Physitemp Instruments TH8 - Manual (Page 8)

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9.0 TEMPERATURE MEASUREMENT WITH THERMOCOUPLE SENSORS
9.1 The thermocouple is a simple and widely accepted device for measuring temperature. It
comprises two wires of dissimilar metals fused together to form a junction which produces an elec-
trical output proportional to temperature. The National Institute of Standards and Technology (NIST
Monograph 125, 1974) has tabulated the voltage/temperature relationships of many commonly used
thermocouple pairs; their tables on copper/constantan form the basis for calibration of Physitemp
thermometers.
9.2 At one time, accurate thermocouple temperature measurements needed elaborate poten-
tiometers and reference to a source of known temperature, such as an ice bath. The advent of mod-
ern solid state devices has made possible the design of an inexpensive thermocouple thermometer
which is direct reading. The first of these was Bailey thermometer BAT-4, which was designed in
1969 and is now in use throughout the world. Your Thermalert is an advanced version of the original equipment
using the latest low power digital technology and compensated electronic reference circuitry.
9.3 As compared with thermistor sensors which were formerly used exclusively in portable ther-
mometers, thermocouples have these advantages:
(a) wide temperature range, e.g. -200°C to over +1300°C.
(b) High stability of output.
(c) Interchangeability - no recalibration required.
(d) Accuracy traceable to NIST calibrations.
(e) Low cost; users can even make their own sensors.
(f) Microscopic size when needed, as in Physitemp microprobes.
(g) Nearly instant response.
(h) Better measurement accuracy due to low mass with smaller heat loss.
9.4 The main disadvantage of the thermocouple low sensitivity was overcome by the develop-
ment of auto zeroing amplifiers which are now used in all Physitemp thermometers. This type of
amplifier is essentially drift-free. It makes possible an electronic thermometer which is permanently
calibrated, just like a mercury thermometer. The following notes may help the user to avoid some of
the errors most frequently made in temperature measurement.
9.5 Faulty measurement technique with any type of thermometer can produce errors of several
degrees. Errors attributed to "out of calibration" equipment can often be corrected by a simple
change of technique.
9.6 Thermocouple probes, like all other temperature sensing devices, must be placed so that
they reach, as closely as possible, the temperature of the material to be measured. Probes are tip-
sensitive, but when measuring the temperatures of liquids, semi-solids or hard surfaces, it is not suf-
ficient to bring only the tip into contact with the material being measured. This is because there
will be loss of heat along both the thermocouple wires and their sheath, so readings will be low.
The effect can be greatly reduced if part of the metal sheath is also placed in contact with the mate-
rial. In liquids and semi-solids, the tip and sheath are simply immersed; on solid surfaces, part of
the sheath is laid against the surface.