Checking a Probe's Accuracy with Ice Water and Boiling Water

A probe is accurate when its reading matches the true temperature. You can't see that by looking at it, but you can compare it with something whose temperature is known. Ice and boiling water are the two usual references, because each holds a fixed temperature under the right conditions.

Why these two references

Ice cubes put in water start to melt when they reach their melting point¹, which is 0 °C (32 °F). A glass that's mostly ice with a little water stays near that temperature while the ice lasts. So a probe in it should read about 0 °C (32 °F). That's the ice check.

Water boils at 100 °C (212 °F) at standard pressure at sea level². That's the boiling check. It comes with a catch, covered below.

What "accuracy check" really means

The word calibration, strictly speaking, means just the act of comparison. It doesn't include any adjustment afterward³. So the two jobs are separate:

1. Compare. See how far the probe's reading is from the reference.
2. Adjust or replace. If the error is too big, either correct it, where the maker allows that, or stop using the probe.

The ice check, in outline

The idea is to put the tip in the middle of a glass of small ice, topped up with cold water. Keep it off the glass sides and bottom. Let the reading settle, then note it. The exact method, wait time and tolerance differ by product, so follow the steps in your own maker's manual.

Wash the probe first. Health Canada advises cleaning a food thermometer in warm, soapy water before each use⁴. Don't dunk a connector, plug or display into water unless the manual says it's rated for that.

The boiling check and altitude

Water doesn't boil at the same temperature everywhere. A liquid under lower pressure has a lower boiling point than the same liquid at atmospheric pressure². Higher places have lower pressure. One source gives water boiling at 93.4 °C (200.1 °F) at 1,905 metres (6,250 ft)².

So if you live well above sea level, water boils below the sea-level figure². To use it as a reference, you have to know your local boiling point. The ice check doesn't have that problem, which is why many people start there.

Boiling water is also very hot. Handle the pot with care, and check the probe's maximum temperature in the manual first. A probe that's rated for the cooker may not be rated for a boil, and some cables and plugs have lower limits than the tip does.

If the probe is off

A small difference may be within what the maker allows. Look for a stated accuracy in the specifications. If the manual gives one, compare your result with it.

When the error is bigger:

• Look for a calibration setting. Some units have one. If yours does, the manual explains how to use it.
• Check the basics. A damaged cable, water in a plug, or a weak battery in a wireless unit can all give odd numbers. See your manual's troubleshooting section.
• Contact the maker or replace the probe. If it can't be corrected, don't use it to judge food safety.

Making an adjustment means correcting the error to an acceptable level³. Without a way to adjust, the choice is between living with a known offset and replacing the probe. For safety checks, replace it.

One more point

A probe that passes the ice check at 0 °C (32 °F) has been checked at one point only. Cooking temperatures are much higher. Where the manual gives a second check, for example in boiling water, use it. A digital thermometer is the kind Health Canada recommends for more accurate readings⁴. After the check, go back to where to place the probe, since a true probe in the wrong spot still misleads.