I’ve read several posts with questions about thermometers reading correctly. There are 2 easy methods to check their accuracy. The ice bath and boiling water bath. Take a styrofoam cup fill with crushed ice and fill with water. Make sure the ice totally fills the container from top to bottom. Let the ice water sit for 5 minutes or so then insert your thermometer. It should read 32°. The boiling method is just as simple…boil distilled water to a rolling boil, insert thermometer. It should read 212°. For both tests make sure the thermometer is at least an inch away from the sides and bottom of the container. I know you can’t adjust most thermometers but you can compensate if you know the number of degrees it’s off.
I wish it was that simple, but after working many years in a field where temperature accuracy is critical, I know it is not.
The ice water test should also used distilled water, and the ice made from distilled water. It should also be constantly stirred.
The boiling water test needs to take place at standard atmospheric pressure, 14.7 psi.
The thermometer probe also needs to be inserted in the test liquid to it’s recommended test depth, usually about 4 inches for liquid-in-glass lab thermometers. It is different for each thermometer type and model, and the manual that came with the thermometer should say for the given unit.
There is also a possibility of non-linear bias. In other words, just because the thermometer is accurate at 32F and 212F, doesn’t mean it will be accurate at 150F. Digital thermometers are especially vulnerable to this particular error type. They will often use calibrations of more than two points, and a mistake at one of those will throw the results off around it, but not around the other calibration points.
Luckily, for brewing you only need decent temperature accuracy around 150F, and it is good enough to do the “thermometers match” test to see if you can trust your temperature reading, because when thermometers are inaccurate, it is highly unlikely that two will match each other in readings. So if you have two thermometers that show the same result around mashing temperature, that is probably pretty close to accurate.
Of course, nothing beats a NIST-traceability certification. But be aware that those only state that “at the time tested” it was accurate, and only at the temperatures measured.
All of the above^^^^!
I have had a couple non linear thermometers. OK at boiling and/or freezing, off at mash temps. Since I don’t mash at 32 or 212, I now use a NIST certified lab thermometer to calibrate them at 150F.