“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 strive for a balance of the temperature difference
A thermometer transports heat via its thermowell and its internal components to the surroundings. Here, it is assumed that the ambient temperature is lower than 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 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 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 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 the same material – also stronger, the larger the effective area is through which the heat is transported. The greater the mass of a thermometer and its thermowell and the greater its thermal conductivity, the greater the heat energy is that can be extracted from the measuring point.
Conditions to prevent heat dissipation errors
- The physical design of the thermometer must be matched to the requirements of the measuring point (so far as is physically possible).
- The thermometer must be immersed sufficiently deep in the medium whose temperature it should measure. If this is not the case, under certain circumstances, so much heat will be transported away into the surroundings that the sensor will not hold sufficient heat energy to adequately detect the medium temperature. It will then be barely possible to maintain the required class accuracy.
- The measuring point should be well insulated as far as possible.