Pressure Measurement

Mechanical pressure transducers and elements

Mechanical Pressure transducers:

In general, transducers are devices which converts one form of energy into another form, a pressure transducer converts pressure into any other measurable form of energy. A mechanical pressure transmitter converts the pressure into mechanical displacement in the system.

Types of Pressure transducers:

  • Bourdon tube
  • Helix and spiral tubes
  • Spring and bellows
  • Diaphragm
  • Manometer

Bourdon tube:

The Bourdon tube works on a simple principle that a bent tube will change its shape when exposed to variations of internal and external pressure. As pressure is applied internally, the tube straightens and returns to its original form when the pressure is released.

The tip of the tube moves with the internal pressure change and is easily converted with a pointer onto a scale. A connector link is used to transfer the tip movement to the geared movement sector. The pointer is rotated through a toothed pinion by the geared sector.

Helix and Spiral Tubes:

Helix and spiral tubes are fabricated from the tubing into shapes as per their naming. With one end sealed, the pressure exerted on the tube causes the tube to straighten out. The amount of straightening or uncoiling is determined by the pressure applied.

The uncoiling part of the tube is mechanically linked to a pointer which indicates the applied pressure on a scale.
This has the added advantage over the C-bourdon tube as there are no movement losses due to links and levers.

Low-pressure elements have only two or three coils to sense the span of pressures required, however, high-pressure sensing may require up to 20 coils

Spring and Bellows:

A bellows is an expandable element and is made up of a series of folds which allow expansion. One end of the Bellows is fixed and the other moves in response to the applied pressure. A spring is used to oppose the applied force and a linkage connects the end of the bellows to a pointer for indication.

The spring is added to the bellows for more accurate measurement. The elastic action of the bellows by themselves is insufficient to precisely measure the force of the applied pressure.



Many pressure sensors depend on the deflection of a diaphragm for measurement. The diaphragm is a flexible disc, which can be either flat or with concentric corrugations and is made from sheet metal with high tolerance dimensions.

The diaphragm can be used as a means of isolating the process fluids, or for high-pressure applications. It is also useful in providing pressure measurement with electrical transducers.



The simplest form of a manometer is that of a U-shaped tube filled with liquid. The reference pressure and the pressure to be measured are applied to the open ends of the tube. If there is a difference in pressure, then the heights of the liquid on the two sides of the tube will be different.


Types of pressure gauges


Instrumentation Engineer

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