?At best, a thermometer only ever measures its own temperature?. Desire to is therefore to equalise the temperature of the sensor element compared to that of the medium to be measured. Only then will Optimistic display the specific medium temperature.
Principles to be considered
Heat always flows from the warmer body to the colder one
Bodies at different temperatures strive 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 lower compared to the medium temperature. In the converse situation, the process medium is heated by the thermometer.
In general because of this each thermometer generates a heat dissipation. The art is 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 the 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
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 larger the effective area is by which the heat is transported. The greater the mass of a thermometer and its thermowell and the higher its thermal conductivity, the higher 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 can be expected
The physical design of the thermometer should be matched to certain requirements of the measuring point (so far as is physically possible).
The thermometer should 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 won’t hold sufficient heat energy to adequately detect the medium temperature. It’ll then be barely possible to maintain the mandatory class accuracy.
Disgusting measuring point ought to 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 actually is it?