Heat Dissipation Error in Resistance Thermometers and Thermocouples
Every resistance thermometer or thermocouple dissipates heat from or supplies heat to the medium to be measured. As a result, the measured temperature will always deviate from the actual medium temperature. This leads to a measurement error, which is known as the heat dissipation error.
What causes the heat dissipation error?
According to the rules of thermodynamics, heat always flows from the warmer to the colder body until the temperature difference is balanced.
If a temperature sensor is installed in a pipeline, for example, the temperature sensor’s protection tube dissipates heat out of the process, thereby cooling the temperature sensor and the medium to be measured.
As a rule, this heat dissipation does not substantially cool the medium to be measured. However, it causes the temperature sensor to record a temperature that is lower than the actual medium temperature. The difference between the measured temperature and the actual temperature is referred to as the heat dissipation error.
Note: This is not the case if the ambient temperature is higher than the medium temperature, in which case heat is transported into the process instead.
How large is the heat dissipation error?
The extent to which heat dissipation influences the measurement error depends on many factors and is not easy to determine in practice.
Important factors include:
- Thermal conductivity of the temperature sensor’s components
- Mass ratios between temperature sensors and the medium to be measured
- Thermal capacity of the medium to be measured and the temperature sensor, i.e. how much energy can be stored
- Temperature difference between the medium to be measured and the ambient temperature
In the temperature sensor, heat is dissipated mainly via the sensor’s metallic components. A generously dimensioned temperature sensor with a large diameter dissipates more heat than a temperature sensor with small dimensions. The greater the mass of the temperature sensor in relation to, for example, the pipeline in which it is installed, the more heat energy can be withdrawn from the measuring point. This heat dissipation continuously cools the temperature sensor, similar to a heat sink. The resulting measurement error can amount to several Kelvin and can significantly increase the response time of the temperature sensor.
If you would like to learn more about response times, be sure to read my blog post on this topic.
How do I minimise the heat dissipation error?
In principle, SIKA designs temperature sensors so that heat dissipation – and thus the heat dissipation error – is as low as possible. Depending on the medium to be measured and the ambient conditions, a compromise must be found between mechanical stability and measurement requirements.
Among other things, the following points should be taken into consideration:
- The temperature sensor should be immersed sufficiently deep into the medium to be measured.
- The diameter of the immersion tube or protection tube should be as small as possible.
- The thermal conductivity of the temperature sensor components.
- The measuring point should be as well insulated as possible.
A good example of this is our solution for the WGF temperature sensor, which measures the air temperature in ventilation ducts. In this case, the temperature sensor is connected to the process via glass fibre reinforced plastic, which has very good insulation properties. As a result, the heat dissipated via the sensor is significantly reduced, and the heat dissipation error is considerably lower.
An optimised solution such as this offers many advantages: Accurate temperature measurement saves operating costs for the user and makes life easier for the designer when configuring the measuring point. Don’t hesitate to get in touch with us about your particular measurement challenge!
