Liquid thermometer
Liquid thermometers are a special type of thermometer and in Germany, in the VDI / VDE guideline 3511 "Technical temperature measurements", a distinction is made between liquid glass thermometers and liquid spring thermometers . The former uses a change in volume as a measuring principle , the latter uses a change in pressure as a measure of a change in temperature . The change in volume of the liquid is attributed to a change in length that is immediately visible in a thin glass vessel. The change in pressure requires an elastic component, the deformation of which is made visible by means of a pointer. The liquid spring thermometer is not dealt with any further below.
Liquid thermometer
This consists of a glass storage vessel filled with a thermometric liquid and a transparent capillary tube connected to it . Since liquids usually expand more than solids when heated , the level of the liquid in the tube takes on a temperature-dependent level. The tube is connected to a scale and allows the direct reading of measured values in a temperature unit , in most countries preferably in degrees Celsius with the unit symbol ° C, in the USA and some other English-speaking countries in degrees Fahrenheit with the unit symbol ° F. There is an expansion space above the liquid column or capillary so that the thermometer is not destroyed if the measuring range is exceeded .
Depending on the thermometer glass and liquid, measuring ranges can be between −200 and +1100 ° C. The measuring and display parts form a solid unit and are close together.
In Germany, construction types, operating conditions, error limits and other details refer to the standardization z. B. in DIN 12770 and refer to the VDI / VDE guideline 3511.
Thermometric liquid
The thermometric liquid must remain liquid in the entire temperature range in which the thermometer is to be used, i.e. it must neither freeze nor boil in the nominal measuring range . Furthermore, it must not have any areas of abnormal thermal expansion within this range . For this reason, water is unsuitable as a thermometric liquid - at least for temperatures below 10 ° C. In addition, the liquid used must not attack the material of the tube. Both wetting (organic) and non-wetting (metallic) thermometric liquids are in use. For safety reasons, they should not be self-igniting, which is why alloys made from alkali metals are out of the question. Since thermometer tubes are sealed, toxic liquids are also used.
The properties of a thermometric liquid are particularly well fulfilled by mercury, which is why mercury was the most widely used liquid in Germany until the 1970s. Since mercury freezes at −38.9 ° C and boils at approx. +356 ° C, mercury thermometers can be used over a wide temperature range. Mercury thermometers can also be designed as contact thermometers and thus used as switching controllers .
The following thermometric fluids are often used:
| substance | Application area | properties |
|---|---|---|
| mercury | from –30 ° C to +350 ° C | Almost temperature-independent coefficient of thermal expansion, toxic, non-flammable |
| toluene | from –80 ° C to +100 ° C | harmful, highly flammable |
| Pentane | from −200 ° C to +30 ° C | for low temperature thermometers, very flammable |
| Ethanol | from −110 ° C to +60 ° C | no remote temperature measurement possible, highly flammable |
| Galinstan | from –11 ° C to over 1000 ° C | non-toxic gallium alloy, strongly wets the thermometer tube |
| petroleum | from -40 ° C to 200 ° C | harmful, highly flammable |
Readability and reading
The organic liquids used are consistently clear and transparent and practically colorless. For better visibility, ethanol, petroleum, pentane etc. are typically colored red, blue or green. The liquid metals have a metallic, silvery sheen.
To make a thermometer sensitive to small changes in temperature, the lumen of the capillary must be small compared to the volume of the vessel.
Thermometer tubes often have a rounded triangular cross-section on the outside, which in principle can be produced by pulling through a suitably shaped caliber or subsequent rolling. Due to the surface tension of the viscous glass, the lumen remains approximately circular in cross section.
The rounding on the front "edge" of the triangle is chosen in such a way that, at a certain viewing angle, the capillary appears widened several times, as if by a strong cylindrical magnifying glass. With vertically oriented tubes and horizontally adjacent eyes of the observer, this effect only occurs for one eye.
Often, thermometer tubes are backed with a light scale made of milk glass or anodized aluminum and the tube and scale are connected to one another in one place so that they cannot move. Laboratory and clinical thermometers are tightly enclosed by a glass envelope into which the scale strip fits with very little clearance, approximately from the top edge of the vessel (inclusion form). Frosted glass offers the option of backlighting and thus also an optical illustration of the thermometer reading.
Rod thermometers made from a very thick-walled capillary typically have an outside diameter of around 5–8 mm and can be sealed in a laboratory vessel with crimped tube connections. They can have a layer of frosted glass on one side and have the etched graduation on the outside of the thermometer.
When reading, look at the thermometer as perpendicularly as possible in relation to the longitudinal axis in order to minimize the parallax error.
The red dye for ethanol sometimes fades to a slightly yellowish color in thermometers that have been exposed to the sun for years outdoors. The blue dye is significantly more UV-stable.
On capillaries for simple thermometers for the living area there is often a delicate horizontal line on the side or at the back, which marks the determined level at 20 ° C (room temperature). The tube is then adjusted and fastened on the scale so that this mark comes to rest on the corresponding scale mark. The household thermometer with cooling zone scale shown in the picture has adjustment marks (recognizable in the photo) at 10 and 35 ° C. This two-point adjustment results in greater accuracy, since, in addition to the simple assignment of the liquid level to a scale value, the double execution of this process also determines the spread (division) of the scale and the thermometer thus reliably displays over larger areas.
Thermal switch
In refrigerators, a thermal sensor - filled with an aromatic, volatile liquid - is usually used to control the thermostat. The cylindrical rigid metal expansion vessel contains about 1 cubic centimeter of liquid and is soldered to a metal capillary up to 1 m long with an outer diameter of only about 1.2 mm, which ends in a small, soft metal bellows that acts as an actuator on a bistable switch, whose operating point can be adjusted by means of a setting wheel via a fine-thread screw. Similar constructions can also be found in room thermostats for heating controls.
Web links
- ChemgapPedia: Exact temperature determination : The structure, calibration and functionality of a liquid thermometer is explained with a short flash animation (German)