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Bourdon Pressure Gauge

Bourdon pressure gauge is used to measure pressure.

Bourdon tubes measure gage pressure, relative to ambient atmospheric pressure, as opposed to absolute pressure; vacuum is sensed as a reverse motion. When the measured pressure is rapidly pulsing, such as when the gauge is near a reciprocating pump, an orifice restriction in the connecting pipe is frequently used to avoid unnecessary wear on the gears and provide an average reading; when the whole gauge is subject to mechanical vibration, the entire case including the pointer and indicator card can be filled with an oil or glycerin. Typical high-quality modern gauges provide an accuracy of ±2% of span, and a special high-precision gauge can be as accurate as 0.1% of full scale.

Basic Principle of Bourdon tube pressure gauge:-

When an elastic transducer ( bourdon tube in this case ) is subjected to a pressure, it defects. This deflection is proportional to the applied pressure when calibrated.

The Bourdon pressure gauge uses the principle that a flattened tube tends to change to a more circular cross-section when pressurized. Although this change in cross-section may be hardly noticeable, the displacement of the material of the tube is magnified by forming the tube into a C shape or even a helix, such that the entire tube tends to straighten out or uncoil, elastically, as it is pressurized.

Diagram Of Bourdon tube Pressure Gauge :

Construction Of Bourdon tube Pressure Gauge:-

The main parts of this instrument is an elastic transducer, which is a bourdon tube which is fixed and open at one end to receive the pressure which is to be measured. The other end of the bourdon tube is free and closed. The cross-section of the bourdon tube is elliptical. The bourdon tube is in a bent form to look like a circular arc. To the free end of the bourdon tube is attached an adjustable link, which is in turn connected to a sector and pinion. To the shaft of the pinion is connected a pointer which sweeps over a pressure calibrated scale.

Operation of Bourdon tube:-

The pressure to be measured is connected to the fixed open end of the bourdon tube. The applied pressure acts on the inner walls of the bourdon tube. Due to the applied pressure, the bourdon tube tends to change in cross-section from elliptical to circular. This tends to straighten the bourdon tube causing a displacement of the free end of the bourdon tube.
This displacement of the free closed end of the bourdon tube is proportional to the applied pressure. As the free end of the bourdon tube is connected to a link – section – pinion arrangement, the displacement is amplified and converted to a rotary motion of the pinion. As the pinion rotates, it makes the pointer to assume a new position on a pressure calibrated scale to indicate the applied pressure directly. As the pressure in the case containing the bourdon tube is usually atmospheric, the pointer indicates gauge pressure.

Applications of Bourdon pressure gauge :-

They are used to measure medium to very high pressures.
For measuring high pressures e.g. in steam boilers, compressors.
For measuring pressures in vehicles tube tire.

Advantages of bourdon pressure gauge:-

These Bourdon tube pressure gauges give accurate results.
Bourdon tube cost low.
Bourdon tubes are simple in construction.
They can be modified to give electrical outputs.
They are safe even for high-pressure measurement.
Accuracy is high especially at high pressures.

Disadvantages of bourdon pressure gauge:-

They respond slowly to changes in pressure
They are subjected to hysteresis.
They are sensitive to shocks and vibrations.
Amplification is a must as the displacement of the free end of the bourdon tube is low.