Thermometer calibration...how to...????

[sargeny1]

Hi All...I have both the Lyman and RCBS Thermometers...BOTH of them show the pointer way below zero when cold...shouldn't the pointer be on zero..??? They were always this way since I bought them...do they need calibration..??? How does one calibrate these thermometers...???
What say ye learned gentlemen..????

Thanks..
Pete

[uscra112]

Thermometers like those won't be linear at such low temps. Not to worry.

Best thing I can think of for us homebrewers to calibrate a lead thermometer would be to melt a batch of known pure lead, then let it cool, taking a reading on your instrument every 15-20 seconds. There will be a "plateau" in the curve just as the lead freezes. Compare what your thermometer reads during that plateau with the handbook value for the melting point of lead. (No, I haven't memorized it - that's why God gave us lazy engineers handbooks - so we wouldn't have to.) This trick is, BTW, helpful in determining alloy content, but only if you already have a calibrated thermometer!

[imashooter2]

Most thermometers in the casting range don't go to zero. They start at 200 degrees and the pointer goes well below that when cold.

The calibration medium most readily available to the home user is boiling water. Find out your elevation and look up the boiling point of water at that elevation (Google is your friend). Hopefully it is over the 200 degree minimum on the thermometer.

Now a question... Do both thermometers read the same in the same pot? If so, they are both good enough. All your casting notes will be made using your measuring tools. Your results will be consistent.