Pyrometers and infrared thermometers
by Chris Woodford. Last updated: July 20, 2017.
If something's too hot to handle, it's no good trying to measure its temperature with an ordinary thermometer. You could try a thermocouple instead—a kind of thermometer that works by generating electricity according to how hot it gets. But what if the thing you're trying to measure is too hot or too inconveniently placed even to measure with a thermocouple? What if it's the inside of a steel furnace or pottery kiln, the roof of a cathedral, or a cloud? In that case, you can measure the temperature remotely with a handy gadget called a pyrometer (from the Greek words meaning "fire" and "measurement"). Infrared thermometers, which sample temperature remotely, are now probably the best known examples. How exactly do they work? Let's take a closer look!
Photo: Hot stuff—setting up a pyrometer experiment at NASA. Photo by courtesy of NASA Glenn Research Center (NASA-GRC).
What is a pyrometer?
You can feel a fire some distance away because it gives off heat radiation in all directions. In theory, if the fire behaves exactly according to the laws of physics, the radiation it produces is related to its temperature in a very predictable way. So if you can measure the wavelength of the radiation, you can precisely measure the temperature even if you're standing some way off. That's the theory behind a pyrometer: a very accurate kind of thermometer that measures something's temperature from the heat radiation it gives out at a safe distance (in some pyrometers) of up to 30m (100ft).
There are two basic kinds of pyrometers: optical pyrometers, where you look at a heat source through a mini-telescope and make a manual measurement, and electronic, digital pyrometers (also called infrared thermometers) that measure completely automatically. Some devices described as pyrometers actually have to be touching the hot object they're measuring. Strictly speaking, instruments like this are really just high-temperature thermometers based on thermocouples. Since they don't measure temperature at a distance, they're not really pyrometers at all.
Until microchips and compact electronic equipment became popular in the 1980s, a manual optical pyrometer was what you used if you wanted to measure the temperature of something extremely hot and relatively inaccessible, such as the inside of a steel furnace or a pottery kiln. It measured the temperature, at a safe distance, by comparing the radiation the hot object produced with the radiation produced by a hot filament (a thin wire through which electricity flows, like the wire in an old-fashioned incandescent light bulb, which glows white when it gets hot).
How does a manual pyrometer work? You look through a telescope eyepiece, through a red filter (to protect your eyes), at the object you're measuring (typically through a spyhole set into a kiln or a Tuyère in a furnace—the nozzle where air is blown in). What you see is a dull red glow from the hot object with a line of brighter light from the filament running right through it and superimposed on top. You turn a knob on the side of the pyrometer to adjust the electric current passing through the filament. This makes the filament a bit hotter or colder and alters the light it gives off. When the filament is exactly the same temperature as the hot object you're measuring, it effectively disappears because the radiation it's producing is the same color. At that point, you stop looking through the eyepiece and read the temperature off a meter. The meter is actually measuring the electric current through the filament, but it's calibrated (marked with a scale) so that it effectively converts current measurements into temperature.
Photo: A NASA scientist uses an optical pyrometer to measure the temperature of a rocket cone in a 1956 experiment. Photo by courtesy of NASA Glenn Research Center (NASA-GRC). Artwork: How it works: 1) Look through the eyepiece at the hot object; 2) Turn the thumb wheel to adjust the filament temperature; 3) As the temperature changes, the glowing red filament slowly merges into the orange background; 4) At this point, the filament is exactly as hot as the object you're measuring—and you can read its temperature off the dial.
Instruments like this are known as disappearing-filament optical pyrometers and were invented at the end of the 19th century by Everett F. Morse. Accurate and convenient, they make it easy to measure temperatures of over 3000°C (5400°F) at a safe distance. But, on the downside, they can be expensive, have to be calibrated properly, need some skill to use, and are affected by ambient (background) temperatures.
Photo: Optical pyrometers haven't changed much. Here's Everett F. Morse's original pyrometer, as he explained it in his patent for an "Apparatus for Gaging Temperatures of Heated Substances" (patent number 696916) from 1902. In this design, you look through a tube (3) at the heat source you want to measure. Using a dial (7) attached to a variable resistor (6), you adjust a light filament until it disappears against the background radiation. At that point, you read the temperature off the meter (9). Picture from US Patent #696,916: Apparatus for gaging temperatures of heated substances courtesy of US Patent and Trademark Office.
Modern digital pyrometers
Photo: Infrared thermometers (pyrometers) being used in a NASA experiment. Photo by Cesar Acosta courtesy of NASA Ames Imaging Library System (AILS).
These days, it's more common for engineers and scientists to use entirely automatic, digital pyrometers. which are quicker and simpler, and use two different types of detectors. Some measure heat by absorbing light, so they're essentially light detectors: semiconductor-based, light-sensitive photocells, a bit like tiny solar cells, but with filters fitted in front so they respond only to a certain band of visible, infrared, or ultraviolet radiation. By sampling radiation far outside the visible spectrum, detectors like this can measure a bigger range of temperatures than older, manual pyrometers. Other pyrometers use detectors that measure heat by absorbing heat, using such things as thermocouples and silicon thermopiles (collections of thermocouples) or thermistors (heat-sensitive resistors).
Photo: Taking the temperature of a ceiling-mounted ventilation system with a handheld digital pyrometer. The red spot you can see is a sighting device that helps the operator position the pyrometer precisely. Photo by Lamel J. Hinton courtesy of US Navy.
Modern pyrometers come in various types and designs. Some devices measure the full spectrum of emitted radiation, so they're called total radiation or wideband pyrometers; they tend to use heat-based detectors such as thermopiles. As their name suggests, narrow-band pyrometers capture a much smaller and more specific band of radiation, typically using photocells. Exactly which radiation band you need to sample depends on the temperatures (and materials) you're trying to measure (the hot alloys in jet engine turbines would be very different from factory cutting tools, for example, which would be different again from something like the temperatures in a chemical plant, and also different from natural things such as steaming geysers and spurting volcanoes). While some pyrometers measure a single band of radiation, others can make more accurate measurements by comparing two wavebands (in other words, two different spectral "colors"); those are called ratio pyrometers or two-color pyrometers.
Some pyrometers are designed to make quick one-off measurements, so they're shaped like pistols, with built-in detectors, signal amplifiers, power sources, and temperature meters. You point them at the object you want to measure and press the trigger.
Many industrial processes rely on constant, precise temperature measurements and control. For those sorts of applications, handheld "pistol" pyrometers aren't suitable. Instead, optical fibers (or similar light guides), permanently fitted to whatever machine or process they're monitoring, are used to gather radiation from a hot area and channel it to a remote detector, typically connected to some sort of electronic control system that automatically regulates the overall process.