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). Read more Calibrating Ordinary Thermometers