In liquid-in-glass thermometers, the thermal development of liquids is supplied for measure up the temperature!


Operating principle

Liquid-in-glass thermometers are based upon the principle of thermal expansion that substances. A fluid in a glass tube (called a capillary) expands when heated and also contracts when cooled. A calibrated scale can then be offered to read off the respective temperature that resulted in the matching thermal expansion. Together thermometers are additionally called capillary thermometers.

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Figure: rule of temperature measurement through a liquid-in-glass-thermometer

In liquid-in-glass thermometers, the thermal development of liquids is offered for measuring the temperature!

Thermometric liquids

The liquids provided in such thermometers should have certain properties in order to be an ideal for use. Because that example, they need to not frozen at short temperatures, i beg your pardon is why water, for example, is no suitable. The liquids should likewise have a saturated high boiling suggest so that they perform not vaporize at high temperatures. Furthermore, the liquid must increase evenly v the temperature in the measuring range used. Otherwise one uneven department of the range on the thermometer would certainly be necessary. Additionally for this reason water is not suitable, due to the fact that water expands unevenly due to the density anomaly.

However, liquids which have actually all the required properties and are therefore perfect for usage in liquid-in-glass thermometers are additionally referred to as thermometric liquids. In the past, the extremely toxic mercury to be used, which has actually a solidification temperature of -39 °C and a cook temperature the 357 °C. Nowadays, usually blue or red fancy ethanol (alcohol) with a melting point of -115 °C and a boiling suggest of 78 °C is used rather of mercury. In this temperature range, the daily temperatures in the selection between -20 °C and +50 °C can be fine covered.

Measuring sensitivity

The measure up sensitivity of liquid-in-glass thermometers rises with the lot of liquid in the thermometer. The more liquid over there is, the an ext liquid will certainly expand and also rise in the glass tube. Because that this reason, fluid thermometers have actually a reservoir to increase the amount of fluid in the thermometer. Too much liquid, however, would cause the thermometer to react really slow to temperature changes, as the heater of the liquid will take longer and also the liquid will certainly take longer to it is adapted to the exterior temperatures to it is in displayed.

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Note

Mercury was/is used not only due to the fact that of the vast temperature selection (theoretically applicable native -39 °C come 357 °C) but also because mercury hardly wets the glass tube. The capillarity is therefore lower and also the temperature can be displayed and read off much more accurately.