Physitemp Instruments TH5 - Manual (Page 9)

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
electrical 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
modern solid state devices has made possible the design of an inexpensive thermocouple ther-
mometer 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 ver-
sion 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
thermometers, 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 devel-
opment of auto zeroing amplifiers which are now used in all Physitemp thermometers. This
typeof 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 sev-
eral degrees. Errors attributed to "out of calibration" equipment can often be corrected by a sim-
ple 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 sufficient to bring only the tip into contact with the material being measured. his 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 material. In liquids and semi-solids, the tip and sheath are simply immersed; on solid
surfaces, the sheath is laid against the surface.
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