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Myth or Reality? My infrared thermometer isn't as accurate as a contact thermometer would be.

My infrared thermometer isn't as accurate as a contact thermometer would be.

Actually, infrared measurement can be very accurate. If you're getting inaccurate measurements, one of two things may be occurring.

1. You may be measuring from too far away. Infrared thermometers don't actually measure at the point indicated by the laser, they measure everything within a circle, surrounding that point.

The farther away you are from the target, the larger that circle gets. If the circle gets too big, then there will probably be too many different individual temperatures within it for the thermometer to make sense of. The more accurate the thermometer, the farther away you can be and still measure only a small circle. This is called distance-to-spot. At a 50-to-1 distance to spot ratio, the Fluke 568 thermometer measures only a 1 inch circle from 50 inches away.

2. If you measure a reflective surface, you may have to adjust the emissivity setting on your infrared thermometer. Emissivity describes how well an object emits infrared energy, or heat. This effects how well an infrared tool can accurately measure the object's surface temperature. Different materials emit infrared energy in different ways. Every object and material has a specific emissivity that is rated on a scale of 0 to 1.0. For infrared tools to report accurate temperatures, the higher emissivity, the better.

Objects that have high emissivity emit heat energy well and are not usually very reflective. Materials that have low emissivity are usually fairly reflective and do not emit heat energy well. This can cause confusion and incorrect analysis of the situation if you are not careful. An infrared thermometer can only accurately calculate the surface temperature of an object if the emissivity of the material is relatively high, and the emissivity level on the thermometer is set close to the emissivity of the object.

Fluke 568 thermometer

Most painted objects have a high emissivity of about 0.90 to 0.98. Ceramic, rubber, and most electrical tape and conductor insulation have relatively high emissivities as well. This makes them ideal for infrared inspection.

Aluminum bus, however, is very reflective, and so are copper and some kinds of stainless steel. These materials are difficult to get accurate temperature readings on, even when you do adjust the emissivity setting on your thermometer.


Because of conduction, heat will travel between objects that are in contact with each other, from the source of the heat outward. Even though you may not be able to get an accurate temperature reading on an aluminum lug or bare conductor, you can usually get a good reading on the insulation around the conductor. Because you know that they are touching, and that the insulation is on the outside, you can confidently say that the inside of the conductor is probably much hotter that the surface temperature that you have found on the insulation. You can also look for something called a "cavity emitter." This is a small hole or crevice (like a lug hole) that acts like a tiny thermal oven, and effectively increases the emissivity of even poor emitting materials. If you take temperature readings from cavity emitters, you get a much more accurate reading than on a flat, reflective part of a low emissivity material.

Another option is to install high-emissivity "targets" on such components as bus bars, tubular bus, any large metal electrical connectors, and most unpainted metals. This will dramatically improve measurement reliability. While there are no standards for how to create such targets, they must be installed while the equipment is de-energized, and they must be able to withstand high operating temperatures. Many plants have reported good success using flat spray paint, especially brands designed for electronic components, and electrical tape. Be careful not to use combustible materials such as black paper or plastic tape.