Thermometer Calibration

It’s spring cleaning time in the brewery. I’ve given the kegerator a good once-over, scrubbed the kettles shiny, replaced all the vinyl tubing, and so now it must be time for instrument calibrations. I check the hydrometer and refractometer every few batches because it’s so easy (use water and a 10% sucrose solution), but it occurred to me that I’ve never checked my thermometers. There are seven types that get regular use in my brewery:

• My newest toy – a NIST-traceable waterproof digital probe model
• A TruTemp-branded compact digital that set me back about $9
• An AcuRite-branded digital “meat” thermometer
• A no-brand kitchen probe thermometer from Walmart
• An ordinary floating alcohol thermometer
• Several LCD strip thermometers
• The bulkhead thermometer in my boil kettle

Since the kettle thermometer requires about three gallons, and the strips five gallons, of liquid in order to be submerged, it didn’t really seem practical to check them against the others. All the others read to 0.1°C, except for the AcuRite (1°C) and the Walmart thermometer, which only does Fahrenheit and which I know not to be accurate anyway.

The other five thermometers were checked at four temperatures: 0.0°C (ice water), ~23°C (room temperature), ~59°C (mixture of ice-cold and boiling water), and 99.2°C (boiling water). That gives me, roughly speaking, fermentation and mash temperature calibrations for each, plus a couple of outliers for pretty curve fits. For the two middle values, the traceable thermometer’s readings were assumed to be correct, which given the readings at freezing and boiling seems justifiable.

Generally speaking, all the thermometers did well. Throwing out the no-name Walmart model and the surprisingly inaccurate alcohol thermometer, the maximum variation was 1.3°C, which isn’t great for lab equipment but is probably fine for brewing. Surprisingly, the AcuRite kitchen thermometer is every bit as accurate as the ISO 17025-calibrated lab thermometer, landing all four readings within its (admittedly larger) resolution. The TruTemp model turned out to be less accurate, albeit with a highly linear discrepancy. This is interesting because it could point to a design problem – assuming a linear voltage response for the thermocouple rather than doing a multi-point calibration. More to the point, it means that it can still be useful for brewing once the proper offset is applied.