?At best, a thermometer only ever measures its temperature?. The aim is therefore to equalise the temperature of the sensor element compared to that of the medium to be measured. Only then will the thermometer display the actual medium temperature.
Principles to be looked at
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 a 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 surroundings. Here, the assumption is 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 which means that each thermometer generates a heat dissipation. The art is to minimise the resulting error.
Factors affecting the magnitude of 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 ultimate analysis, the consideration of the sum of the 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 the same material ? also stronger, the bigger the effective area is through which the heat is transported. The greater the mass of a thermometer and its own thermowell and the greater its thermal conductivity, the higher the heat energy is that can be extracted from the measuring point.
Conditions to avoid heat dissipation errors
Example of a measuring point that a considerable heat dissipation/measuring error should be expected
The physical design of the thermometer must be matched to the requirements of the measuring point (so far as is physically possible).
Dynamic must 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 be transported away in to 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 required class accuracy.
The measuring point should be well insulated so far as possible.
Note
Information on our temperature measuring instruments are available on the WIKA website.
See also our article:
Temperature ? what happens to be it?