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35 Field Instrumentation GEN Rev) III.
Bourdon metallic devices
Pressure balance systems

I.
Dead weight tester

It works on the basis of Pascal’s law any pressure communicated to the surface of a confined liquid is transmitted unchanged to every part of the liquid.
II.
Manometer:

In this method the pressure created by a column of liquid is used to balance the pressure to be measured. Pressure reading is the difference in height from the top of the pressure column to the top of the vented column. Mercury is normally used in manometers
Types of manometers

Single leg manometer: It is used to measure barometric pressure. It is a closed tube manometer with a vacuum on sealed side. Pressure applied to open end forces liquid up into the sealed end. A barometer when moved vertically indicates changes in elevation, becoming an altimeter.
U- Tube manometer: It is used to measure differential pressure.
Inclined tube manometer (slant manometer): It is used to provide greater reading accuracy through the use of a longer tube.

36 Field Instrumentation GEN Rev)
U-Tube manometer and slant manometer
Force balance systems

I.
Bourdon tube
One end of the tube is sealed the other is connected to the process. As the pressure in the process increases, tube tends to straighten out. The resultant motion is transferred through a linkage or rack-and-pinion mechanism to an indicating pointer. A bourdon tube is basically a spring that stretches as pressure is applied.
Bourdon tube metals must not be subject to hysteresis i.e. the metal must not stretch a different amount for increasing or decreasing pressure, causing the gauge two different reading for one pressure. The choice of metals depends on corrosion resistance, flexibility, hysteresis characteristics, pressure range, and cost. Figure 2.3 Bourdon tube

37 Field Instrumentation GEN Rev) It exists normally in any of the three shapes.

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