TITLE.PM5

34 ENGINEERING THERMODYNAMICS

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M-therm/th2-1.pm5

Atmospheric pressure is measured with the help of barometer.

Fig. 2.14. Schematic diagram showing gauge, vacuum and absolute pressures.

2.16.2. Unit for pressure

The fundamental SI unit of pressure is N/m^2 (sometimes called pascal, Pa) or bar. 1 bar

= 10^5 N/m^2 = 10^5 Pa.

Standard atmospheric pressure = 1.01325 bar = 0.76 m (or 760 mm) Hg.

Low pressures are often expressed in terms of mm of water or mm of mercury. This is an

abbreviated way of saying that the pressure is such that which will support a liquid column of

stated height.

2.16.3. Types of pressure measurement devices

The pressure may be measured by means of indicating gauges or recorders. These instru-

ments may be mechanical, electro-mechanical, electrical or electronic in operation.

1. Mechanical instruments. These instruments may be classified into following two groups :
   — The first group includes those instruments in which the pressure measurement is made
   by balancing an unknown force with a known force.
   — The second group includes those employing quantitative deformation of an elastic member
   for pressure measurement.


2. Electro-mechanical instruments. These instruments usually employ a mechanical
   means for detecting the pressure and electrical means for indicating or recording the detected
   pressure.


3. Electronic instruments. Electronic pressure measuring instruments normally de-
   pend on some physical change that can be detected and indicated or recorded electronically.
   2.16.4. Mechanical-type Instruments
   The mechanical-type instruments are classified as follows :


4. Manometer gauges
   (i) U-tube manometer (ii) Cistern manometer
   (iii) Micro-manometer etc.


5. Pressure gauges
   (i) Bourdon tube pressure gauge (ii) Diaphragm gauge
   (iii) Vacuum gauge.
   2.16.4.1. Liquid manometers
   Low pressures are generally determined by manometers which employ liquid columns. It is
   difficult and costly to construct manometers to measure high pressures, as otherwise the liquid
   column will become unwieldy and temperature corrections will also be difficult. Their use is,
   therefore, restricted to low pressures only, and for such purposes they are quite accurate.