Dual-use thermometers let the user take both contact and noncontact temperature measurements with the same unit.
The advent of low-cost, infrared thermometers (IRTs) that can measure temperatures in foodservice environments without direct item contact has revolutionized the food safety inspection process.
Food safety trainers and health inspectors are accelerating their use of these devices to speed and expand their inspection process. That's driving chefs and foodservice staff to acquire and learn how to more effectively use this technology in helping to ensure food safety.
But while IR technology brings new temperature-measuring capabilities, they operate differently, and are not always a replacement for, traditional contact thermometers. In fact, it has become even more important for foodservice professionals to understand the technical aspects of how temperature is measured and to learn the strengths and weaknesses of both device types.
Contact vs. Non-contact
First, it's critical to know that IR thermometers only measure surface temperatures. In some situations, it is important to measure interior temperatures (e.g. to test "doneness,") and in such cases, a traditional probe thermometer should be used.
It's also very important to note that IR thermometers can't take accurate readings through panels, covers or glass. Shiny surfaces, such as stainless steel, also present certain measurement problems to IR thermometers, but these can be overcome with the right techniques.
There are still many reasons IR thermometers will come into increasing use. For one thing, they are much more accurate and easier to use than probe thermometers when surface temperature measurements are needed. Because they do not require physical contact, they can be used to quickly spot check servery holding areas, plated food, and the interior zones of heating and cooling equipment.
Also, health inspectors are paying more attention to surface temperatures because foodborne bacteria usually land on the surface of food first. When it is important to check internal temperatures as well, some of the latest IR thermometers also have built-in probes that take contact temperature measurements as well.
Tips and tricks
To get the most out of an IR thermometer, it pays to understand a bit about the technology that is involved and how it affects temperature measurement practices. Here are a few things to keep in mind.
Understand the basics of " emissivity." IR thermometers are calibrated on the assumption that they are reading the temperature of what is technically known as a "high emissivity surface." In simplified terms, that means a surface that efficiently radiates the internal temperature of an object. Almost all food falls into this category.
On the other hand, certain surfaces, especially shiny ones like stainless steel, have low emissivity. They act more like mirrors, reflecting the temperature of their surroundings, which are typically at room temperature. If you were to take an IR temperature reading of the outside of a stainless steel pot filled with boiling water, for example, you might only get a reading of about 130° F, even though water boils at 212° F.
Similar situations occur if you are taking the readings from the inside wall of a stainless steel cooler. There's a simple solution, however: give the thermometer a high-emissivity surface to read instead. You can do this by attaching a piece of masking tape to the shiny area and using that as a measurement target. Coating a shiny surface with oil will also give it high emissivity characteristics.
Watch out for "evaporative cooling" effects. Faulty readings are also common in a another kind of situation: if you try to measure the temperature of the surface of some types of boiling liquids. Consider boiling water: as it boils, the liquid evaporates. This tends to create an evaporative cooling effect at the surface of the water, where an IR thermometer reads the temperature. Try it yourself. Take a reading from the surface of boiling water; then, shut the burner off and agitate the surface of the liquid with a fork while you are taking a reading. The new reading will be much closer to the water's actual temperature. (Some liquids, like oils, do not evaporate at cooking temperatures and do not have this problem).
Knowan IR thermometer's strengths. With such exceptions noted, IR thermometers are much more effective than probe thermometers in many other instances. For example, it is often very difficult to measure the temperature of the thin oil coating in a sauteè pan; yet because oil has high-emissivity characteristics, an IR thermometer can measure its temperature accurately, helping the attentive chef avoid over-or under-heated oil.
Know a unit's accuracy and operating range. Not all IR units are created equal: in foodservice applications, you will generally want a device that is accurate within ±2° F throughout the HACCP-critical temperature range of 40° F to 140° F.
Also, when selecting a range, consider the application. Will you be using the IR thermometer primarily to scan hot or cold holding areas for quick HACCP safety checks? Will you also be using it for precise cooking temperature measurements for items such as candy, chocolate, sauces and soups? If scanning is the primary use, look for IR thermometers that are very accurate in the 40° to 140° F HACCP zone. If you also need precise temperature measurements below 400° to 500° F, be sure the IR thermometer you're considering handles that temperature range.
To measure temperatures above 500° F for pizza ovens or other very hot cooking, there are high temperature, industrial models that can and are used for this purpose.
Don't try to use IR to read "through" a glass window. While an IR thermometer's target indicator light can be transmitted through glass, the actual "read" will be of the glass surface and surroundings rather than that of the target on the other side of the glass. A handy "workaround" technique that will give you an approximate reading of a case's internal temperature is to flip open the case top or front and quickly take a reading from its inside surface, which should approximate the temperature of circulating air in the case.
Understand your instrument's "field of view." How much of a target an IR instrument takes its reading from is also important. Some units use a light emitting diode and lens to cast a flashlight-like illumination over the actual target area you are measuring. Others use a laser dot that indicates the center of the target area, but not the boundaries.
In either case, the actual, measured area becomes signifcantly larger as the distance between it and the unit increases. The LED type system offers "what you see is what you get" operation and is generally considered more user friendly, but the illumination is only visible for a fairly short distance, typically up to three feet. A laser type unit can "spot" at longer distances, but requires that you mathematically calculate the area of coverage using a formula based on the distance (the same kind of calculation can let you use the LED type at a distance). In most foodservice applications, IR thermometers work best when used at a distance of between one inch and 10 inches from their target.
Will Menchine is a Product Manager for the Raytek FoodPro and FoodPro Plus thermometer product lines.