Using infrared thermometers

At the machine shop, it is often necessary to know the temperature of a workpiece. In this column, I’ll describe a convenient way to take this temperature.

Because of thermal expansion, dimensions of solid materials change when temperature changes. This change of dimension is material-specific, so plastics expand more than metals, for example. A piece of mild steel that measures 100 mm (3.937") at 20 degrees Celsius (68 degrees Fahrenheit) will measure 100.08 mm (3.94") at 70 C (158 F). For 6061 aluminum, a 100 mm dimension will increase 0.11 mm (0.004") over 50 C (122 F), and for 6/6 nylon the increase in dimension is 0.45 mm (0.018"). As can be seen from these numbers, if you are making precision parts, you need to know the temperature of your workpieces. And it would be great if you could read the temperature of a turning workpiece on a lathe without having to touch it.

Temperature sensor companies have come to the rescue with sensor technology that is low-cost, accurate and very convenient to use. I’m talking about pistol grip infrared thermometers.

Because infrared light is invisible to human eyes, a laser projects a beam of visible light to aid data acquisition. Image courtesy of B. Taylor

A few companies make these gadgets with various features, so you have choices. Available from industrial and tool suppliers, the thermometers are self-contained and battery-powered. They work by reading the wavelength of infrared light emitted from a target. Because infrared light is invisible to human eyes, a laser projects a beam of visible light to aid data acquisition. You hold the thermometer like a pistol, pull the trigger to send a beam of light that makes a spot of light on a surface, aim at what you want to measure and read the temperature on the readout. (See the photograph.)

This brings us to my No. 1 feature: data storage. This means that the device records the temperature when the trigger is released and holds that temperature reading on the readout for a few seconds. Think about it: Without data storage, you simultaneously are aiming the laser beam at a possibly moving target and trying to read the readout, and when you release the trigger, the readout turns off. That’s kind of iffy. With data storage, you put the spot of light where you want it to be on the target, then you release the trigger. The spot disappears, and you read the temperature on the readout of what you were aiming at when the laser beam spot turned off. With data storage, your reading is more reliable. I recommend it.

Helpful feature No. 2 is the ability to read the temperature of shiny surfaces. If you make parts that are shiny from a high level of surface finish, you will want this option. Otherwise your temperature reading will be lower than the actual temperature of your target — sometimes way lower.

Feature No. 3 to discuss is temperature range. Some units read temperature high enough for those of us who do heat treating, but you’ll pay more money for that ability. Also, the thermometers all give readings in either Celsius or Fahrenheit. I perform a lot of custom lathe work, so I keep a thermometer in the tool rack beside the lathe. This is a timesaver.