Most of our gauges are constructed with bourdon tubes to measure pressure and vacuum. The bourdon tube, which is a hollow metallic tube sealed at one end, flexes when pressure is applied.
It flexes because it naturally wants to straighten out, but cannot because it is linked to a geared movement. As it tries to flex, this linear movement is changed to a rotational one by means of small gears. They in-turn cause the pointer to indicate the measured pressure. Gauges like this are designed for clean, non-clogging liquids and gases.
How Bourdon Pressure Gauges Work
The Bourdon tube pressure gauge is the most commonly used type in steam systems. It consists of a coiled or 'C' - shaped tube that is sealed at one end, and open at the other. The open end of the Bourdon tube is exposed to the process fluid, allowing it to flow into the tube. Any increase in pressure causes elastic distortion of the tube, causing it to unwind. The resulting displacement of the closed end of the tube is translated by a series of gears to an angular displacement of the pointer.
The pointer position is therefore proportional to the pressure applied at the gauge's pressure connector. Typically, the maximum deflection of the Bourdon tube corresponds to a pointer angular displacement of 270°.
The tube can be constructed out of a number of different materials, depending on the application; generally, brass or bronze is used for higher pressures, whereas stainless steel is used for lower pressures.
Fig. 12.6.1 'C'-shaped (a) and coiled (b) Bourdon tubesBourdon tube pressure gauges often have the option of being liquid filled. The area surrounding the Bourdon tube is filled with a transparent liquid, normally glycerine. This protects the internal mechanisms against damage from severe vibration and to keep out ambient corrosives and condensation.
This also damps the movement of the pointer making the gauge less susceptible to small transient pressure fluctuations.
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