How accurate is a floating dairy thermometer for measuring strike/sparge temps?

I've been using a dairy thermometer in lieu of a meat thermometer for measuring water temps pre-mash and then in the mash. It seems accurate to me, but I worry there's enough inaccuracy that I might be mashing a few degrees too low, as evidenced by my last batch. I'm also using the same thermometer to measure the temperature of my swamp cooler, which also seems to hold accuracy. I have nothing to measure this against, as my meat probe thermometer is certainly out of range. Any way to measure this for calibration without anything to judge it against? Or are they fairly accurate right out of the box? It came from the LHBS by the way.

I use a floating thermometer too and like it very much. If you wanted to see how accurate it is, you could test it against boiling water (212*).

...and ice water if it goes low enough..

Pappers_ said:

I use a floating thermometer too and like it very much. If you wanted to see how accurate it is, you could test it against boiling water (212*).

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* adjusted for elevation Water Altitude Boiling Point Calculator

Thanks all. Well I did some searching and found Craftsman makes a voltage tester with a temperature probe and bought that (giftcard!). Did a boil test with the dairy thermometer, voltage tester, and a third thermometer I found in the kitchen! Voltage tester was HIGHLY accurate, within one degree. Extremely impressed, and very quick reading and quick changing. The one from the kitchen was slow and about three degrees off or dead-on, just depended on the test.

The dairy thermometer, on the other hand, has a problem. Even as the boil climbed to 214-216, it said it was under 210. In the swamp cooler it says 64 degree water is 59 degrees. So the dairy thermometer has been giving me mash temps 3-5 degrees too low! A culprit is found! I'm hoping this Denny Conn Rye IPA recipe won't be too far effected, my porter before it has a light body but still very drinkable.

Now at least I have an accurate, reliable thermometer and know to only use the dairy thermometer in situations where I can swing a few degrees. Thanks all!

How did you get a boil temp of 214-216? Water boils at 212, without putting it under pressure or adding things to it, that is as high as it goes.

^ It starts boiling at 212, but it can get higher. That's the problem with calibrating against boiling water. It's at 212 exactly as it starts boiling.

I just purchased a floating thermometer to calibrate my dial thermometer with two days ago. My dial was reading 2 degrees high, which really isn't bad at the homebrew level anyway.

Airborneguy said:

^ It starts boiling at 212, but it can get higher. That's the problem with calibrating against boiling water. It's at 212 exactly as it starts boiling.

I just purchased a floating thermometer to calibrate my dial thermometer with two days ago. My dial was reading 2 degrees high, which really isn't bad at the homebrew level anyway.

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Yeah I just dialed up and down my stovetop over and over to capture the boiling point. The volt tester caught it right away, but the dairy thermometer never broke 200, so I know not to rely on it for near-boiling temps.

I am guessing the dairy thermometer has good accuracy in it's intended range of 80-120deg

Airborneguy said:

^ It starts boiling at 212, but it can get higher. That's the problem with calibrating against boiling water. It's at 212 exactly as it starts boiling.

I just purchased a floating thermometer to calibrate my dial thermometer with two days ago. My dial was reading 2 degrees high, which really isn't bad at the homebrew level anyway.

Click to expand...

After 212 degrees at sea level water is a vapor unless one adds pressure or dissolves something into it to raise the temp. Add more heat and your boil becomes more rolling, which is just the water turning to steam at a faster rate. Any temp increase you are seeing in pure water is the temp of steam bubbles, tho I find it hard to fathom you having a thermometer accurate and fast enough to measure this.

No thermometer I have ever tested in boiling water has ever stopped right at 212. I'm not saying I've seen 220, but 214-216 is pretty common. I don't know the science behind it though so I'm sure the steam explanation is possible. This is why I stopped calibrating my dial thermometer with boiling water.

So what method is best to calibrate?

Water with sugary wort will get to higher temp of 212. Maple syrup ends up at 230. All depends on the sugar content.