Expansion thermometer
Expansion thermometers are special thermometers with which the temperature is determined by measuring the different changes in length or volume of two bodies as a result of a change in temperature. Typical examples for an expansion thermometer are shown in the picture.
Expansion thermometers can be divided into, depending on their physical state
- Liquid thermometer
- Gas thermometer
- Solid state thermometer
The measuring principle of these thermometers is based on the difference in the thermal expansion of different materials.
The limits are set by the respective properties of the materials, for example the boiling and solidification temperatures of liquid thermometers. Measurement deviations can be caused, for example, by thermal after-effects of the glass tube and non-uniform expansion of the liquid and the surrounding glass vessel over the entire temperature range.
Liquid thermometers consist of a small glass flask for receiving the thermometer liquid with a fused calibrated glass capillary in front of a scale . In proportion to the temperature change, the volume of the liquid and the glass vessel change differently, which means that the level in the capillary increases with increasing temperature.
The temperature can be read from the level and the label on the scale. Depending on the measuring range, different liquids are used, e.g. B. Ethanol (-120 ° C to 60 ° C), toluene (-90 ° C to 110 ° C) or n- pentane (-200 ° C to 35 ° C) and earlier very often mercury (-35 ° C) up to 357 ° C), which today is almost only found in the scientific field because it is still preferred because of its property of not wetting glass. The first usable mercury thermometers were built by Daniel G. Fahrenheit . An alloy of gallium, indium and tin ( Galinstan ) has established itself as a non-toxic substitute for mercury .
A special type of liquid thermometer is the minimum-maximum thermometer . It consists of a U-shaped glass capillary with two separate scales, one for the maximum (rising from the bottom to the top) and one for the minimum temperature (falling from the bottom to the top). There are flasks at both ends of the capillary: at the "minimum end" filled with alcohol, at the "maximum end" evacuated or filled with alcohol vapor.
In the bend of the capillary there is mercury, which is pushed back and forth by the expanding or contracting alcohol. The alcohol serves as the actual temperature-measuring liquid, the mercury shows the temperature on both scales. The temperature displayed on both scales should be the same, otherwise the scales are not correctly positioned.
There are steel needles over both ends of the mercury, which are shifted accordingly by the mercury. There is a magnet on the back of the thermometer that holds the needles in their respective positions. This allows the respective maximum and minimum temperature since the thermometer was last reset. The maximum / minimum display can be reset by briefly removing the magnet (usually with a reset button on the thermometer).
Liquid spring thermometers consist of a vessel to which a manometer is coupled via a capillary tube . The vessel is filled with a liquid; mercury, xylene, toluene or others are used. When the temperature changes, the pressure in the system rises or falls , which is indicated by the pressure gauge. The application range of these thermometers is between –60 ° C and 500 ° C.
Gas spring thermometers (or gas thermometers ) are constructed like liquid spring thermometers. Instead of a liquid, they are filled with a gas ( nitrogen or helium at around 50 bar ). The application range of these thermometers is from –200 ° C to 800 ° C.
Solid-state thermometers are, for example, bimetal strip thermometers , in which two strips of different metals are attached to one another. Due to their different expansion coefficients, they expand in different ways when the temperature changes. This causes the bimetal strip to bend. This is used as a thermal switch in the iron, for example .