Page 309 - Introduction to Marine Engineering

Instrumentation and control 283

The piezoelectric pressure transducer is a crystal which, under
pressure, produces an electric current which varies with the pressure.
This current is then provided to a unit which displays it as a pressure
value.

Temperature measurement by instruments will give a value in degrees
Celsius (°C). This scale of measurement is normally used for all readings
and temperature values required except when dealing with theoretical
calculations involving the gas laws, when absolute values are required
(see Appendix).

Liquid-in-glass thermometer
Various liquids are used in this type of instrument, depending upon the
temperature range, e.g. mercury -35°C to +350°C, alcohol -80°C to
4-70°C. An increase in temperature causes the liquid to rise up the
narrow glass stem and the reading is taken from a scale on the glass
(Figure 15.6). High-temperature-measuring mercury liquid thermo-
meters will have the space above the mercury filled with nitrogen under
pressure.

Scale-
-•-Glass
stem

• i- Liquid

• Bulb
Figure 15.6 Liquid-in-glass thermometer