What does a thermometer measure in the first place

thermometer

thermometer, Temperature sensor, Devices for measuring the temperature of objects. They can be based on very different physical, including chemical, principles.

A distinction is made between primary thermometers, which are used to determine the Temperature fixed points (Fixed points) are used, and more user-friendly secondary thermometers that measure the temperature range in between. Magnetic thermometers are mainly used in the ultra-low temperature range. Above approx. 4 K, steam pressure thermometers and metal resistance thermometers are used. From around room temperature, expansion thermometers and thermocouples predominate, and they also work up to very high temperatures (around 3 500 K).

According to the measuring principle, a distinction is made between two groups: Touch thermometer after reaching the equilibrium state always only measure their own temperature, but with optimal thermal contact they practically assume the temperature of the measuring object. Depending on the physical effect, these thermometers work, for example, on the basis of changes in state - e.g. as a result of the thermal expansion of a substance such as the Expansion thermometer (see fig. 1 + 2), Steam pressure thermometer, Gas thermometer (see Fig. 3), metal expansion thermometer -, the change in electrical resistance (resistance thermometer), thermoelectric properties of material pairings (thermocouple), the color change (temperature measurement colors), the change in the natural frequency of a sensor caused by temperature changes, such as with Quartz thermometer, thermoluminescence, the change in the thermal noise of electrical resistances as in noise thermometry (3-1 100 K) or on the basis of a change in the magnetic susceptibility of certain salts.

At the non-contact thermometer or. Radiation thermometer If the thermometer does not come into contact with the object to be measured, it measures the temperature via the radiation emitted by the measuring body (pyrometer, spectral pyrometer 700-2500 K, total radiation pyrometer 220-420 K). Others use the interference between a light beam diffracted next to the warm body with a second light beam of the same beam (Light interference thermometer), the radio waves emitted by a measuring medium(Microwave thermometer), the Raman scattering of a laser beam (temperature measurement with lasers) or the propagation of sound in the measuring body (acoustic thermometer, 2-20 K).

Thermometers also have different designs and accuracy requirements depending on the application and intended use. (Temperature measurement, temperature scale, international temperature scale from 1990)