?At best, a thermometer only ever measures its 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 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 internal components to the environment. Here, it is assumed that the ambient temperature is lower than the medium temperature. In Off-limits , the process medium is heated by the thermometer.
In general therefore each thermometer generates a heat dissipation. The art would be to minimise the resulting error.
Factors affecting the magnitude of 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 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 the different parts of the thermometer; however, the air enclosed in the thermometer also transports heat ? though to a much lesser degree. Heat dissipation is ? for exactly the same material ? also stronger, the bigger the effective area is by which heat is transported. The higher the mass of a thermometer and its own thermowell and the higher its thermal conductivity, the greater heat energy is that may be extracted from the measuring point.
Conditions to avoid heat dissipation errors
Exemplory case of a measuring point for which a large heat dissipation/measuring error should be expected
The physical design of the thermometer must 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 isn’t the case, under certain circumstances, so much heat will undoubtedly be transported away in to the surroundings that the sensor will not hold sufficient heat energy to adequately detect the medium temperature. It will then be barely possible to keep the required class accuracy.
The measuring point ought to be well insulated as far as possible.
Note
Info on our temperature measuring instruments can be found on the WIKA website.
See also our article:
Temperature ? what happens to be it?