A thermometer calibration log records each time you check a thermometer against a known temperature: the date, the thermometer, the method, what it should read, what it did read, and what you did about it. The standard check is the ice point: a glass packed with crushed ice and topped up with cold water sits at 32°F at any altitude, and a good probe reads within its tolerance. The boiling point works too, but only if you know yours: water boils at 212°F at sea level and lower as you climb, about 202°F in Denver, so a probe reading 202 there is right, not 10 degrees off. The tolerance comes from the FDA Food Code: a food thermometer scaled only in Fahrenheit should be accurate to ±2°F, an air or water thermometer, like the one in a walk-in, to ±3°F. For manufacturers, the FDA's food safety rule asks for instruments that are accurate, precise and maintained, and a facility with preventive controls keeps a written method and frequency for calibrating them and the records for at least two years. In the example below, a Denver sauce maker checks seven thermometers in one morning. Below are the methods, the rules, the example, and a free template.
The short version
- A calibration log records each check: date, thermometer, method, reference, reading, action.
- Ice point: crushed ice and cold water, 32°F at any altitude.
- Boiling point: 212°F at sea level, about 202°F in Denver; know your altitude.
- Food thermometers in Fahrenheit: within ±2°F. Air and water thermometers: within ±3°F.
- Out of tolerance: recalibrate if it can be, replace if it can't.
- Facilities with preventive controls write down the method and frequency, and keep records 2 years.
- TaroStack keeps each batch's readings on its batch record. More below.
The methods
| Method | How | Reads |
|---|---|---|
| Ice point | Fill a glass with crushed ice, top with cold water, stir, wait, then hold the probe in the middle without touching the glass | 32°F, at any altitude |
| Boiling point | Probe in water at a full rolling boil, not touching the pot | 212°F at sea level, lower as you go higher |
| Against a reference | Read the thermometer beside a reference probe you've checked, in the same water or food | The reference's reading |
Thermometers that can't be checked in ice, like a walk-in's or a fill line's, are checked against a reference probe. How the readings themselves are logged is in temperature log template.
The rules
| Rule | Where |
|---|---|
| Food thermometers | ±1°C, or ±2°F if scaled only in Fahrenheit (Food Code 2022, 4-203.11) |
| Air and water thermometers | ±1.5°C, or ±3°F if scaled only in Fahrenheit (4-203.12) |
| Calibration | Food thermometers calibrated per the manufacturer as needed for accuracy (4-502.11) |
| Manufacturers | Instruments for temperature, pH, acidity or water activity accurate, precise and maintained; a thermometer in each freezer and cold store (21 CFR 117.40) |
| Preventive controls | Calibrate, or check the accuracy of, monitoring and verification instruments, with a written method and frequency (21 CFR 117.165) |
| Records | Calibration records kept, at least 2 years (117.190, 117.315) |
Where the method and frequency belong is in what is a food safety plan, and how long records are kept in how long should HACCP records be kept. This is general information, not legal advice.
An example: a Denver sauce maker
Mile High Green Chile Co., a made-up green chile sauce maker in Denver, Colorado, I use in these examples, hot-fills jars for grocers. Its process authority's minimum fill temperature makes the fill line thermometer a monitoring instrument, and on a Monday morning the owner checks all seven:
| Thermometer | Method | Should read | Read | Tolerance | Result |
|---|---|---|---|---|---|
| Probe A, digital | Ice point | 32.0°F | 32.6°F | ±2 | Pass |
| Probe B, digital | Ice point | 32.0°F | 34.8°F | ±2 | Recalibrated, rechecked at 32.2°F |
| Dial stem thermometer | Ice point | 32.0°F | 31.0°F | ±2 | Pass |
| Probe A, digital | Boiling point, 5,280 ft | 202.0°F | 202.1°F | ±2 | Pass |
| Fill line thermometer | Against probe A at the fill | 186.5°F | 185.0°F | ±2 | Pass |
| Walk-in thermometer | Against probe A, water glass in the cooler | 37.4°F | 39.0°F | ±3 | Pass |
| Freezer thermometer | Against probe A | −2.0°F | 3.0°F | ±3 | Replaced |
Probe B reads almost 3 degrees warm, so jars it measured would look hotter than they were; it's recalibrated and checked again before use. The boiling check shows why altitude matters: against 212°F, probe A would look 10 degrees cold and get "fixed" wrong. The probes are checked weekly, the cooler and freezer monthly, and the log goes in the binder with the batch records. The acidified food side of a hot-filled sauce is in FCE registration.
Get the template
Download the thermometer calibration log. One sheet: each check's date, thermometer, method, the reference and the reading, the tolerance and how often it's checked; the difference, pass or adjust, and the next check due fill in, with out-of-tolerance checks in red. It's an ordinary Excel file with no macros, and it works in Google Sheets, LibreOffice and Numbers.
Where this stops working
The log says probe B read 2.8 degrees warm on Monday. It doesn't say which batches probe B measured last week, and that is the question an auditor, or a process authority, asks next.
How TaroStack does it
TaroStack keeps each batch's readings on its batch record, with the thermometer that took them.
Your own fields on the batch record. A batch record can carry fields of your own, such as the fill temperature and which thermometer read it, recorded on a phone at the line.
Holds kept out of stock. A doubtful batch can be put on hold, apart from what's available to sell until it's released, so jars from a doubtful week don't ship while you decide.
Where it went. Each grocer's order records the lots it got, so if a batch has to come back, one recall report shows every store it reached.
| The job | Binder | In TaroStack |
|---|---|---|
| Fill temperature for a batch | A paper sheet | A field on the batch record |
| Which thermometer read it | Often not written | A field beside the reading |
| Doubtful jars | Set aside, hopefully | A hold, kept out of stock |
| Which stores got a batch | Invoices | One recall report |
Batch records with your own fields, holds, lots and recalls are on every plan, from $49 a month. The first 30 days are free, with no credit card. If you'd like a hand setting up, ask, and we'll do it with you.