Page 286 - Mechanical Engineer's Data Handbook

214 MECHANICAL ENGINEER'S DATA HANDBOOK

7.4 Temperature measurement

7.4. I Liquid-in-glass thermometers thermocouples connected in series, known as a 'ther-
mopile', gives an e.m.f. proportional to the number of
Mercury thermocouples. Practical thermocouples are protected
by a metal sheath with ceramic beads as insulation.
The commonest type of thermometer uses mercury The advantages of thermocouples are: they are
which has a freezing point of - 39 "C and a boiling simple in construction, compact, robust and relatively
point of 357"C, although it can be used up to 500°C cheap; they are suitable for remote control, automatic
since the thermometer may contain an inert gas under systems and recorders since they have a short response
pressure. time.
The advantages of this thermometer are: good The disadvantages are that they suffer from errors
visibility; linear scale; non-wetting; good conductor of due to voltage drop in the leads, variation in cold-
heat; and pure mercury is easily available. junction e.m.f. and stray thermoelectric effects in leads.
The disadvantages are: it is fragile; slow cooling of
glass; long response time; and errors arise due to 7.4.3 Thermocouple circuits
non-uniform bore and incorrect positioning.
Basic thermocouple circuit
Alcohol
V= Constant x Temperature (usually)
Alcohol can be used down to - 113 "C, but its boiling
point is only 78 "C. The alcohol needs colouring. It is Galvanometer e.m.f. Y= Vh - Vc
cheaper than mercury, and its low-temperature oper- where: Vh=e.m.f. for 'hot' junction, Vc=e.m.f. for
ation is an advantage in a number of applications.
'cold' junction
Mercury in steel
Metal a
This thermometer employs a mercury filled capillary
tube connected to a Bourdon-type pressure gauge junction junction
which deflects as the mercury expands with tempera-
ture. It is extremely robust and can give a remote
indication.
Thermocouple circuit with ice bath
7.4.2 Thermocouples
A bath of melting ice is used for the cold junction.
When a junction is made of two dissimilar metals (or Temperature is given relative to 0 "C.
semi-conductors) a small voltage, known as a 'thermal
electromotive force (e.m.f.)' exists across it, which
increases, usually linearly, with temperature. The basic
circuit includes a 'cold junction' and a sensitive
measuring device, e.g. a galvanometer, which indicates
the e.m.f. The cold junction must be maintained at a
known temperature as a reference, e.g. by an ice bath
or a thermostatically controlled oven. If two cold
junctions are used then the galvanometer may be
connected by ordinary copper leads. A number of G =galvanometer, C =cold junction, H = hot junction