Here is a useful rough rule: Heat leakage effects are substantially reduced when an
amount of probe equal to 10 or more sheath diameters is immersed or laid on the suface.
For example, with a probe of 1/16” diameter, 10 x 1/16” = 10/16 = .625 = the minimum
immersion depth.
9.7
Errors between thermocouple probes. All Physitemp probes and sensors are made with
thermocouple wire that has been specially tested to meet our own stringent standards. Our
probes are guaranteed accurate to within 0.1°C in the range 0-50°C. Copper-Constantan (type
T) thermocouples from other manufacturers are normally close to this accuracy provided they
are ordered to "special limits of error," and so are suitable for use with our thermometer. This
interchangeability of sensor, including microprobes, is a major advantage of Physitemp thermo-
couple thermometers.
9.8
Measurements in Liquids. These are quite easy to make, because
there is good thermal contact between liquid and probe. The latter quickly
reaches liquid temperature and readings can be taken within a few seconds.
However, a liquid which has been heated above or cooled below ambient
will be losing or gaining heat, and convection currents will give rise to
temperature variations of up to several degrees. These variations can be
reduced by vigorous stirring. This simple precaution must always be taken.
9.9
Measurements of Air Temperature. Temperature can vary widely in different parts of a
room; differences of at least several degrees will usually be noted. When a microprobe is used
to indicate air temperature, readings will often fluctuate rapidly, responding to actual tempera-
ture changes caused by air currents. Breathing near the microprobe will produce wide fluctua-
tions. These effects indicate the sensitivity of the Thermalert/microprobe combination, due to
high discrimination of the instrument and almost instant response of the probe. Fluctuations can
easily be eliminated by bringing the probe into contact with a metallic object, thus increasing its
effective mass and slowing the response. Using a larger probe will have the same results.
9.10
Measurements on Solid Surfaces. These are most easily made with surface probes such
as our BT-1 and MT-D. The right-angled tip provides the 10 diameters of probe contact speci-
fied in Section 9.6. Straight probes may also be used,
provided that sufficient shaft length is in contact with
the surface to be measured. In general, the smaller the
probe, the more accurately it will measure the surface
temperature of a solid. For instance, an MT-29 micro-
probe, because of its small size, needs to be in contact
with the surface for as little as 1/8”. SST-1 has a 1/4”
gold disc sensor. Gold is an excellent conductor, and
is non-allergenic and non-polluting. It makes a fine
skin surface probe.
10
SST-1
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