Use one thermometer in the middle of the box, away from the door and the coil airstream, and check the warmest food with a probe. Log the display and thermometer at least twice a day with the time and your initials. A log turns small changes into visible trends and helps you decide about food after a failure.
A temperature reading only tells you something if you know where it came from. Air near the door swings with every opening. Air in front of the evaporator is the coldest in the box. Food in the middle of a large container may be warmer than its surface. Placement and habit matter as much as the instrument.
Where to place them
| Location | What it tells you |
|---|---|
| Middle of the box, shelf height, away from door and fans | A steady, representative air temperature |
| Near the door | The warmest air, useful for spotting door effects |
| In front of the evaporator | The coldest air. Do not use this for judging food. |
| In a container of water or glycol | A reading that behaves more like food, less jumpy than air |
| Inserted into food | The real food temperature |
Types of thermometer
- Built-in display. Reads a sensor. Convenient, but the sensor can be misplaced or drift.
- Door thermometer. Often mounted outside, reading inside the box. Easy to see.
- Independent air thermometer. Sits on a shelf as a cross-check.
- Probe thermometer. For food. Clean and sanitize it between uses.
- Data logger. Records readings continuously, and some alert your phone.
Check accuracy now and then by placing the probe in ice water. It should read close to freezing. Replace or recalibrate an instrument that does not.
A log that works
- Record the date and time.
- Record the display reading and the independent thermometer.
- Note the door, floor and coil in a few words.
- Initial each line, so someone is accountable.
- Do it at fixed times, such as opening, mid-shift and closing.
- Write a note whenever something is unusual, with the time.
Keep the logs for as long as your inspector or your policy suggests. Ask what is expected.
Reading the log
| What you see | Possible meaning |
|---|---|
| Steady, small differences | Normal. Keep going. |
| Display and thermometer drift apart | Sensor or instrument error |
| Afternoon highs every day | Heat load or a dirty condenser |
| Rises after deliveries | Door and load habits |
| Slow steady rise over weeks | Developing fault: coil, charge or gasket |
| Sudden jump | Failure or door left open |
Why it matters for food safety
Cold-held food in a food business must stay at 41 degrees or below, and a log shows you kept it there. After a failure, the log shows when the box was last in range. See the food-safety guide for how records fit into an inspection, and the temperature swings page for what a moving log means.
The routine sits inside the daily and weekly staff checks and the wider maintenance checklist.
Questions about thermometers and logs
How often should I log?
At least twice a day. More often during hot weather, after repairs and during any suspected problem.
Is a wireless monitor worth it?
It can be, especially if nobody is on site overnight. Choose one with alerts and battery backup, and still keep a manual check.
What if the display and thermometer differ by a few degrees?
Check both against a third reading. Have the controller sensor calibrated or replaced if it is the one that is off.