Thermometer calibration log: how to check a probe and what to record

A thermometer calibration log records that each food thermometer read within 2°F (1°C) of the true temperature when you checked it. The usual check is the ice-point method: a probe in a slush of crushed ice and water should read 32°F (0°C). Each row needs the date, which thermometer, the reading, who checked it and what you did if it was off. This page shows both methods, how often to check, a filled-in sheet and what to do with food read on a probe that turned out to be wrong.

If you want a blank sheet to print alongside your other records, the free sheets are on the logs page. Read on for how to use it so the entries mean something.

Why a calibration log matters

Every other temperature log in your kitchen depends on one thing: the thermometer told the truth. A hot holding reading of 138°F means nothing if the probe reads 6°F high and the food is really at 132°F. A cooling log, a receiving log and a cook-to-165°F check all rest on the same small tool, and it is the one most likely to be dropped, left in a hot pan or run through the dishwasher.

The FDA Food Code requires food temperature measuring devices to be accurate to within ±2°F or ±1°C and to be kept in good repair and adjusted so they stay accurate [1]. Inspectors often ask a cook to show how they calibrate. A log with dated rows answers that question in ten seconds, and it is also the record a HACCP plan normally lists as the verification for every temperature control point.

Method 1: the ice-point method

This is the standard method and the one most health departments teach. It costs a cup of ice [1][2].

1. Fill a large glass or container to the top with crushed ice. Crushed or small cubes work better than big cubes because there are no air gaps.
2. Add clean tap water until the container is full, so ice and water are in about equal parts. The mix should be slushy, not ice floating in a lot of water.
3. Stir and wait about 3 minutes so the whole mix settles at the same temperature.
4. Put the probe in so the sensing area is fully under the surface. On a stem thermometer that is usually the last 2 inches; on a thin thermocouple, the tip and a little more. Do not let the probe touch the sides or the bottom.
5. Stir gently and wait until the reading stops moving, about 30 seconds for a digital probe and up to a minute for a dial.
6. Read it. It should show 32°F (0°C). Anything from 30°F to 34°F is inside the ±2°F limit, and most kitchens write down the actual number and adjust anyway if it is more than 1°F off.

If it is outside the range, adjust the thermometer (below) and repeat the test. Do not skip the second test; it is what proves the fix worked.

Method 2: boiling point

Bring clean water to a rolling boil and put the probe in without touching the pot. At sea level the reading should be 212°F (100°C) [2]. Water boils at a lower temperature as you go up, by roughly 1°F for every 500 feet, so a kitchen at 5,000 feet should expect about 202°F. This is why the ice-point method is the one most kitchens use: it does not change with altitude, it is safer, and it checks the range where you make food-safety decisions. Use boiling water only as a second check, or for a thermometer that reads high temperatures such as a fryer or candy probe.

How to adjust each type of thermometer

If a thermometer cannot be adjusted and is more than 2°F off, take it out of service and mark it. A probe with a loose tip or a cracked display should go with it.

How often to calibrate

The Food Code says calibrated "as necessary to ensure accuracy" and does not give an interval, so your plan sets it [1]. These are the points most plans and inspectors accept:

• When a new thermometer arrives. Before it is used for the first time.
• At the start of every shift, or once a day for the probe used for critical limits, such as cooking and reheating checks. It takes about three minutes.
• After a drop or a hard knock, even if it looks fine.
• After a big temperature swing, such as from a hot pan to a freezer, or a probe left in a steam table.
• Whenever a reading looks wrong. A "cold" walk-in that suddenly reads 50°F while the food is cold to the touch is a calibration question first.
• At least weekly for ambient thermometers inside coolers and freezers, plus a comparison against a calibrated probe.

Write the interval in your plan and then keep it. A plan that says daily and a log that shows monthly is worse than a plan that says weekly and a log that shows every week.

What a filled-in calibration log looks like

Keep the columns few enough that a cook fills them in standing at the pass. Here are five rows from one week:

The third and fifth rows are the ones that earn trust. They show that the check finds problems and that someone acts on them. A sheet with every box perfect for a year looks invented.

What to do with food read on a bad probe

This is the part most calibration sheets leave out. When a probe fails a check, the question is what it was used for since the last good check.

1. Work out which way it was off. A probe that read 35°F in ice water reads 3°F high. Food it called 137°F was really about 134°F, and cold food it called 40°F was really about 37°F, which is fine. A probe that reads high is a hot-side risk; a probe that reads low is a cold-side risk.
2. List what it checked since the last good test: cooked-to-165°F items, hot holding, cooling and receiving.
3. Re-check hot and cold food that is still on hand with a good probe. If it is out of range, apply the usual corrective action: reheat to 165°F, or discard.
4. Write it in the Action column. "Rechecked 4 hot-hold pans with blue probe, all at 138°F or above" is a full answer.
5. Do not assume a failed probe was fine for the whole week. If you test daily, you are only looking back one day, and that is the reason to test daily.

Where the log sits in your HACCP plan

In a HACCP plan the calibration log is a verification record, not a control point. The control points are cooking, holding, cooling and reheating, and the monitoring record for each is a temperature log. Verification is how you prove the monitoring tool is reliable. In plan terms, add one line: "Food thermometers are calibrated by ice point daily and after any drop; record in the calibration log." The critical control points guide shows how monitoring and verification rows sit side by side. The records that rely on the probe are covered in the temperature log sheet guide, the receiving temperature log and the cooling log guide.

Keep calibration records with your other temperature logs. The Food Code asks for HACCP records to be kept at least 90 days, and many health departments want a year or more, so ask yours [1].

Common mistakes

• Testing in a glass of ice with a little water. Ice with air pockets reads warm or cold depending on where the tip sits. Fill with water and stir.
• Not waiting for the reading to settle. A digital probe that is still dropping reads high.
• Touching the glass. The side of a glass on a warm counter is warmer than the slush.
• Calibrating once and forgetting. A probe that was right in June can be wrong in July.
• Sharing one probe for raw and ready-to-eat food without sanitizing it. Wipe and sanitize between uses.
• Using a probe past its life. If it needs adjusting every week, replace it.

Questions

How accurate does a food thermometer need to be?

Within ±2°F (±1°C) for food temperature measuring devices under the Food Code [1].

What should the reading be in ice water?

32°F (0°C). A properly made slush of crushed ice and water, stirred and left about three minutes, is the standard.

How often do I have to calibrate a food thermometer?

The Food Code does not name an interval. Most plans use daily or at the start of each shift, plus after any drop and when a reading looks wrong.

Do I have to log it?

The rule is to keep thermometers accurate. A log is how you prove it, and a HACCP plan normally lists it as a verification record.

Do infrared thermometers need calibrating?

They are checked differently and only read surface temperature. Follow the maker's steps, and use a probe in the food for any decision.

Can I use boiling water instead of ice?

Yes, as a check at 212°F at sea level, lower at altitude. Ice water is safer and works at any altitude.

Where do I get a calibration log?

Use the columns above on any sheet, or take the free sheets on our logs page. See also all our food safety guides.

Sources

1. FDA Food Code 2022 (4-203.11 and 4-203.12 temperature measuring devices, 4-502.11 good repair and calibration), U.S. Food and Drug Administration
2. Kitchen thermometers, USDA Food Safety and Inspection Service

Related: all food safety guides.