Calibrating bimetallic thermometers

[AiredAle]

I stumbled across this webpage today while looking for something completely different. From time to time on this forum we have threads regarding calibrating dial-type (bimetallic) thermometers. I have always calibrated mine by checking the freezing point, this site claims that's wrong, and has added some comments from thermometer manufacturers to back up his assertions.

He claims these thermometers are only accurate within +/- 20 degrees of the calibration temperature. So if you calibrate at the freezing or boiling point, and want to use the thermometer at mash temperatures, you may have problems.

Interested to know what others think, or have experienced.

http://www.hi-tm.com/Documents/Calib-boil.html

 

[jdoiv]

Funny, I found a very similar write up the other day that said the same thing. He suggested calibrating your therms with a certified traceable therm for the temps you'll operate at. I just ordered this one from Professional Equipment and plan on using it to calibrate all my therms to it.

 

[david_42]

I use cheap vodka. When heated, it sits at 172F until all of the alcohol is gone.

 

[jdoiv]

How can you tell when the alcohol is all gone? How long will it stay at 172?

 

[AiredAle]

The alcohol boils at 173 F because it is an azeotrope, that is it's boiling off as a mixture of 96% alcohol/4% water. When the alcohol is boiled off, and pure water is left, the boiling point will go up to 212 F. With a 40% alcohol content vodka, you'll need to boil off just about half of your total volume to start kicking up the temp.

To be most accurate, the thermometer should be in the vapor phase, with the alcohol/water azeotrope condensing on it.

Edit: these temps are based on sea level/standard barometric pressure, if your altitude or barometric pressure are much different the boiling points will be lower, normally.