Checking Whether Your Thermometer Is Accurate

You can check a thermometer against a known point on the temperature scale, or against a second thermometer you trust. Either way you end up with an offset to write down, or a reason to replace the tool.

Two ways to check

Thermometers can be calibrated either by comparing them with other calibrated thermometers or by checking them against known fixed points on the temperature scale¹. The best known fixed points are the melting and boiling points of pure water, though the boiling point of water varies with pressure, so it has to be controlled¹. That makes ice the easier choice at home.

Be clear about one limit before starting. An ice bath tests a thermometer at one low point. It says nothing direct about how the same tool reads at oven temperatures. It's a sanity check, not a full calibration.

The ice-water check, where the manual allows it

Do this check only if the gauge's manual says its sensor can be removed and put in water, and its scale reaches the freezing point. Many oven dials start far above freezing, and many aren't made to be dipped in water. Don't remove or force anything fixed to your oven, and let the oven cool fully. Otherwise, compare against a second thermometer at oven temperature, or ask the maker.

The method is to immerse the sensing portion in a stirred mixture of pure ice and water at atmospheric pressure¹. Then mark the point it shows once it settles. In steps:

1. Fill a glass with crushed ice, then add cold water until it's slushy.
2. Stir the mixture.
3. Put the sensing part in the middle, clear of the glass walls and the bottom.
4. Wait until the reading stops moving.
5. Write down the number, and compare it with the freezing point in your own scale.

If the number is off by a little, you now know the offset. If it's off by a lot, or if the reading won't settle, the gauge is a candidate for replacement. Ask the maker for a part, and follow the manual for fitting it.

Checking an infrared gun

A gun can't be dipped in water, and you shouldn't try. Its sensor reads surfaces. There are two safer checks.

Against a contact thermometer. Point the gun at a surface whose temperature a second thermometer is measuring. If the gun has an adjustable emissivity setting, you can tune it until the gun matches the contact reading². Your gun's manual may describe another method, so follow it. Any result holds for similar surfaces only².

Against itself. The question in the source is whether the same thermometer gives the same reading for the same temperature¹. Aim at the same clean spot several times and see how closely the numbers agree. Wide scatter points to a technique problem, a dirty lens or a fault. Spot choice is covered in the aiming guide.

What if the two tools disagree

Disagreement doesn't always mean a faulty tool. They may be measuring different things. A gun reads a surface from a distance, and a dome gauge reads its own tip. Two common reasons are explained in floor versus dome readings and in the emissivity note.

Before blaming a tool, run through this list:

• Is the spot clean and non-shiny?
• Is the aiming distance the same each time?
• Has the gauge had time to settle?
• Are the batteries fresh, if the tool uses any? Check the manual.
• Is the lens clean?

Why even a good check has limits

Contact probes have their own limits. The main limitation with thermocouples is accuracy³. So a second thermometer is a reference, not proof. Two tools that agree are more convincing than one, but your manual and the thermometer's own spec sheet say what accuracy to expect.

What to do with the result

• Small offset on a gauge: write it on a note kept with the oven and apply it by hand.
• Large offset or erratic reading: stop relying on it and contact the maker.
• Gun scatter: clean the lens as the manual describes, then try again.

Whatever you find, keep using the same method each session. A steady method matters more than a perfect number, since you're comparing readings against your own oven's pattern. If you need the manual's stated limits, see Reading Your Maker's Manual on Temperatures and Limits.