Thermometer calibration - how do YOU do it?

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frazier

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I've been getting some poor attenuation for several batches now, and I came to the conclusion that my thermometer was at least 2 to 4 degrees low. So I was mashing at 158 instead of 154, for example. With no time to get to a store on a busy brewday, I just compensated on-the-fly.

So how best to calibrate?

1. Boiling water, it should read 212 (or compensated for altitude).
2. Buy a laboratory instrument, with a certificate of calibration and traceable to the National Bureau of Standards;
3. What is this "calibration" thing of which you speak?

Anyway, I guess I'm just venting. But I do want to trust my thermometer.
 
Ice/water mix at 50/50 mix. Swirl your thermo for a few minutes and you should get within .5 deg or right at 32
 
I like to do a two point calibration for my mash thermometer, boiling and ice water, since mash temp is much closer to boiling than ice.
I buy ~15$ thermometers from restaurant supply stores that have hex on the back that lets you calibrate them. I recommend either using two to prevent errors or calibrating in the boil each time.
 
The down and dirty way is what has been stated. A water/ice mixture should give you 32 degrees, and boiling (assuming you are not at high elevation) should give you about 212 degrees.

If you want to get more precise then I'm afraid you are as you stated in the initial post needing a laboratory grade instrument. Comparing your thermometer to a higher accuracy thermometer with a known good calibration is where you want to be.

You could also send it to a calibration laboratory, but unless it is a high reliability instrument, you have no guarantee that it will hold it's accuracy anyway. In the end it is best to spend a bit more on a thermometer with a proven track record, and compare it against other good thermometers when the opportunity arises.
 
When calibrating, make sure that your measurements are taken in DI, not wort. Dissolved substances raise the boiling point and lower the freezing point, which could throw off your calibration.
 
I have tried calibrating my thermometer in boiling water but I don't really know what is considered boiling. Is it when you see the first bubbles rise to the surface or when you have a rolling boil? I get 206 when I first see those tiny bubbles and hit 212 when it is a rolling boil.
 
I have tried calibrating my thermometer in boiling water but I don't really know what is considered boiling. Is it when you see the first bubbles rise to the surface or when you have a rolling boil? I get 206 when I first see those tiny bubbles and hit 212 when it is a rolling boil.

those first bubbles are gases being released from the liquid. the rolling boil is where you want to be calibrating.
 
In reality, the boiling/freezing comparison is not accurate when you're calibrating a dial thermometer. It should be calibrated at the range in which you expect to use it. Unfortunately this means you need a good digital thermometer like a thermapen.

I recalibrate my thermometers every time I brew, and I usually find that they do need a small adjustment (1 degree or so).
 
In actuality, it is accurate to calibrate over a range wider than the one you expect to use.
 
But what do you do when you calibrate it at boiling, and then check it at freezing and it doesn't read 32? Do you adjust it again, and then go back to boiling? The dial thermometers perform differently at different temps. I calibrate all my thermometers on my strike water, when it gets to about 150-160. The mash is probably the point with the greatest need for accuracy. For pitch temp, I don't care if it's off by 1 or 2 degrees.
 
How does one calibrate all of their thermometers with strike water at around 150-160? I thought the point was that at freezing and at boiling we are sure of the temperature of the water? How can you possibly know what the actual temp of some warm water is?

Anyways, I have two of those glass alcohol thermometers and both read 0 at freezing water, but one is 220 at boil and one is 209 at boil. (or similar)
So one has to scale the freeze to boil with 180 (212 - 32) degree range.

the first is reading 220, so 220-32 is 188, so I have to scale down my reading by 180/188 after I've accounted for 32 degrees off of zero for freezing. (Really celcius is a better scale for this)

so if my thermometer reads 160, I take [(160-32) * (180 / 188) ] + 32 and know that it's actual temperature is 154.5.

I have a chart with the calc for 10 or so temps around mash temperature.
Anyways, that's what I do, with my cheap $6 thermometers.
 
But what do you do when you calibrate it at boiling, and then check it at freezing and it doesn't read 32? Do you adjust it again, and then go back to boiling? The dial thermometers perform differently at different temps. I calibrate all my thermometers on my strike water, when it gets to about 150-160. The mash is probably the point with the greatest need for accuracy. For pitch temp, I don't care if it's off by 1 or 2 degrees.

Dial thermometers are not the most accurate temperature instruments out there. Even a good one is allowed by it's specifications to be off by 1 minor division anywhere across it's range. Cheaper ones can have larger errors and still be within their design limits.

So what I am saying is you could set it up to read 212 at boiling and have it read 30 or 28 at freezing and this may be the best you will get. In this case the standard calibration practice would be to split the difference and set it up to read maybe 213 at boiling and 31 at freezing, or however it works out.
You can also document the error as a correction chart to use when reading the instrument.
 
You can get a third point at 0 degrees F by adding significant amount of rock salt to the ice water bath and stirring the heck out of it. This is how Mr. Fahrenheit established the zero point on his scale since it was as cold as he could get water. I'm still not sure how he got 212 though...
 
I just calibrate in ice water.

Boiling points can change with altitude and barometric pressure. Make sure you take this into consideration when calibrating with a boil. I'm at 4200' so my boil temperature is around 204f.

Use this calculator.. http://www.csgnetwork.com/h2oboilcalc.html

thanks for the link. I was thinking my digital thermometer was off since it showed me boiling at 210°. With my altitude adjustment I'm right at my boiling point.
 
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