Thermometer FAQ?

As you may be aware, my day job is with a thermometer company. It’s an important responsibility, helping restaurants in the northern hemisphere keep their food at a safe temperature.

As part of an interview for a Food Safety publication, I was asked a bunch of questions about thermometers. I only consider it fair to share those with you.

What accounts for differences in price – is it accuracy

Speed, accuracy, and durability play a huge role in the price of a probe thermometer, particularly in digital technology. There are three key technologies used in restaurants for thermal validation:
Thermocouple- (Best) uses a difference in voltage that is created when two dissimilar metals are joined and one end is heated or cooled relative to the other end. This difference in temperature creates a voltage that is well characterized and used to provide a temperature value. This technology is generally faster (3-5 seconds in some cases) and more accurate (varies by probe and manufacturer, but Thermoworks are generally +/- 0.7° F) across a broad temperature range. This is the technology typically used in a restaurant. They can be calibrated, but for the most part, they only require validation/certification in an ice bath or boiling water as they don’t normally drift from temperature accuracy when used in a restaurant/home setting. For example, the industry leading Super-Fast Thermapen® is a thermocouple. http://thermoworks.com/products/thermapen/
Thermistor- (Better) uses a semi-conductor. As the semi-conductor changes temperature, the amount of current that passes through the sensor changes as well. This is the technology you’ll typically see in entry level digital thermometers. It tends to be slower (6-15 seconds generally) and not as accurate as a thermocouple (+/-2° F). In addition, the temperature range on a thermistor is much smaller than a thermocouple, and as temperature increases or decreases dramatically, they tend to drift further from accuracy. In the food safety range this isn’t an issue, as they are generally +/-2° F between 32° and 165° F. In theory, they can be calibrated, but the technology in a thermistor generally only reads incorrectly if the sensor is damaged due to abuse. And if it’s damaged, calibration is risky, as it’s hard to verify that the probe is accurate across the typical food safety range. Therefore, it’s recommended to validate/certify that the device is accurate. They are generally cheaper than a thermocouple. Restaurants usually use these for spot checking temps, or in situations where it’s likely the probe will be damaged/lost. For example purposes, the RT600C is a great thermistor based thermometer: http://thermoworks.com/products/low_cost/rt600c.html
Bi-Metal Stem/Analog Dial (Good) – uses two dissimilar metals pressed together and molded into a spring. As the temperature of the metals change inside the temperature probe, the spring twists, causing the dial on the thermometer to turn. These devices are the cheapest of all technologies, but also take the longest to register an accurate temperature, usually 15 seconds or greater, and are generally accurate to +/- 2° F. Because of their nature, and the fact that there are moving parts, they require calibration, as the spring may change its orientation in the temperature probe. Although they are used in some restaurants, the time to an accurate temperature, and the frequency with which they go out of calibration makes them less desirable than their digital counterparts.

Do you have any advice for how consumers should decide their price point? (type of meat or poultry they typically cook, how often they cook meat or poultry, whether they’re at high-risk for foodborne illness complications, etc.

I recommend a thermistor or thermocouple thermometer with a reduced tip probe like the Thermapen, or RT600C. This gives the consumer the most flexibility. A bi-metal stem is too thick to accurately temp the inside of thinner meats, such as hamburgers and chicken cutlets. You’ll generally pay more for a faster response time. Be careful with the term “Instant Read,” as some thermometer manufacturers use that term to describe the frequency with which the display is updated, but not necessarily the speed that the thermometer display reflects an accurate temperature.

Are digital or analogue thermometers more accurate than the other?

Most low cost digital (>$30) will have accuracy similar to an analog. This accuracy is generally +/-2° F, but in some cases may be up to 0.9°F. Higher end digital thermometers will be much more accurate than an analog. Digital thermometers also tend to be more stable over the long term because there are fewer parts to wear. In fact, some models have no moving parts.

How deep does the thermometer have to go to get an accurate reading?
The goal is to get the thermometer sensor into the thickest part of the food item being measured. This depends on the thermometer being used. Thermocouple probes usually have the sensor on the very end of the probe. Thermistor sensors are slightly larger, and are usually 1/8th of an inch away from the end of the probe tip. Bi-metal stem thermometers may start near the probe tip, but the coil may stretch up to an inch away from the probe tip.

Should you be aiming for the middle of the meat? Let’s say I have a chicken breast that’s about two inches thick – should I pierce it through to the pan or only about an inch deep?

When taking a temp of cooked food, the goal is to get the lowest possible temperature reading in the food. That way you can validate that the coldest part of the food is still at a food safe temperature. For chicken breast, I recommend piercing the side of the breast near the thickest part of the meat.

Is it important to wash thermometers? Before you re-insert into the same meat and between meals?

YES. The probe should be cleansed/sanitized after each reading of food that is considered TCS (TCS = food that needs time and temperature control to prevent the growth of microorganisms and/or toxins).

Is there anything else that’s particularly important to remember when cooking?

Have fun, be safe. Verify that your food is cooked to a temperature in accordance with FDA recommendations (http://www.fda.gov/downloads/Food/ResourcesForYou/Consumers/UCM257049.pdf)

Do all thermometers need to be calibrated?

Bi-metal stem/analog thermometers should be calibrated. Digital thermometers should be tested to verify that the temperature reading is accurate. One of the reason thermometers from Thermoworks rarely need recalibration is that they are made from high-quality thermocouple materials and thermistors. These components can remain stable for years when used at cooking temperatures. It should be noted that some digital thermometers do require recalibration.

Why is calibration necessary?

With bi-metal stems, it’s possible that the mechanical works holding the probe and dial arm in place may shift, particularly as the metal housing expands and contracts as it is exposed to temperature variation. With digital thermometers, it’s important to validate that the sensor probe hasn’t been damaged, and therefore still within appropriate specifications.

How would a consumer calibrate their thermometer at home?

The best way to calibrate is by using an ice bath: http://thermoworks.com/learning/thermapen101_creating_an_icebath.html

How often do they need to calibrated?

Most restaurants validate their thermometers daily, and the FDA recommends they be calibrated “at a frequency to ensure accuracy” (2013 FDA Food Code Guide 3-B, section 36). The recommendation for home users is less specific, it’s good practice to check and verify that the thermometer is within accuracy specs at least once per month, prior to a big cooking event (Thanksgiving, for example), or if the thermometer had been subjected to an over temperature condition or dropped.

When should consumers replace their thermometer?

If the thermometer cannot be made to read within +/-2° F of the correct temperature, it is time to replace it.

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