Non-contact temperature sensors (Thermal Imaging)

Thermal imaging

A particularly useful application of non-contact sensor technology is thermal imaging, where a dense array of infrared radiation sensors provides a graphic display of objects in its view according to their temperatures. Each object shown on the digital display of a thermal imager is artificially colored in the display on a chromatic scale that varies with temperature, hot objects typically registering as red tones and cold objects typically registering as blue tones. Thermal imaging is very useful in the electric power distribution industry. Thermal imaging is also useful in performing “energy audits” of buildings and other heated structures, providing a means of revealing points of heat escape through walls, windows, and roofs.

In such applications, relative differences in temperature are often more important to detect than specific temperature values. “Hot spots” readily appear on a thermal imager display and may give useful data on the test subject even in the absence of accurate temperature measurement at any one spot.

A digital image taken with a thermal imaging instrument by maintenance personnel at a municipal water treatment facility shows “hot spots” on an electric motor. A color scale on the right-hand side of the image serves as a “legend” for interpreting color as temperature. In this below image, dark blue is 68.1 degree F and white is 152 degree F:

This electric motor is in a vertical orientation, with the electrical connection box in the upper-left corner and two prominent hot spots on both the near and the left-facing sides of the case. A manual valve handle appears in the foreground, silhouetted in dark blue against a lighter blue (warmer) background. A lifting “eye” on the motor case appears as a green (cooler) shape in the middle of a white (warmer) area. The two “hot spots” correspond directly to stator windings and iron pole faces inside the motor.

Thermal imaging is particularly useful for detecting hot spots on equipment unsafe to directly touch, as is the case with many “live” electrical components. This next thermal image shows an operating three-phase motor starter (contactor and overload block):

The bright spot in the center of the contactor is the higher temperature of the electromagnetic coil, providing magnetic force to actuate the contactor mechanism. The middle heater’s screws register slightly higher temperatures than the screws on either of the other two heater elements.

Large temperature differences may indicate poor electrical connections (i.e. greater resistance) at the hot spots. It is possible that the elevated temperature of this overload heater is simply due to it having a less open surface area for it to radiate heat since the two overload heaters flanking it enjoys the advantage of having more air cooling. If three people pack themselves into a narrow bench seat, the middle person is going to be warmer than either of the outer two!

Another noteworthy detail in this image is the “Spot Difference” measurement provided by the thermal imager. A cross-hair cursor on the display serves to locate a particular spot in the image, which in this case is contrasted against a reference spot chosen in an earlier step.

A thermal image of a three-phase circuit breaker shows a much more even distribution of temperature:\

The hottest objects in this image are the three load screw terminals, appearing as white/red against a blue/green background. Note the range of the temperature scale on the right of the image: This image only spans a temperature range of 61.3 degree F to 68.6 degree F. This narrow temperature range tells us the differences in temperature shown by colors in this image are nothing to worry about.

Emissivity

The main disadvantage of non-contact temperature sensors is their inaccuracy. The emissivity factor varies with the composition of a substance, but beyond that, there are several other factors (surface finish, shape, etc.) that affect the amount of radiation a sensor will receive from an object. For this reason, emissivity is not a very practical way to gauge the effectiveness of a non-contact pyrometer. Instead, a more comprehensive measure of an object’s “thermal-optical measurability” is emittance.

A perfect emitter of thermal radiation is known as a blackbody. Emittance for a blackbody is unity (1), while emittance figures for any real object is a value between 1 and 0. The only certain way to know the emittance of an object is to test that object’s thermal radiation at a known temperature.

This assumes we have the ability to measure that object’s temperature by direct contact, which of course renders void one of the major purposes of non-contact thermometry: to be able to measure an object’s temperature without having to touch it. Not all hope is lost, though: all we have to do is obtain an emittance value for that object one time, and then we may calibrate any non-contact pyrometer for that object’s particular emittance so as to measure its temperature in the future without contact.

Nevertheless, non-contact pyrometers have been and will continue to be useful in specific applications where other, contact-based temperature measurement techniques are impractical.

List of Prominent Manufacturers: Advanced Energy, Ametek, Capital Instrument, Chauvin Arnoux, Chino, Dahua, Dewalt, Dias, Durag Group, Dwyer, Ecom, Flir, Fluke, Guide Sensmart, Jenoptik, Keysight, Laserliner, Metrel, MSA, Optris, Orlaco, PCE, Pepperl+Fuchs, Rigid, Savox, Scott Bafety, Tempsens, Testo, Testboy, Thermoray, Trotec, Workswell, Xenics

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