This six-minute video by AquaLab captures the correct way to check, verify and if necessary, adjust some ordinary thermometers using an ice bath.
Most people think that it’s easier to do so at the boiling point of water, but the ice point (very nearly 0.01 ° C at all normal atmospheric pressures) is far more stable than the boiling point that varies considerably with atmospheric pressure and consequently the altitude of the location at which the measurement is made.
What’s more, the ice point was, until about 1990, the fundamental reference point for the Centigrade and Fahrenheit scales. However, when the internationally agreed definition of the Celsius scale replaced the Centigrade scale, as a part of The International Temperature Scale (ITS-90) .
The new reference became the triple point of water precisely 0.01 ° C lower than the ice point. The ice point of water, a well-stirred mixture of ice and water is still as stable as ever, for ice and water that are free of contaminants (that is, both made up of distilled, de-ionized water).
The fine distinction is made because of a very small shift in the definition of the scale reference point from the freezing point of water in the 1968 scale (IPTS-68) to the triple point of water, a difference of 0.01 °C in the 1990 scale (ITS-90). The °C symbol was redefined as meaning “Degrees Celsius”, in honor of the Swedish scientist who developed the original 100 unit scale and the term “Centigrade” scale was considered as “obsolete” after that.
Also, in the prior international scale, The International Practical Temperature Scale of 1968 (IPTS-68)”, the unit for absolute temperature degrees was the Degree Kelvin, or °K whereas in the ITS-90 scale it was redesginated as the Kelvin.
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