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-   -   PSA-Check your thermometers regularly (http://www.homebrewtalk.com/f36/psa-check-your-thermometers-regularly-326340/)

d_striker 05-05-2012 12:53 AM

PSA-Check your thermometers regularly
As in before every batch.

The bonehead that I am, it took me two batches before I realized my digital Thermo was way off. I was mashing about 8 degrees warmer than I thought. Not a huge deal but makes for a very noticeable difference in taste and mouthfeel.

It takes no time to stick it in a glass of ice water before EVERY batch.

Polboy 05-05-2012 01:00 AM

its a good warning, i have two thermometers (one in hlt and 2nd digital i check mash tune with) so i dont think i have to check it each time but will do it more often :)

Minjin 05-05-2012 03:27 AM

Why icewater?

BrewKnurd 05-05-2012 03:32 AM


Originally Posted by Minjin
Why icewater?

Because ice water and boiling water are the two easiest constants you have. A proper ice bath will be exactly 32 degrees.

mabrungard 05-05-2012 12:31 PM

Since the freezing and boiling points of water are fairly distant from our typical mashing temperatures, getting a certified thermometer is the best way to check the calibration of your working thermometers. Many brewers have found that their thermometers can be way off, even though they checked them at freezing and boiling.

zeg 11-26-2012 02:30 PM

Reviving this thread because I just checked my thermometer and found it was about 6 off at boiling! It was reading 218 instead of 212F!

Guess my mashes have been on the low side... fortunately I've mostly been doing high mash temps, so I think they've all been in the sane region, but this probably explains my higher than expected attenuations.... I've also been doing partial mashes, so that limits the impact of screwing up the temperature.

One thing to add to the thread---with most dial thermometers, you only have one degree of freedom. You can turn the knob to set the offset to the needle, but there's no way to adjust the scaling. Because of this, a two-point calibration is not really possible. You can use it to check that your scale is roughly correct, but you can only match the true temperature at one point unless the scale is naturally correct. You may be able to do a bit better by manually working out the scale factor and applying it, I suppose. In any case, I usually do this at boiling, simply because a true 212F boil is easier to get than a true 32F solution.

If you're going for a cold calibration, you want to make sure the ice you're using is crushed very finely. You're aiming for water that is in equilibrium between solid and liquid. Dropping standard ice cubes into water can give you liquid that's a few degrees warmer.

But, given the overall accuracy of the thermometers, and the inability to truly calibrate the dial type, it's not that critical to get exactly 32F. As mabrungard says, the best would be to match the dial to a known temperature right at the working temperature, so somewhere around 150F. Unfortunately, it's difficult to get a good calibration standard there unless you buy a rather high quality thermometer.

TNGabe 11-26-2012 02:54 PM

6 degree ain't nothing. I mashed at about 120 once. Then dumped a bunch of syrup in when I realized gravity was 1.020 after the boil. Wasted a vial of 550 on it and ended up with Belgian natty lite. I would have pitched it, but a friend legged it up. Quit drinking before the boil after that.

SpeedYellow 11-27-2012 01:45 PM


Originally Posted by zeg (Post 4621863)
Reviving this thread because I just checked my thermometer and found it was about 6 off at boiling! It was reading 218 instead of 212F!....

Did you toss in some pebbles or a pinch of chalk to the boil? If not, the water could read several degrees hot. The exact thing happened to me -- my lab thermometer read IIRC 217F at boil, then I tossed in a pinch of chalk and suddenly it dropped to 212F.

zeg 11-27-2012 02:00 PM

Interesting, worth a try. I don't have any chalk around, but I'll get some pebbles or gravel to see.

My suspicion is that the thermometer really was off because I saw the same temperature in boiling wort well after it had reached a steady boil, and with an immersion chiller and a spoon in the kettle. I'd expect those to have a similar effect. At first I didn't think anything of it, because I expected the boiling point to be raised a bit by the increased gravity. It didn't sit quite right, so I looked it up and a 218F boiling point would mean I was boiling some very, very heavy sugar syrup, far more appropriate for making hard candy than beer!

For the record, the purpose of the chalk or pebbles is to keep the boil smooth. Without them, pockets of superheated water can be bouncing around in the violent, rolling boil.

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