Pyrometers

Last updated: September 11, 2009.

If something's too hot to handle, it's no good try to measure its temperature with an ordinary thermometer. You need 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"). What are pyrometers and how 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 radiation, you can precisely measure the temperature even if you're standing some distance away. That's the theory behind a pyrometer: a very accurate kind of thermometer that measures something's temperature from the heat radiation it gives off.

How does a pyrometer work?

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 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.

Optical pyrometers

Until microchips and compact electronic equipment became popular in the 1980s, an optical pyrometer was what you used if you wanted to measure the temperature of something hot, 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).

Photo: Measuring heat generated during space flight with an optical pyrometer. Photo by courtesy of NASA Dryden Flight Research Center (NASA-DFRC).

How does an optical 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 so that it converts current measurements into temperature.

Photo: Left: 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). Right: How it works.

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 sourced from US Patent and Trademark Office.

Infrared thermometers and digital pyrometers

These days, it's more common for scientists to use entirely automatic, digital pyrometers. They measure heat (infrared) radiation from hot objects using semiconductor-based, light-sensitive photocells (similar to tiny solar cells, but designed to respond to both visible and infrared radiation). Pyrometers like this are often 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. A laser sight switches on and fires a light beam so you can be sure you're pointing the detector at the right thing. At the same time, a heat source (such as a hot filament) built into the pyrometer fires up and starts shooting infrared radiation toward the detector chip. Meanwhile, incoming radiation passes through a lens on the front of the detector. An optical chopper (a rotating disc with holes in it driven by an electric motor) interrupts the beam dozens of times each second so the detector is alternately receiving snippets of radiation from the internal heat source and the external hot object. The detector chip can't measure absolute amounts of radiation, only differences, so it works by comparing the radiation from the two sources. By subtracting the measurements it makes of its own, known heat source from the alternating measurements it makes of the unknown heat source, it can very accurately figure out the temperature of the object you're trying to measure.

Photo: Left: Infrared thermometers (pyrometers) being used in a NASA experiment. Photo by Cesar Acosta courtesy of NASA Ames Imaging Library System (AILS).

Photo: Right: Taking the temperature of a ceiling-mounted ventilation system with a handheld digital pyrometer. The red spot you can see is a laser sight that helps the operator position the pyrometer precisely. Photo by Lamel J. Hinton courtesy of US Navy.