Analog vs Digital Thermometers: How do You Calibrate a Thermometer?
In a professional kitchen, maintaining your tools and equipment is an essential part of your routine. You keep your knives sharp and your food surfaces clean. You degrease the drip trays and season the griddle. Another key item on the list? Calibrating your food thermometers.
Using an inaccurate or improperly calibrated thermometer presents a food quality issue and, more importantly, a food safety threat.
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Why is Thermometer Calibration Important?
Between 41° and 135°F, pathogens double every 20 minutes, so it's important to control and limit the amount of time that food remains in this "temperature danger zone" (TDZ).
In a commercial kitchen, thermometers and temperature logs ensure that food is cooked to a minimum safe temperature to kill most pathogens and/or held outside the TDZ to prevent pathogen growth.
Temperature should be checked and maintained at every step, starting with inspecting your shipments. If your thermometer is even a few degrees off, you could end up serving food with a dangerous bacterial load.
In terms of more tangible consequences, inaccurate thermometers can also get you dinged on inspection.
When Should You Calibrate a Food Thermometer?
Your thermometer calibration schedule will depend on the type of food thermometer and the manufacturer's recommendations.
Thermometer calibration is necessary:
- When the thermometer is new
- After a thermometer has been dropped
- After a thermometer has experienced an extreme temperature change
You should also calibrate your thermometers regularly. It's best to use the manufacturer's recommended schedule, but generally speaking:
- Bimetallic thermometers, which use the properties of two different metals to detect temperature, need to be calibrated before every shift.
- Digital thermometers need to be calibrated weekly or monthly.
How Do You Calibrate a Thermometer?
You calibrate a food thermometer in two steps. First, you test the thermometer's accuracy with a known temperature, and then you'll adjust the thermometer's readout to match that temperature.
The most common methods for testing thermometer calibration in a kitchen are to use the freezing and boiling points of water.
How to Calibrate a Thermometer with the Freezing Point Method
The freezing point method, also called the ice point method, is typically the safest way to test a thermometer's calibration. It's also best for a thermometer you intend to use on cold foods.
To use the ice point method:
- Fill a container with crushed ice, then add tap water and stir.
- Let the water sit for at least two minutes to allow the water to cool.
- Immerse the thermometer probe without letting it touch the sides or bottom of the cup. The container's material may be higher or lower than the temperature of the water, which will skew your calibration.
- Wait until the reading stabilizes – if perfectly calibrated, it should read 32°F (0°C).
- If your thermometer reads more than 2°F (1°C) away from freezing, you need to adjust it. Keep the probe immersed as you recalibrate.
The method for adjusting your thermometer reading varies, so we'll talk about that in the minute.
How to Calibrate a Thermometer with the Boiling Point Method
The boiling point method can be problematic for a few reasons. First, it's more dangerous because it would be easy to burn yourself. Secondly, the boiling point of fresh water changes by atmospheric pressure and elevation.
Because of these complications, it's best to only use this method when you need to confirm that an already-calibrated thermometer remains accurate at high temperatures.
The easiest way to calculate the boiling point is just to use your elevation. At sea level, water's boiling point is 212°F (100°C). It drops 1°F for every additional 500 feet.
So, for example, if you're 1,000 feet above sea level, your boiling point will be 210°F. At a mile high (just over 5,000 feet), it will be 202°F (94°C).
To use the boiling point method:
- First, you need to know the boiling point where you are.
- Heat a pot of water to a rolling boil.
- Immerse the thermometer's probe without touching the container.
- After at least one minute, the reading should be stable – if perfectly calibrated, it will match your local boiling point.
- If your thermometer reads more than 2°F (1°C) away from your boiling point, you need to adjust it. Keep the probe immersed as you recalibrate.
How to Adjust the Calibration of Your Thermometer
As with calibration frequency, it's best to check your manufacturer's instructions for how to adjust your particular thermometer.
Bimetallic analog thermometers are usually adjusted by turning a calibration nut just beneath the display dial. You may need to use pliers or a wrench to do the job. As you turn the calibration nut, you'll see the dial reading change. Adjust until it's accurate.
The best way to learn how to calibrate a digital thermometer is by the manufacturer's instructions, but it typically involves a reset button. Some digital thermometers don't have a reset button and can only be recalibrated by the manufacturer.
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