A Contact or Non-Contact Approach?
Non-contact temperature sensors such as infrared pyrometers are relied upon for their unique ability to monitor temperature and dictate process control in a multitude of manufacturing processes. The non-contact approach offers an unparalleled safety element for many of these industrial processes where product (which may be moving) is heated and exposed to elevated temperatures. These conditions presents a distinct danger to workers fabricating these products.
Unlike contact methods which require thermal contact, pyrometers can be operated from a safe distance away from the process. This advantage allows for constant temperature monitoring without any need to slow down or stop the line to make a contact measurement with a device such as an RTD or thermocouple. The continuous measurement from a pyrometer ultimately yields improvements in process efficiency, product quality and safety.
Contact measurement devices however can be highly accurate. For example, thermocouples can be relied upon for general surface measurements. However, thermocouple readings can be influenced by a number of other factors such as:
- the ambient temperature of the air
- thermal inertia (starting temp of probe)
- thermal mass of the probe
- surface oxidation
- loss of heat through conduction
- the applied pressure of the thermocouple on the surface
All of these factors can cause a thermocouple to read in very large errors. In fact, two-point thermocouples are bulky and massive and often times are not adequate because the heat flux flowing from the product surface may not be sufficient to heat the probe to a temperature level equal to the product. Ribbon thermocouples have similar shortcomings as they tend to buckle and bend, and therefore they do not always contact the surface with adequate pressure.
The Non-Contact Advantage
Infrared pyrometers should be used whenever contact methods are not appropriate. Pyrometers are more accurate and reliable than thermocouples when measuring small, moving, or fragile targets in a hostile/dangerous environment. However, not all pyrometers are built equally. Pyrometers can come equipped with many different wavelength technologies. Careful consideration of wavelength will determine how accurate a certain pyrometer will be for a certain application.
Pyrometers operate by measuring the infrared energy emitted by the target and collected by the senor’s detector filtered at a specified wavelength. Hotter objects emit more energy at each wavelength of measurement and cooler objects emit less energy at each wavelength of measurement. The temperature reading is determined by comparing the measured IR energy in comparison to the theoretical amount of IR energy emitted by a perfect emitter or blackbody at the same temperature. This relationship is otherwise known as emissivity.
Emissivity can be defined as the percent of IR energy emitted by an object compared to the theoretical amount of infrared energy emitted by a perfect emitter at the same temperature. For practical purposes emissivity is the opposite of reflectivity and emissivity can vary across wavelengths. For accurate measurements with a pyrometer, the selection of an appropriate wavelength technology is paramount.
Two Images, One Difference – Wavelength
Careful wavelength selection allows a pyrometer to effectively view through any potential optical obstruction such as steam, oil, flames, plasma and more. A practical example of this would be looking at an image of a person’s chest taken with a camera and one taken with an x-ray. The image taken with a camera will show the surface of the person and whatever else can be seen with visible light (clothing, skin, etc..) whereas the x-ray will show the structures inside of their body, particularly their bones, muscles, and fat. The differences that result from the same image are dictated by the wavelength of the sensor being used where the camera’s sensor has a wavelength in the visible light spectrum and the x-ray’s wavelength is not. The same theory of operation is what allows pyrometers to effectivley view through certain optical obstructions if the correct wavelength is chosen. This is why Williamson is Where Wavelength Matters!