Contact thermometer

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Adjustable contact thermometer (0… 100 ° C); is set with an attached permanent magnet . Length 300 mm
Contact thermometer with fixed switching temperature (75 ° C) glued into an aluminum block
Contact thermometer with fixed switching temperatures of 40 and 42 ° C

Contact thermometer are mercury - liquid thermometers , the means projects into the mercury column of electrical contacts as a temperature switch are formed.

In the past, contact thermometers were often a component of temperature regulators or monitors and can switch small loads (e.g. an aquarium heater , now replaced by a rod heater , or about 0.1 amps at 230 volts AC) directly. To switch larger loads, they were often combined with switching amplifiers, e.g. B. supplemented with a thermal mercury switch . The service life or the permissible number of switching operations increase as the electrical switching load of the contact thermometer decreases. This means that> 10 6 switching operations can be achieved at a voltage of 300 V and a switching capacity of 0.15 VA. Under pressure and in quartz glass, some of these could be used up to 700 ° C.

Contact thermometers were manufactured for fixed or adjustable switching temperatures. For fixed temperatures, two wires are fused into the glass of the capillary as contacts . For adjustable temperatures, the top of the wires protrudes into the capillary from above and can be adjusted using a spindle drive housed in the hermetically sealed glass housing. The adjustment is made from the outside by a rotating permanent magnet, whose field acts like a magnetic coupling through the glass. In 1926, the Juchheim company from Ilmenau applied for a patent for the first glass contact thermometer, with which variable temperature settings were possible using adjustable metal threads.

Today contact thermometers have been replaced by electronic temperature sensors and controllers. In the case of low demands, temperature switches , etc. a. with bimetals an alternative.

Disadvantages of contact thermometers are their fragility and the mercury they contain. Advantages are the visual recognizability of the actual and switching temperature as well as the very precise switching point compared to other energy-free temperature switches, which is only slightly affected by hysteresis.

Individual evidence

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