Minimum-maximum thermometer

from Wikipedia, the free encyclopedia
A minimum-maximum thermometer

A minimum-maximum thermometer for Six (also maximum-minimum , minima maxima or maximum-minimum thermometer ) is used in a longer period of time (minutes, hours or days) occurring minimum and maximum to record temperatures and at the same time display the current temperature.


The minimum-maximum thermometer (old design) consists of a long U- shaped bent glass tube in which there is ethanol / mercury . The mercury collects at the bottom of the U and runs relatively far up the left or right leg of the U when the temperature changes . Magnetic , colored floating bodies (drag pins) float on the left and right ends of the mercury column . In addition, the left leg of the U- shaped tube contains a transparent liquid medium with a high coefficient of expansion , for example ethanol. This liquid medium is the actual measuring liquid. On the left leg there is a temperature scale that has the lowest temperature values ​​at the top and the highest temperature values ​​at the bottom. The opposite is true for the right thigh. The scale begins at the bottom with the lowest temperature values ​​and ends at the top with the highest temperatures. In the right leg of the U there is a gas above the mercury that exerts a slight permanent pressure on the mercury thread. This gas pressure can push the mercury column back into the left thigh when the temperature drops.

Example of a measurement

If the temperature rises, the liquid measuring medium expands at the top of the left arm. Because the diameter of the capillary tube is larger than that of the steel rod, the newly created additional volume of alcohol flows around the left drag pin, so it is not moved, but held in place by a permanent magnet behind the scale . The ethanol pushes the mercury column down in the left leg of the U-tube and up in the right leg. The mercury pushes the right drag pin up in front of it until the highest temperature is reached. There it is held by a permanent magnet at this maximum value. The lower edge of the right drag pin shows the highest temperature reached.

If the temperature falls, the volume of the measuring liquid decreases, the measuring medium contracts a little and sucks the mercury column higher up into the left leg of the U-tube. The measuring medium between the left mercury column and the left drag pin flows around the left drag pin and flows upwards. Because of its high surface tension, the mercury cannot flow around the drag pin, but pushes it upwards as the temperature drops until the lowest temperature is reached. A permanent magnet holds the drag pin in this position. The lower edge of the left drag pin shows the lowest temperature reached.

At temperatures between these two extreme values, the mercury no longer touches the drag pins. The maximum and minimum temperatures of the observation period are indicated by the lower edges of the magnetically held drag pins. The current temperature can be read off the dials at the ends of the mercury column.

To restart an extreme value measurement, the thermometer is reset with a push button located between the two legs of the tube. This has the consequence that the permanent magnets are pushed backwards. As a result, the drag pins are no longer attracted and each fall on one end of the mercury column. If the push button is released, the magnets come forward again and can hold the drag pins again. The thermometer is ready to measure again.

inside view

MinMax thermometer disassembled

The picture shows a disassembled minimum-maximum thermometer of conventional design. On the right in the picture you can see the glass tube. It consists of the U- shaped middle part with the capillary, in which the mercury and the indicator pins are located, and the two pear-shaped extensions at the top. The tube is inserted into a plastic carrier on which the scale in degrees Celsius is also printed. The tube has been adjusted and calibrated at the bottom with glue. In the enlargement, the glass bulbs A (right) and B (left) can be clearly seen on the area highlighted in red. B contains the measuring liquid, which expands when the temperature rises. The glass bulb A is only about half filled with liquid, above is a gas cushion. A acts like a pressure equalization vessel. As the temperature drops, the gas pushes the mercury thread more into the left capillary. The back wall can be seen in the middle of the picture. It carries a resilient plate on which two permanent magnet strips are glued. These magnetic strips hold the small indicator pins in place when this plate is directly behind the U-tube. To reset the thermometer, this plate is briefly pushed backwards. (To show that the strips are magnetic, a paper clip is attached to them in the picture.) The plastic housing can be seen on the left. The plate with the U-tube is inserted there and the thermometer is then closed with the rear wall.

The thermometer shows a current temperature of 19 degrees Celsius, a minimum of 6 degrees and a maximum of 29 degrees.

Because of the dangerous mercury, such thermometers are now manufactured according to EU guidelines only for commercial use. Otherwise these are now produced free of mercury. Alternatively, these are now also available in digital , electronic and bimetallic versions.