Brewing Thermometers

This will be a little bit different of a post this week to discuss the thermometer, an important but often overlooked and misunderstood piece of brewing equipment.  So get that meat thermometer out of your mash tun and read on!

Chances are, unless you have some sort of instrumentation background, you’re probably under the impression that the temperature readout on a digital thermometer is the temperature of whatever its in, and the more digits its reads out, the more accurate it is.  Unfortunately, that’s not true: determining the absolute temperature of something over a useful range of temperatures turns out to be very hard to do.  In fact, these digits are resolution, not accuracy.  A thermometer able to read out 100.1 degrees might be able to tell you when the temperature increases by 0.1 degrees, but not what the actual temperature is.  Many ovens, even with their digital readouts, can be 50F/25C off in either direction easily.

food thermometer

Photo by Steven Jackson Photography

Even the best of the thermometers sold by brewing supply companies are only only specified to be accurate to within 2F/1C.  That means if you are conducting a mash and hit 150F/65.5C, the actual temperature of the mash could be anywhere from 148F to 152F/ 64.5C to 66.5C.  This will make a considerable difference in the final beer, even assuming the thermometer is kept calibrated and is not even further off.  Calibrating a thermometer at best returns it to its rated accuracy.

Lab grade thermometers (in a reasonable price range) do about twice as well and are generally specified as being accurate to within 0.9F/0.5C.  That’s about where the average home brewer is going to top out, and is the accuracy of my personal thermometer (though for just a bit more I have found accuracy within 0.4C).  If you have large rolls of money burning holes through your pockets, you can get much better lab grade thermometers, including those meant to properly calibrate others.  But, I suspect at that point the limiting factor on accuracy in getting a mash tun temperature is going to be the temperature difference from one spot to another, and higher thermometer accuracy isn’t going to do you much good.

Here are some important factors to consider when choosing a thermometer for brewing:

  • Accuracy/ Resolution: You should be able to find, perhaps with some effort, +/-0.9F/0.5C for a very reasonable price, and all but the worst thermometers read out plenty of digits.
  • Temperature range: It would probably be difficult to find a digital thermometer that is unable to read in the brewing temperature range, but its something to keep in mind.  Also, thermometers are typically only good to their maximum rated accuracy within a more limited range than their full working range.
  • Calibratable: No matter how good your thermometer is, it can drift and need recalibration.  A good thermometer can be calibrated at least with ice water, and others with ice and boiling water.  Ice water is better to use, as water boils at a different temperature depending on your elevation.
  • Time to read: My old thermometer took forever to settle into the actual temperature, and it was incredibly frustrating!  Look for a thermometer that can give you an accurate reading in a few (say, under 10) seconds or better.
  • Stem length: A longer stem will keep your hands farther from hot things, and let you probe deeper into the mash.
  • Low/ high temperature hold: If you have to read the temperature of something inside of where you cannot easily see (an overhead hot liquor tank, a dark mash tun), a thermometer that displays the hottest temperature it encounters until it’s reset is for you.
  • Water/ shock (drop) resistance: The steam from my brew pots and mash tun made my old thermometer inoperable by the end of the brew day, and I would have to crack open the case and let it dry for a few days.
  • Sleak design: Its nice to have a cool looking gadget, but having a thermometer without nooks, crannies, and crevices will also reduce places for gunk to build up and bacteria to hide.

For what its worth, my current thermometer is a ThermoWorks RT301WA-N.  I won’t go so far as to endorse it because I haven’t had it long, but I do like it so far.  I wish the stem was longer, but otherwise I’m quite happy with it.  As of this writing, you could get one for $19.00 (USD) plus shipping.

For no good reason, here’s how three common types of thermometers work:

  • Digital: Some digital thermometers work by running a small electrical current through a precision resistor, and measuring the voltage drop.  This voltage drop is caused by the (aptly named) resistor resisting the flow of electricity, like stepping in the middle of a garden hose.  The resistor is a special type that changes resistance very reliably with a change in temperature.  Thus, as the temperature changes, so does resistance and the resultant voltage drop across the resistor.
  • Mercury/ alcohol: This old type of thermometer is also perhaps one of the simplest.  The mercury—or more likely today, alcohol—simply expands when it heats up, and forces its way further up a marked stem.
  • Bi-metalic strip:  This is probably the type on your grill or in those long thermometers for turkey friers. Two strips of metal, each one a different metal, are formed into a single strip and coiled into a flat spiral.  The center of the spiral is attached to a needle that displays the temperature on the dial face.  These metals are chosen to have different rates of expansion with temperature, so one side of the strip gets longer faster.  This causes the coil to get tighter or looser as the temperature changes, rotating the needle.

– Dennis,
Life, Fermented


About Dennis
Home brewer, home chef, garage tinkerer. Author of Life Fermented blog.

2 Responses to Brewing Thermometers

  1. Anonymous says:

    I believe you’ve neglected to mention another type of thermometer – one using a thermocouple. This is how the Thermapen works, a popular device amongst chefs. I use one for my homebrewing and have found it quite good, although it lacks a few features like the min/max temp memory and long stem.

    • Dennis says:

      Actually I neglected quite a few types of thermometers! You are correct though, thermocouple types are rather popular, and I made it sound as if all digital thermometers used temperature-variant resistors. I updated the post to correct that.
      – Dennis

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