Page 312 - Introduction to Marine Engineering

286 Instrumentation and control

Thermistor
This is a type of electrical thermometer which uses resistance change to
measure temperature. The thermistor is a semi-conducting material
made up of finely divided copper to which is added cobalt, nickel and
manganese oxides. The mixture is formed under pressure into various
shapes, such as beads or rods, depending upon the application. They are
usually glass coated or placed under a thin metal cap.
A change in temperature causes a fall in the thermistor resistance
which can be measured in an electric circuit and a reading relating to
temperature can be given. Their small size and high sensitivity are
particular advantages. A range of measurement from — 250°G to
+ 1500°C is possible.

Level measurement
Float operated
A float is usually a hollow ball or cylinder whose movement as the liquid
surface rises or falls is transmitted to an indicator. A chain or wire
usually provides the linkage to the indicator. Float switches may be used
for high or low indication, pump starting, etc., where electrical contacts
are made or broken, depending upon the liquid level.

Sight or gauge glasses

Various types of sightglass are used to display liquid level in storage
tanks. The simple boiler gauge glass referred to in Chapter 4 is typical of
such devices.

Pneumatic gauge
This is a device which uses a mercury manometer in conjunction with a
hemispherical bell and piping to measure tank level. The arrangement is
shown in Figure 15.10. A hemispherical bell is fitted near the bottom of
the tank and connected by small bore piping to the mercury manometer.
A selector cock enables one manometer to be connected to a number of
tanks, usually a pair. A three-way cock is fitted to air, gauge and vent
positions. With the cock at the 'air' position the system is filled with
compressed air. The cock is then turned to 'gauge' when the tank
contents will further pressurise the air in the system and a reading will
be given on the manometer which corresponds to the liquid level. The
cock is turned to Vent' after the reading has been taken.