How to Check the Accuracy of Your Kitchen Thermometer

Every week, baking expert Alice Medrich is going rogue on Food52 -- with shortcuts, hacks, and game-changing recipes.

Today: Could your thermometer be telling a lie? Alice spells out how to get an answer you can trust. 

That thermometer you have in the kitchen drawer? The one you use for meat, for candy, for frying, for tempering chocolate, for bread making, for custards and sauces, and heaven-knows-what? It won’t do you any good unless it is at least close to accurate.

A thermometer should be checked when brand new, and rechecked every six months or so, after you drop or otherwise traumatize it, when you haven’t used it in a while, and whenever you just aren’t sure it’s telling you the truth. You can test a thermometer in boiling water or an ice bath; doing both is not a bad idea either. 

To test in boiling water:
Water at sea level boils at 212° F* or 100° C. Bring a pot of water** to a rolling boil. Insert the thermometer stem at least an inch deep in the water, without letting it touch the sides or bottom of the pot. Wait for the thermometer to register; this usually takes a minute or less. To avoid scalding your hand, hold the thermometer with tongs or slip the stem into a long handled slotted spoon -- anything secure enough to prevent its falling into the water. The thermometer is accurate if it registers 212° F or 100° C.  An error factor*** of up to 2 degrees Fahrenheit in either direction is not uncommon for kitchen thermometers. (All three of my thermometers read 1/2 to 2 degrees high in boiling water.)

To test in an ice bath:
Water freezes at 32° F or 0° C. For testing, you need as much ice as possible in a glass of water. To that end, fill a glass to the top with crushed ice. Set the glass in a pan or bowl to catch overflow. Pour cold water over the ice to the rim of the glass. Pile more ice on top, letting the water overflow.  Insert the thermometer stem at least an inch deep in the ice water without letting the stem touch the glass. Wait for the thermometer to register; this usually takes a minute or less.  The thermometer is accurate if it registers 32° F or 0° C. (My three thermometers are within 1 degree of accuracy at this end.) 

Here are your options if your thermometer is not accurate:

Option 1: Live with it
If your thermometer is off by 1 or 2 or even a few degrees and you can't or don’t want to recalibrate it, make a note to self or mark the thermometer, "Reads 3 (or whatever) degrees high." When using, aim for a reading that is 3 degrees higher than the temperature wanted. If your thermometer is off at both ends by significantly different amounts or in different directions, consider Option 3 below -- Contact the Manufacturer.

Option 2: Recalibrate
Dial thermometers: These may have a hexagonal nut under the dial. Use a tiny wrench to turn this nut -- very very slightly -- to adjust the dial. It’s quite difficult to do this while the thermometer is in boiling or freezing water, and the temperature will be in flux as soon as you remove it. The solution is to note that the thermometer reads (say) 4 degrees high in boiling water, then set the thermometer aside until it registers a steady room temperature.  Then twist the stem until the dial reads 4 degrees lower. Then retest in boiling water. If there is no calibration nut, go to Option 3.

Digital thermometers: These may have a reset or calibration button; if so, follow the manual to recalibrate. Otherwise, go to Option 3. Note: If your digital thermometer is the type with a remote probe, you can purchase a replacement probe for less than a new thermometer. 

Mercury or liquid-in-glass-tube thermometers: Forget about it! Techniques for recalibrating are technical and time-consuming. Toss the thermometer, or go to Option 3.

Option 3: Contact the Manufacturer
Go to the brand website and explain the problem. You may be asked to send the thermometer back to them, probably with a receipt. They may repair or replace the thermometer. What, you didn’t save the receipt? You’ve had that thermometer for a zillion years? What do you have to lose? Pack it up anyway and send it with a note saying how much you use and depend on their marvelous product, how disappointed you are that it's no longer accurate, and is there anything they or you can do? Who knows...

Final advice? Always save receipts for thermometers and polish up your heartfelt letter-writing skills. With luck, you might parlay them into a continuing supply of accurate thermometers. Because, honestly, most of the manufacturers could be doing a better job with thermometers.

NOTES
*If you don’t live at or close to sea level, go online to find the temperature at which water boils at your elevation. Use that temperature for the boiling water test.
**You may have heard that distilled water is recommended for testing thermometers, because water with a high mineral content may slightly affect the temperature at which it boils or freezes. For normal cooking, baking, candy-making, home brewing, and so forth, we don’t need to worry about that degree of accuracy.
***The manufacturing standard for ordinary kitchen thermometers is accuracy within 2 degrees Fahrenheit. You are unlikely to get repair or replacement if your thermometer falls in that range.

More: Test out that spruced-up thermometer with salted pumpkin caramels.

Get excited about Alice's forthcoming book Flavor Flours: nearly 125 recipes -- from Double Oatmeal Cookies to Buckwheat Gingerbread -- made with wheat flour alternatives like rice flour, oat flour, corn flour, sorghum flour, and teff (not only because they're gluten-free, but for an extra dimension of flavor, too). 

Photos by James Ransom