Physitemp Instruments TH8 - Manual (Page 9)

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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 ther-
mocouple 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 thermocouple 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 temperature changes
caused by air currents. Breathing near the microprobe will produce wide fluctuations. 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 bring-
ing 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 specified 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
m o r e a c c u r a t e l y i t w i l l m e a s u r e t h e s u r f a c e t e m -
p e r a ture 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.
SST-1