We recently heard a story where a commercial brewery was not obtaining the final gravities they were expecting. After fingers were pointed at equipment manufacturers and at yeast suppliers the end result was the gauge thermometer they were using in their Mashtun was 5 degrees off. Whoops. This prompted us to write down some tips on how to properly calibrate your dial thermometers so that you don't have the same problem.
First off the common street knowledge about calibration is to submerse your thermometer in ice water or a boil and recalibrate to at 32F or 212F accordingly. Don't do that.
When calibrating you should use a set point that is as close to your testing temperature as possible. If you use the thermometer for a wide range of reading, choose a calibrating temperature nearest to the mid-point of the scale. For example our Tel-Tru Dual Gauge Thermometers come with a dial range of 0 to 220F. They are calibrated at 100F, close to midway through the sacle, and then checked at the low and high end to ensure that they are within an accuracy of 1 Fahrenheit throughout the range.
While a 1F level of accuracy in a thermometer is completely acceptable for all aspects of brewing, if you were calibrating a thermometer specificallly for use in a Mashtun that has a range of 140F to 160F you could ideally choose to calibrate at 150F. Likewise if you were calibrating a Hot-Liquor tank thermometer you might want to calibrate at 170F. The idea is to apply the best accuracy to the range in which you will be working.
So to calibrate at a specific temperature instead at 32F or at boil, you need an accurate thermometer you can trust. We suggest purchasing our MT360 Spirit Filled thermometer.
If you don't have a spirit filled thermometer and just want to test accuracy you can use an ice bath (32F) or boiling water(212F at sea level) . However, again, just don't re-calibrate your thermometer at 32F or 212F because you can throw the accuracy off in the middle of the range. For example a therometer calibrated at 32F will likely be out of accuracy tolerance when checked at 212F because the accuracy has been applied to the low end of the range.
Our MT500 thermometer with 6" probe should be submersed at least 2.5 inches into the fluid to get an accurate reading (there is a mark on the stem to indicate this level). Our MT502 thermometer with 2.5" probe should be submersed all the way to the nut on the back of the housing to get an accurate reading.
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