Tools for Environmental Health.

Infrared Thermometers Again: Temperature Verification, Other Useful Information, and a New Model

Ever since we first evaluated noncontact infrared thermometers two years ago, we have been listening to your apprehensions about using these units in the field. So we want to emphasize that the infrared thermometer is only a screening tool. It is not - nor will it ever be - a substitute for contact thermometers and good common sense, particularly in regulatory settings.

One of your primary concerns has been the inability to field-calibrate these units. The bad news is that the units cannot be calibrated since they are preset at the factory. The good news, however, is that you can easily verify their accuracy in the field. Here is an easy method for doing so.

Temperature Verification

You need the following materials:

* an empty aluminum beverage can (your choice of beverage),

* a can of matte-black spray paint or matte-black 3M[R] self-adhering dots,(*) and

* a temperature-standard thermometer (available through any scientific catalog).

First, prepare the aluminum can. Spray-paint at least one side of the can (and wait for the paint to dry), or apply a 3-inch dot to the side of the can (not on the seam).

We believe in two-point verification for all temperature-measuring equipment, including thermocouple probe and bimetal thermometers. Preferably, the calibration points will be at 41 [degrees] F and 140 [degrees] or 165[degrees] F. After all, these are the safe-food target temperatures and the most critical. The verification procedure is easy. Simply fill the aluminum can with refrigerated water or cold tap water, insert the temperature-standard thermometer, and allow the can and thermometer to come to equilibrium; this will take a minute or so. Next, take a reading with the infrared thermometer. You will find the spot-size-at-distance ratio (e.g., 1:6, 1:10) molded into the thermometer housing or on a permanently affixed label. This information will tell you how far from the can you should hold the infrared thermometer. For instance, if your unit has a spot-size-at-distance ratio of 1:6, holding the unit 6 inches away from the target will measure the temperature of a spot 1 inch in diameter. Therefore, to verify the temperature accuracy of your infrared unit, you must hold it close enough to the water-filled can that the spot size does not exceed the diameter of the painted side or of the black-dot applique. You can now simply compare the temperature reading on the infrared thermometer to the reading on the temperature-standard thermometer. Take a second reading using hot (tap water) or tempered boiling water. At both the hot and cold temperature verification points, the infrared thermometer should give a reading within two percent or within [+ or -] 3 [degrees] F ([+ or -] 2 [degrees] C) of the reading on the temperature-standard thermometer.

Because this test is so easy and so rapid, we strongly recommend that you verify the temperature accuracy of your infrared thermometer before going into the field. Or, if you are using the infrared thermometer for an extended time, as you might on a mass cook-chill line, verify the temperature accuracy every three hours.

By the way, since you already have the temperature-standard thermometer in place, this method also provides an opportunity to calibrate your stem thermometer and verify any other electronic one-piece or thermocouple-probe thermometers you are using.

The Black Dot Infra-Spot

A word about the advantage of using the 3M matte-black, self-adhering dots: These appliques are sold in packages that contain 10 1-inch, five 2-inch, and two 3-inch dots and are marketed under the name "Black Dot Infra-Spot Checks." The 1- and 2-inch dots are ideal for any flat surface inside temperature-holding equipment or on the exterior of food bins, pots, and hotel pans. Using a black dot allows you to make a noncontact temperature measurement at the same location each time. The dots allow for rapid, accurate readings and are well worth the cost of a few cents each.

The Raytek[R] MiniTemp[TM]

The nice folks at Raytek have let us evaluate their latest contribution to the infrared thermometer market: the MiniTemp MT4(**) noncontact thermometer. This unit is about two-thirds the size of Raytek's standard Raynger[R] series, with little loss of accuracy or utility. The MT4 has laser sighting and a 1:6 spot-size-at-distance ratio, making this unit one of the most convenient and versatile instruments for food safety evaluations that we have seen to date. A companion model, the MT2, is identical except that it does not have the laser-sighting device and is a bit less expensive - $100 versus $140 for the MT4.

The temperature detection range of the MiniTemp is 0 [degrees] to 500 [degrees] F, and accuracy is comparable to that of most infrared thermometer units: within two percent at temperatures over 30 [degrees] F and within three percent at temperatures under 30 [degrees] F - with good repeatability. The operating range is between freezing and 120 [degrees] F. Needless to say, this thermometer is not immune to thermal shock and must be "conditioned" to the ambient temperature when going from one temperature extreme to another. It's also not a good idea to keep the unit in the trunk of a car during the dog days of summer. Otherwise, the MiniTemp is quite rugged and will tolerate the usual working conditions without any problems. The belt-attachment carrying case is a plus.

Although the MiniTemp has the advantages of size and convenience, it lacks some of the features that we have grown to enjoy in the Raynger. Like its larger cousin, the MiniTemp is backlit, but not as brightly, and the display is noticeable only in very low-light conditions. Additionally, the low-battery icon comes on only when the battery is very low - we missed it the first time around. The MT series does, however, provide the following features: a hold mode when the trigger is released, easy switching from Fahrenheit to Celsius scales, and easy changing of the 9-volt alkaline or nickel-cadmium battery. In addition, the battery life (approximately 12 hours in use for the MT4 and 22 hours for the MT2) is a bit better than with the Raynger series. Overall, the MiniTemp is an excellent value, and we give it a two thumbs up.

* Black Dot Infra-Spot Checks, Model #sp-1, available from Same Day Distributing, 5082 Bolsa Avenue, #106, Huntington Beach, CA 92649. Telephone: [phone] or [phone].

** Raytek Corporation, 1201 Shaffer Road, P.O. Box 1820, Santa Cruz, CA 95061. Telephone: [phone]. Web site: <www.raytek.com>.