and Kathy Klein, Temperature Products Product Manager,
(as seen in American Laboratory magazine, November 2000).
In the past, surface temperature measurement has been a slow and involved process. To take a temperature measurement, a probe needed to be held in contact with the objects surface. What if the object was too hot to approach? What if the surface was too far away, too small for a probe to be inserted, or moving? How is it possible to monitor surface temperature continuously? These problems have been solved with advancements in temperature measurement utilizing infrared (IR) technology.
IR thermometers vary in shape, size, and function. However, all IR thermometers provide several advantages over previous surface temperature measurement techniques. These advantages include noncontact measurement at varying working distances, high accuracy, a wide measuring range, and a fast response time.
To understand the benefits of IR thermometers, it is important to understand how they function. All objects emit IR energy. The hotter an object is, the more active its molecules are, and the more IR energy it emits. Optics located inside an IR thermometer collect the infrared energy emitted by an object and focus the energy onto a detector. The detector then converts the energy into an electrical signal, which is amplified and displayed as a temperature reading (Figure 1).
The greatest advantage of IR thermometers is the ability to take temperature measurements of hot, hazardous, or hard-to-reach objects without contact (Figure 2). With standard IR meters, measurements can be taken from within a few inches to approx. 10 ft away from the object. IR thermometers are usually available with lasers, which are used to help the user define the area they are measuring. Units with Class II lasers use less than 1 mW of power and can take measurements up to 50 ft from the object. Units with Class IIIa lasers use less than 5 mW of power and can take measurements up to 100 ft away. Most IR thermometers are limited to a measuring distance of approx. 100 ft due to atmospheric considerations. However, even with their limitations, IR thermometers still surpass standard thermometers regarding the distance from the object that is necessary for temperature measurement.
IR thermometers come in multiple styles ranging from portable, handheld instruments to permanently mounted in-line units. Handheld meters are usually gun-shaped with triggers, making them easy to handle and aim. In addition, in-line sensors can be used for continuous monitoring such as on a conveyer belt for quality control. These sensors are available with several output options such as a 4-20 mA or voltage output (10 mV/°C). Sensors with a thermocouple (type J or K) can be used with standard thermocouple meters that a user may already have.
IR thermometers are highly accurate compared to other temperature measurement methods. Most IR meters operate in an accuracy range of ±1.0-3.0% of reading, while thermocouple probes have an accuracy of ±1.8-7.9 °F, plus the uncertainty of the meter to determine overall system accuracy.
Another common feature of an IR thermometer is a large temperature range. With thermocouple thermometers, measuring range is limited by the type (J, K, T, etc.) of meter and probe being used. For example, a standard type T meter has a measuring range up to 400°C. If higher temperatures need to be measured, the user would have to switch to a type J or K meter and probe. With IR thermometers, basic units can measure up to 538°C, while specialized IR meters can take readings up to 3000°C (Figure 3).
IR thermometers also reduce the long response times typical of meter/probe systems. It is not uncommon to have to wait 30 sec or more when taking a surface measurement with a conventional surface probe. Basic IR thermometers feature response times as low as a half a second. Users simply aim and shoot to receive almost instantaneous readings.
Although there are numerous benefits of IR temperature measurement, there are also a few disadvantages that need to be mentioned. It is difficult to take temperature measurements of reflective surfaces using IR thermometers. They also pick up both emitted and reflected energy unless they are adjusted to read emitted energy only. The emissivity of certain materials can be looked up in published tables and adjusted on some meters to help ensure accurate measurements. In addition, measuring an object through glass generally gives the surface temperature of the glass, unless the glass is made of a special IR transmitting material such as germanium.
IR thermometers offer flexibility, ease of use, and improved performance over conventional probe and meter systems. There are IR thermometers available to fit just about any application. With specialized meters for applications ranging from measuring thin-film plastics to monitoring temperatures in hazardous locations, IR technology is the ideal solution for surface temperature measurement challenges.