Thermometer issues

roblanderson · Nov 21, 2013

During a recent brew we tried 3 different thermometers to verify our mash temp. 2 digital and one mercury.

Target strike temp was 156 and these were our readings: 152, 156, 160. The mercury thermometer was the high one. These temp differences were consistent throughout the entire mash.

Sticking these thermometers in ice water, they all read 32 degrees. Put them in boiling water and they all read 212.

Has anyone seen this before? I'm not sure how to calibrate a thermometer at a temp other than freezing or boiling.

We went with the mercury thermometer as being most accurate for this brew, but we're really not sure which to trust.

stpug · Nov 21, 2013

I did a similar test using three different thermometers (cheap digital, analog dial, mercury). The cheap digital and analog dial nearly the same while the mercury read hotter by about 4 degrees (coincidence I guess). I ended up buying a thermoworks for ~$25 because I needed something I could trust, and then redid the test. It turns out the cheap digital and analog dial were correct, the mercury read higher.

Not really any kind of solution to your dilemma but take it for what it is. Borrow some friends and/or neighbors. Put several in the same glass of really hot water (140-160 range) and see which read the same temps.

corax · Nov 21, 2013

One of my favorite quotes (supposedly from a Physicist):

"If you have a thermometer, you know the temperature. If you have two thermometers, you're confused."

IME, you can substitute "pH meter" for "thermometer" above and have an equally accurate statement.

doc5md · Nov 21, 2013

Agree with above poster who said check in water. Check in ~150 degree water and see what you get. if all the same, maybe the mash wasn't well mixed.

Wynne-R · Nov 21, 2013

I think when you say strike target temp you mean mash target temp. Mash temps are hard to measure because the mash is not heterogenous and the sensor area on a digital is pretty small. Even a well stirred mash can give you crazy numbers. My glass analog lab type thermometer works pretty well.

If you want to compare thermometers, I would do it in the strike water. Turn off the heat and stir it, then watch the thermometers as the water cools. That should eliminate hot spots.

If your thermometer is within a degree Fahrenheit thats about as good as it gets. Even the much vaunted Thermapen is ± .7°F.

roblanderson · Nov 22, 2013

Wynne-R said:
I think when you say strike target temp you mean mash target temp. Mash temps are hard to measure because the mash is not heterogenous and the sensor area on a digital is pretty small. Even a well stirred mash can give you crazy numbers. My glass analog lab type thermometer works pretty well.

If you want to compare thermometers, I would do it in the strike water. Turn off the heat and stir it, then watch the thermometers as the water cools. That should eliminate hot spots.

If your thermometer is within a degree Fahrenheit thats about as good as it gets. Even the much vaunted Thermapen is ± .7°F.

Sorry I wasn't clear - we were checking the strike water initially and that is where we saw the 8 degree spread between 152 - 160. After adding the grain we continued to see the 8 degree spread over the next hour.

I'll heat up some water and check all three again in the same 155 temp range. I also have a meat thermometer I can include as well. If I continue to have a large gap I'll look into buying a better thermometer.