7
6.6 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. 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 material.
In liquids and semi-solids, the tip and sheath are simply immersed; on solid surfaces,
the sheath is laid against the surface.
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 surface. For example, with a probe of 1/16” diam-
eter, 10 x 1/16” = 10/16” = .625” = the minimum immersion depth.
6.7 Errors between thermocouple probes. All Physitemp clinical probes and sensors are
made with thermocouple wire that has been specially tested to meet our own stringent
standards. Our clinical probes are guaranteed accurate to within 0.1oC in the range 0-
50oC. Copper-Constantan (type T) thermocouples from other manufacturers are not
normally this accurate. Probes made from wire to “special limits of error” may be
accurate to ±0.5°C in this range. This interchangeability of sensor, including micro-
probes, is a major advantage of Physitemp thermocouple thermometers.
6.8
Measurements in Liquids.
These are quite easy to make, because there is
good thermal contact between liquid and probe. The latter quickly reaches liquid tem-
perature 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.
6.9
Measurements of Air Temperature.
Temperature can vary widely in different
parts of a room; differences of at least several degrees will be noted. When a micro-
probe 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 BAT-12/microprobe combination, due to high discrimination of the instrument
Summary of Contents for BAT-12
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