?At best, a thermometer only ever measures its own temperature?. The aim is therefore to equalise the temperature of the sensor element to that of the medium to be measured. Only then will the thermometer display the actual medium temperature.
Principles to be considered
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, the assumption is that the ambient temperature is lower compared to the medium temperature. In the converse situation, the process medium is heated by the thermometer.
In general which 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 the 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 own components)
Heat penetration coefficient of the material of the thermometer
Thermal conductivity of the thermometer (and its components)
Mass ratios (thermowell, neck tube, medium to be measured)
The physical design of the thermometer is, in the ultimate analysis, the consideration of the sum of all of 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
Heat dissipation thus occurs predominantly via the metal components of the thermometer; however, the air enclosed in the thermometer also transports heat ? though to Healthy . Heat dissipation is ? for exactly the same material ? also stronger, the larger the effective area is by which the heat is transported. The greater the mass of a thermometer and its thermowell and the greater its thermal conductivity, the greater heat energy is that may be extracted from the measuring point.
Conditions to prevent heat dissipation errors
Exemplory case of a measuring point for which 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 should be immersed sufficiently deep in the medium whose temperature it should measure. If this isn’t the case, under certain circumstances, so much heat will undoubtedly be transported away into the surroundings that the sensor won’t hold sufficient heat energy to adequately detect the medium temperature. It will then be barely possible to keep up the mandatory class accuracy.
The measuring point should be well insulated so far as possible.
Note
Information on our temperature measuring instruments can be found on the WIKA website.
See also our article:
Temperature ? what happens to be it?