| Sensor Selection Guide
A rule of thumb for applying infrared thermometers suggests that 20% of the problem is picking the right sensor, and 80% is installing it correctly. As the figure below illustrates, emissivity, intervening optical obstructions (smoke, steam, dust, mist, dirty windows, etc), misalignment, and reflections are enough to threaten the results of any installation.
Single-wavelength infrared thermometers filtered in the long wavelength region of 8-14 microns are the most widely used sensors for process control. These general-purpose sensors are appropriate for measurement of non-reflective materials and relatively low temperature applications. However, applications with the following characteristics require a more advanced sensor design in order to obtain an accurate and reliable temperature measurement.
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Low-emissivity (high-reflectivity) materials, especially metals
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Variable-emissivity (variable-reflectivity) materials
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Small or moving parts
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Hostile operating conditions involving high ambient temperatures or optical obstructions such as dust, smoke, and steam
Advanced infrared thermometers can be divided into three distinct designs: short-wavelength, dual-wavelength (also known as ratio or two-color), and multi-wavelength. Optimum temperature measurement results are obtained by selecting the most appropriate sensor technology for the process application. The table below summarizes some of the application characteristics appropriate for each sensor design.
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Infrared Thermometer Selection Guide
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Technology
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Application Characteristics
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Single Wavelength
Long-Wavelength
> -50 F (-50C)
Short-Wavelength
> 125 (50°C)
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Single wavelength sensors provide an average temperature value for the target area. For most applications, a short wavelength sensor will provide the greatest accuracy.
Long Wavelength Sensors are recommended for
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Low temperature measurement of non-reflective materials with a clear optical path
Short Wavelength Sensors are recommended for
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Low temperature measurement of reflective, low-emissivity materials,
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Low or high temperature measurement of non-reflective materials
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Moderate optical obstruction from dust, steam, or dirty optics.
Custom Wavelength Sensors
Glass, plastic, through flames, coated materials
Long Wavelength (8-14um) Sensors for general purpose applications
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Dual Wavelength
>
300°F (150°C)
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Dual wavelength sensors tend to measure the hottest temperature viewed within the target area, and they provide automatic compensation for emissivity variation for grey body materials. These sensors measure both the target temperature and emissivity.
Dual-Wavelength Sensors are recommended for
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Low or varying emissivity at all temperatures,
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Viewing through optical obstructions such as dirty optics, scale, steam, dust, water, or other mechanical obstruction such as an induction coil,
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Measuring a small or wandering object, or an object with a small heated area or significant temperature gradient.
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Multi Wavelength
>
300°F (150°C)
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Multi wavelength sensors provide ‘aim and read’ capabilities for non-grey body materials. These materials are not compatible with the general purpose temperature measurement algorithms associated with traditional single- or dual-wavelength sensors. These sensors measure both the target temperature and emissivity.
Multi-Wavelength Sensors are recommended for
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Non-Grey body materials such as aluminum, brass, chrome, copper, electrical steel, molybdenum, stainless steel, super alloys, tin, titanium, tungsten, and zinc,
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Unusual emissivity properties due to coatings or contamination,
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Compensation for background interference or other irregularity.
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