure, the tube tends to straighten itself. Since the tube
is encased in a circular cover, therefore it tends to become circular instead of straight.
With the help of a simple pinion and sector arrangement, the elastic deformation of the
Bourdon’s tube rotates the pointer. This pointer moves over a calibrated scale, which directly
gives the pressure.
Advantages of Bourdon Tube Pressure Gauge
1. Construction is simple and the cost is low.
2. It has excellent sensitivities.
3. It has high accuracy.
4. This gauge is available for various ranges
Disadvantages of Bourdon Tube Pressure Gauge
1. Slow response to a change in pressure.
2. Prone to shock and vibrations.
3. It is subjected to hysteresis.
4. Not available for the low-pressure condition.
Application of Bourdon Tube Pressure gauge
1. Gas distribution system.
2. Used in the hydraulic installation.
3. Used in pneumatic installation.
3. Bellows Pressure Gauge
It is a device, the pressure responsive element is made of a thin metal tube with a deep
circumference when the pressure changes as the elastic elements increase or decrease.

3. A pointer is connected to it which indicates reading on the circular dial. It expands when the
pressure increases and when the pressure is low the elastic elements in the pressure gauge
are contracts.
Dead Weight Pressure Gauge/ Tester
It is the most accurate pressure gauge which is used for the calibration of the other pressure
gauges in a laboratory. It consists of a piston and cylinder of a known area. One end of the
tube containing fluid (generally water) is connected to the cylinder. A pressure gauge to be
calibrated is fitted to the other end of the tube. Dead weight is placed on the top of piston. Let
W= weight placed on the piston A= cross- sectional area of the piston Then, intensity of
pressure, p=W/A This pressure is transmitted to the pressure gauge to be calibrated. The
pointer of the pressure gauge moves and takes up a steady position on the dial. The position
of the pointer is marked as 'p'. In this way, by loading the piston by weights (loads) of other
magnitudes, other intensities of pressures are marked on the dial.
The dead weight tester apparatus consists of a piston cylinder combination fitted above the
chamber as shown in Fig.The chamber below the cylinder is filled with oil. The top portion of
the piston is attached with a platform to carry weights. A plunger with a handle is provided to
vary the pressure of oil in the chamber. The pressure gauge to be tested is fitted at an
appropriate place as shown in the Fig.

Fig. Dead weight tester

4. To calibrate a pressure gauge, an accurately known sample of pressure is introduced to the
gauge under test and then the response of the gauge is observed. In order to create this
accurately known pressure, the valve of the apparatus is closed and a known weight is placed
on the platform above the piston. By operating the plunger, fluid pressure is applied to the
other side of the piston until the force developed is enough to lift the piston-weight
combination. When this happens, the piston weight combination floats freely within the cylinder
between limit stops. In this condition of equilibrium, the pressure force of fluid is balanced
against the gravitational force of the weights plus the friction drag on the piston. The pressure
which is caused due to the weights placed on the platform is calculated using the area of the
piston. Now the pressure gauge to be calibrated is fitted at an appropriate place on the dead
weight tester. Now the valve in the apparatus is opened so that the fluid pressure P is
transmitted to the gauge, which makes the gauge indicate a pressure value. This pressure
value shown by the gauge should be equal to the known input pressure P. If the gauge
indicates some other value then the gauge is calibrated, adjusting the pressure on the gauge
so that it reads a value equal to input pressure. Gauge tester is used to calibrate all kinds of
pressure gauges and a wide range of pressure measuring devices. It is simple in construction
and easy to use.