When you have to measure the temperature of the untouchable and have ruled out using the thermocouple and the RTD, your best choice is usually one of the many infrared thermometers(also known asoptical pyrometers). The sensing technology is the same for hand-held as for fixed-position units that monitor continuously.
In this series, I'll look at point-and-shoot, hand-held designs that usually look like a hair-dryer or a stick that clips in your pocket.
Typical targets for infrared thermometers include:
- Moving parts on a conveyor.
- A moving web of material.
- A hot billet in a strong AC magnetic field.
- Material too hot for contact sensors.
- Ovens and furnaces.
- Vessel or process surfaces, including detection of areas of heat loss or waste.
- Outside walls seen from inside a building, to detect unwelcome invasions of cold.
A basic model has a lens to focus on the target and an infrared radiation sensor to convert the received energy into an electrical signal. From there, the severely non-linear signal is converted to a digital readout in °F or °C. You point the instrument to your target and see its temperature in a fraction of a second. Looks easy, yes?
You may think thatthermocouple and RTDapplications are tricky and confusing. Wait until you enter the minefield ofinfrared thermometerapplications.
Figure 1 shows the radiation intensity of ablack-body(emissivity equals 1) heat source, as a function of wavelength and temperature.
Emissivity IssuesYou will usually pick up less than the theoretical radiant energy from your target because the target's emissivity is almost invariably less than 1. If you pick up only half the energy, this defines the emissivity at 0.5.
Many infrared thermometers and controllers have an emissivity adjustment. Some have it set permanently at 0.95 in thehopethat your target's value is somewhere close. Hope is not a reliable parameter.
If you know for sure the target's emissivity -- and you have an adjustment -- you will turn the setting down to that value. This boosts the signal just enough to bring it up to a value corresponding to an emissivity that equals 1, which your instrument is expecting.
For many materials, you can find approximate values or a range of values in emissivity tables in physics references. Extreme examples are 0.02 for polished aluminum and 0.97 for asphalt.
Shiny metal targets suffer not only very low and wrong temperature readings due to their very low emissivity. They suffer reflection errors too. For example, a boiling chrome-plated tea-kettle, obviously at 212°F (100°C), can reflect the walls back into your lens and show wall temperature of say 70°F (~20°C). There will be little or no contribution from the hot kettle. That's not all. If some neighboring very hot body is reflected into the lens, you could see gross errors on the high side.
For transparent materials such as glass or plastic film, an instrument with a high infrared bandwidth could sense temperatures of objects behind the target or reflected from it, and give deceptive readings. In such cases, an optical filter is applied that selects a narrow band in the infrared spectrum where the material is opaque.
Handling Emissivity IssuesRather than depend on published emissivity figures for your material, it is better to measure the temperature of the target when it is stationary or accessible, using a calibrated thermocouple and indicator. Then, you can adjust the emissivity setting to bring your infrared thermometer reading into agreement. Some infrared thermometers incorporate a second range to suit a plug-in thermocouple for just this purpose.
For temperatures up to 500°F (260°C), you can stick tape with a known emissivity of 0.95 onto the target and turn the emissivity setting to 0.95. Then, when you point to the untaped surface, adjust the emissivity to give the same reading. You can do the same with matt black paint.
You can also create a black body. If you drill a hole in the material some six or more diameters deep, that dark tunnel is your black body. No adjustment to a lower than 1 setting is needed.
Dual-Wavelength ThermometersThis design put two sensors in one enclosure, each one with its own optical filter selecting a different part of the infrared spectrum radiating from a single target. The circuit derives a signal equal to theratio of the two outputs. This represents the target temperature, independently of emissivity.
This design is used when the emissivity is unpredictable; the target does not fill the field of view; or the target is partially and intermittently obscured by smoke, dust or vapor.
In my next column, I will deal with targeting and advanced features developed for hand-held infrared thermometers.