Heat dissipation

?At best, a thermometer only ever measures its own temperature?. Desire to is therefore to equalise the temperature of the sensor element to that of the medium to be measured. Only then will diaphragm seal display the specific medium temperature.
Principles to be looked at
The heat always flows from the warmer body to the colder one
Bodies at different temperatures shoot for a balance of the temperature difference
Heat flow from the warmer to a colder body
Striving for a balance of the temperature difference
Heat transport between thermometer and environment
A thermometer transports heat via its thermowell and its own internal components to the environment. Here, it is assumed that the ambient temperature is leaner compared to the medium temperature. In the converse situation, the process medium is heated by the thermometer.
In general this means that each thermometer generates a heat dissipation. The art would be to minimise the resulting error.
Factors affecting the magnitude of the heat dissipation
Temperature difference between your medium to be measured and the ambient temperature of the thermometer
Heat capacity of the medium to be measured
Heat capacity of the thermometer (and its components)
Heat penetration coefficient of the material of the thermometer
Thermal conductivity of the thermometer (and its own components)
Mass ratios (thermowell, neck tube, medium to be measured)
The physical design of the thermometer is, in the final analysis, the consideration of the sum of all the mentioned influencing factors.
Thermometer components with high heat dissipation
With electrical thermometers: thermowell, outer sheath of the MI cable, wires
With gas-actuated thermometers: thermowell, stem, capillaries
With bimetal thermometers: thermowell, spindle
The heat dissipation thus occurs predominantly via the metal components of the thermometer; however, the air enclosed in the thermometer also transports heat ? though to a much lesser degree. The heat dissipation is ? for exactly the same material ? also stronger, the larger the effective area is through which heat is transported. The higher the mass of a thermometer and its own thermowell and the higher its thermal conductivity, the greater the heat energy is that may be extracted from the measuring point.
Conditions to prevent heat dissipation errors
Example of a measuring point that a large heat dissipation/measuring error can be expected
The physical design of the thermometer should be matched to certain requirements of the measuring point (as far as is physically possible).
The thermometer must be immersed sufficiently deep in the medium whose temperature it will measure. If this is not the case, under certain circumstances, so much heat will undoubtedly be transported away into the surroundings that the sensor will not hold sufficient heat energy to adequately detect the medium temperature. It’ll then be barely possible to maintain the mandatory class accuracy.
The measuring point should be well insulated as far as possible.
Note
Info on our temperature measuring instruments are available on the WIKA website.
See also our article:
Temperature ? what actually is it?

Scroll to Top