Page 292 - Instrumentation Reference Book 3E

276 Temperature measurement

in series. All the hot junctions are on the object 14.5.3.9 Therniocozple compensating cable
whose temperature is to be measured, and all the Ideally a thermocouple connects back to the read-
cold junctions are kept at a constant and known ing instrument with cables made of the same
temperature. Where a quick temperature res- metals as the thermocouple. This does, however,
ponse is required these thermocouples can be of have two disadvantages in industrial conditions.
very thin wire of about 25 pm diameter. A speed First, many thermocouple metals have high elec-
of response of the order of 10 milliseconds can be trical resistance. This means that on long runs,
achieved. Typical applications of thermopiles are which on a big plant may be up to 100 meters or
to be found in infrared radiation measurement. more, heavy gauge conductors must be used. This
This subject is dealt with in Section 14.6. is not only expensive but also makes the cables
difficult to handle.
14.5.3.8 Portable tlzerniocozpb instruments Second, in the case of precious metal thermo-
With the development over the last decade of couples, types B, R, and S for instance, the cost
would be very high indeed. To overcome these
microelectronic equipment, portable electrical
thermometers have become very popular. They problems compensating cables are used; see
are available with either analog or digital read- Figure 14.41. These cables are made of base metal
outs. The analog instruments are about the size of and are of lower resistivity than the thermocouple
an analog multi-meter; the digital instruments are material. The alloys used have thermoelectric
about the size of a pocket calculator. While most properties that essentially match the thermo-
couples themselves over a limited ambient tem-
of these instruments use type K thermocouples
they are available for use with other thermo- perature range.
Examples of compensating cables are:
couple materials. There are also portable therm-
ometers available using resistance thermometer or
thermistor sensors. However, the thermocouple
instruments are on the whole the most popular. Type Conipositiori Tlzermo- Temperature
couples
limitations
The more sophisticated instruments have the conipensated
option to use more than one type of thermocouple:
a switch on the instrument sets it for the type in U CoppedCopper-
use. They are also available with a switched nickel Rand S 0-50°C
option to read out in Celsius or Fahrenheit. A Vx CoppedConstantan K 0-80°C
range of hand-held probes are supplied for use
with these instruments. Figure 14.40 shows some
of the options available. The spring-loaded thermo- Other base metal thermocouples, such as
couples are for surface contact measurements; types J and T, comprise relatively inexpensive
hypodermic probes are supplied for such applica- and low resistance metals. They are therefore
tions as temperature measurements in food, such normally installed using cables consisting
as meat, where it may be an advantage to know of the same metals as the thermocouples
the internal temperature of the material. themselves.

General purpose thermocouple

Spring loaded thermocouple
for surface temperature measurement

Hypodermic thermocoupk
for internal temperature measurement of soft plastic, etc.

Figure 14.40 Hand-held thermocouple probes