Tools for Environmental Health.

This article is provided by NEHA for informational purposes only. It is designed to better inform our members about topical and relevant goods and services available to the environmental health professional. Opinions expressed about any product or service in this column either expressly or implied are solely and completely those of the author/s and do not necessarily represent the views or opinions of the National Environmental Health Association.

Thermometry 

If you subscribe to the free Food-Safe listserv run by the U.S. Department of Agriculture ([less than][e-mail][greater than]), you probably notice the periodic flurry of messages that occurs every time the pros and cons of various temperature-measuring devices are mentioned. Falling between the roiling vilification and exalted praise of infrared and bimetal dial thermometers (not necessarily respectively) is a quiet acceptance of the electronic temperature-measuring device. To add to the confusion, however, a variety of terms describing operation and use always get batted about before the e-mail messages run their course. We must admit, that we, too, have lost our way once or twice in this morass of definitions. So, with the kind help of Kristin Bartholomew from Atkins Temptec and of Omega Engineering, Inc., through its Web site ([less than]www.omega.com[greater than]), we delineate below the distinction between the types of electronic temperature sensors we use most commonly in our practice.

How We Measure Temperature

The temperature-measuring devices we take with us into the field use sensors of several different types. Each sensor infers temperature by sensing some change in a physical characteristic. In our work, we routinely use six sensors: thermocouples, resistive temperature devices (RTDs and thermistors), infrared radiators, bimetallic devices, liquid expansion devices, and change-of-state devices. Since previous columns have covered infrared radiators, bimetallic devices (dial thermometers), liquid expansion (temperature standard thermometer), and the change-of-state devices (color-change paper thermometers), we are dedicating this column to the first two on the list, which are probably the most misunderstood.

Thermocouples

The temperature sensors most frequently encountered in our arsenal of tools are thermocouples. Thermocouples have two dissimilar alloy strips or wires that are joined at one end. Changes in the temperature at the juncture of these different metals induce a change in electromotive force (EMF) between the other ends of the strips or wires. Basically, thermocouples produce a direct-current (DC) output voltage that changes with a change in temperature. As the temperature goes up, the output EMF rises.

Two types of thermocouples are suitable for our work. The first is the Type K (chromel/alumel) thermocouple. Overall, this is the best general-purpose thermocouple because of its wide measurement range. It is ideal for use in high-humidity atmospheres on materials with high moisture content, as occurs in food service. The Type T (copper/constantan) thermocouple is particularly suitable for measurement of low and subzero temperatures, and in atmospheres with high moisture content. This thermocouple is ideal for use in refrigerated warehouses and freezer lockers and, surprisingly, for measuring ambient temperatures in buildings. The "type" of a thermocouple has to do with the types of metals used to manufacture the probe. If you are using a K-type probe, it will be compatible only with a K-type instrument; the same holds true for the T-type thermocouple.

Thermocouple thermometers are the most difficult to engineer. The thermocouples themselves exhibit three distinct disadvantages, for which both the design and the manufacture of the instrument must compensate. First, the temperature response for thermocouples is nonlinear across the temperature scale. Second, thermocouples may be affected by "incidental thermocouples"--the tin-lead-solder joint wire connections to the tinned-copper printed circuit boards. Because the incidental thermocouples consist of metals, they, too, respond electrically to changes in temperature. Third, the signal from a thermocouple is extremely small--in the millivolt range. The smallness of the signal allows circuit drift or a change in DC voltage to affect its accuracy. Therefore, it can be truly stated that you get what you pay for. We have found that accuracy and performance are directly proportional to cost. We have also found that the best units that meet our criteria for portable instrumentation have a total-system-accuracy (th at is, the accuracy of the probe and instrument combined) of [+ or -]0.5[degrees]C ([+ or -]0.9[degrees]F). So, if you are already spending top dollar on an accurate instrument, you may want to ensure its continued utility by sending it back to the manufacturer for annual recalibration.

Thermistors

Thermistors are part of a larger family of temperature-measuring instruments known as resistive temperature devices. The materials that make up these devices respond to temperature changes by exhibiting changes in electrical resistance. Two key types are the metallic devices (commonly referred to as RTDs) and thermistors. As their name indicates, RTDs rely on resistance change in a metal, with the resistance rising more or less linearly with temperature.

The word "thermistor" is a contraction from "thermally sensitive resistor." A thermistor is made of a temperature-sensing element composed of sintered semiconductor material that exhibits a large change in resistance proportional to a small change in temperature. Most of the thermistors we use have negative temperature coefficients; the resistance drops nonlinearly with temperature rise.

Selection Guide

Thermocouples have an excellent response time, are inexpensive, and are quite durable. For any given thermocouple type, such as the K- or T-type alloy combinations that we routinely use, the voltage output relative to temperature can be characterized and is highly repeatable. This means that a large variety of thermocouple probes can be used with the same instrument, even though the probe geometries differ.

Additionally, thermocouples can be used over a wide temperature range. This feature makes them ideal devices for evaluating temperatures encountered in food storage, preparation, and service.

Handheld, pocket thermocouple thermometers are larger and heavier than other food thermometers and are normally powered by a 9-volt transistor battery with a typical operating life of about 500 hours.

Overall, thermistors are more stable than thermocouples, but their temperature range is not as broad, nor their response time as rapid. The more restrictive temperature span of thermistors is typically between --40[degrees]C and [+ or -]150[degrees]C, which makes them poor candidates for measuring the temperatures of cooking devices such as ovens, deepfat fryers and griddle tops. At the temperature extremes encountered with these devices, the thermistors will loose their accuracy thermistors do, however, offer a high accuracy in the regulatory temperature range and therefore are ideal for use as compliance tools. They are particularly suitable for measuring food temperatures, such as the internal temperature of a cooked hamburger patty.

The thermistors we routinely use are small and lightweight; they fit easily into a shirt pocket. Typically, a 1.5-volt replaceable photoelectric battery with an average one-year operating life powers them. Because the sensor tip is a ceramic bead potted in a highthermal-conductivity epoxy, it must be immersed about one-half inch into the food to compensate for thermal conductivity down the stem. The volume of food required for measuring is about onefourth inch around the tip.

Be aware that from an engineering standpoint, thermistors are indeed resistive devices and that, accordingly, they function by passing a current through a sensor. Even though a very small current is used, it creates a certain amount of heat that may throw off the temperature reading under certain conditions, particularly in fluids that are not agitated. This problem does not arise with thermocouples, which are essentially zero-current devices.

Epilogue 

Although this discussion has been a slight departure from our usual presentation of new and unique equipment, we thought that it was appropriate--because next month, we will present a unique HACCP Monitoring Thermometer Kit that includes both thermocouple and infrared temperature-sensing devices, We simply wanted to make sure that we're all singing from the same hymnal.