Radiation error

The radiation error is the measurement error that occurs when measuring the temperature of a gas due to the absorption or emission of thermal radiation on the thermometer .

Mode of action

If a thermometer is placed in a gas, heat transfer between the thermometer and the gas takes place until both are in thermodynamic equilibrium . As long as no radiation sources or sinks act on the system of gas and thermometer, which add additional energy to this system, both have the same temperature.

If, however, thermal radiation  hits the thermometer - for example through exposure to sunlight - some of this radiation is absorbed by the thermometer and converted into heat , depending on the degree of absorption . The thermometer warms up above the gas temperature until an equilibrium is reached between the radiation absorbed and the heat given off to the gas. In this way the measured temperature is higher than the actual temperature of the gas.

If, on the other hand, energy is withdrawn from the thermometer by the emission of thermal radiation, it can cool down below the temperature of the gas and thus measure a temperature that is lower than the actual temperature. This is the case on cold nights, for example, when the atmospheric counter-radiation is less than the thermal radiation that the thermometer emits when it is at the same temperature as the gas. The thermometer cools down until the heat absorbed by the air is equal to this radiation loss.

Countermeasures

For precise temperature measurements it is therefore advisable to provide the thermometer with a reflective housing and to shield it from the direct influence of radiation sources and sinks. In meteorology , thermometers are therefore placed in so-called weather huts , which, thanks to their design, keep the radiation away from the thermometer and at the same time ensure good heat transfer by convection .