Mechanical Engineering Principles

270 MECHANICAL ENGINEERING PRINCIPLES

(ii) can be made very small and compact,

(iii) is robust,

(iv) is easily replaced if damaged,

(v) has a small response time,

(vi) can be used at a distance from the actual
measuring instrument and is thus ideal for use
with automatic and remote-control systems.

Sources of error

Sources of error in the thermocouple, which are
difficult to overcome, include:

(i) voltage drops in leads and junctions,

(ii) possible variations in the temperature of the

cold junction,

(iii) stray thermoelectric effects, which are caused
by the addition of further metals into the’ideal’
two-metal thermocouple circuit.
Additional leads are frequently necessary for
extension leads or voltmeter terminal connec-
tions.

A thermocouple may be used with a battery- or
mains-operated electronic thermometer instead of
a millivoltmeter. These devices amplify the small
e.m.f.’s from the thermocouple before feeding them
to a multi-range voltmeter calibrated directly with
temperature scales. These devices have great accu-
racy and are almost unaffected by voltage drops in
the leads and junctions.

Problem 1. A chromel-alumel

thermocouple generates an e.m.f. of 5 mV.

Determine the temperature of the hot

junction if the cold junction is at a

temperature of 15°C and the sensitivity of

the thermocouple is 0.04 mV/°C.

Temperature difference for 5 mV

=

5mV

0 .04 mV/°C

= 125 °C

Temperature at hot junction

=temperature of cold junction

+temperature difference

= 15 °C+ 125 °C= 140 °C

Now try the following exercise

Exercise 127 Further problem on the

thermocouple

1. A platinum-platinum/rhodium thermocou-
   ple generates an e.m.f. of 7.5 mV. If the
   cold junction is at a temperature of 20°C,
   determine the temperature of the hot junc-
   tion. Assume the sensitivity of the ther-
   mocouple to be 6μV/°C [1270°C]

24.4 Resistance thermometers

Resistance thermometersuse the change in elec-

trical resistance caused by temperature change.

Construction

Resistance thermometers are made in a variety of

sizes, shapes and forms depending on the applica-

tion for which they are designed. A typical resis-

tance thermometer is shown diagrammatically in

Figure 24.4. The most common metal used for the

coil in such thermometers is platinum even though

its sensitivity is not as high as other metals such

as copper and nickel. However, platinum is a very

stable metal and provides reproducible results in a

resistance thermometer. A platinum resistance ther-

mometer is often used as a calibrating device. Since

platinum is expensive, connecting leads of another

metal, usually copper, are used with the thermometer

to connect it to a measuring circuit.

The platinum and the connecting leads are shown

joined atAandBin Figure 24.4, although some-

times this junction may be made outside of the

sheath. However, these leads often come into close

contact with the heat source which can introduce

errors into the measurements. These may be elimi-

nated by including a pair of identical leads, called

dummy leads, which experience the same tempera-

ture change as the extension leads.

Principle of operation

With most metals a rise in temperature causes an

increase in electrical resistance, and since resistance

can be measured accurately this property can be

used to measure temperature. If the resistance of

alengthofwireat0°CisR 0 , and its resistance at