Fluid Pressure Measurement by Mechanical Instruments

Hi all!

Today’s post covers the various mechanical instruments used to measure the fluid pressure.

The first type of device is the Bourdon tube pressure gauge. Bourdon tube pressure gauges are extensively used for local indication. This type of pressure gauges were first developed by E. Bourdon in 1849.

Bourdon tube pressure gauges can be used to measure over a wide range of pressure: from vacuum to pressure as high as few thousand pounds per square inches. It basically consists of a C-shaped hollow tube, whose one end is fixed and connected to the pressure tapping, the other end free. The cross-section of the tube is elliptical.

When pressure is applied by the fluid on accommodating the area within the tube, the elliptical tube tries to acquire a circular cross-section; as a result, stress is developed and the tube tries to straighten up. Thus the free end of the tube moves up,depending on magnitude of pressure. A deflecting and indicating mechanism made up of sector gear meshing with a spur gear is attached to the free end by a connecting link that rotates the pointer.bourdon tube1

Though the C-type tubes are most common, other shapes of tubes, such as helical, twisted or spiral tubes are also in use.

Bourdon Tube pressure gauges are used in Steam vessels, boiler equipment and gas cylinders.

The second type of mechanical measuring instrument is the Diaphragm type of pressure gauge.

In this instrument, the diaphragm replaces the Bourdon tube. The diaphragm is directly connected to the gear and pointer arrangement. When the fluid exerts pressure, the diaphragm rises and pushes the internal linkage, rotating the sector gear and showing a proportionate reading on a calibrated dial.

UntitledHowever, this type of pressure gauge becomes inaccurate after the diaphragm loses its elasticity. Thus it is not very widely used.

The third and final pressure measuring device is the Aneroid barometer.

The aneroid barometer uses the principle of pressure flow that says that the pressure always flows from the high pressure to low pressure region.

The aneroid barometer consists of 5 major elements : the vacuum chamber, the spring and lever arrangement, and the dial and pointer arrangement.


The aneroid barometer can measure small changes in the pressure by the expansion and contraction of the vacuum chamber due to those pressure changes. The change in the size of the barometer causes the movement of the lever arms, which magnify even the smallest changes in pressure. The change in pressure can be directly read out from the dial itself.

The aneroid barometer is used in airplanes for pressure measurement and also for altitude measurement. This is made possible due to the fact that the air pressure changes in proportion to the changes in the height. Thus, if we know the exact variation , we can use the aneroid barometer as a height measurement device as well.

That’s all for now! Cheers! 🙂




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