Imho buy a laboratory glass thermometer. It will cover -50 to +150 degrees
C it is *very* reliable (unless you break it) and accurate to +/- 1 degree
(which is +/-0.5% for that span). It will serve you well for many years if
you take care of it. I spent $20 on mine and never looked back. For an
accurate calibration point, a digital human temperature thermometer is
very good as others have pointed out.

Boiling liquids and stirring slush will only get you within +/- 2 degrees
or so and there will be plenty of error sources. I have tried this in
several ways (including using standard laboratory practice cf. manuals)
and it is not easy. The glass thermometer is the way to go imho.

Peter

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