Williamson’s Advanced Infrared Temperature Measurement Technology
The Advantages of Infrared Pyrometers
Temperature is commonly measured in manufacturing operations to monitor and control product quality and process productivity. Many applications use contact devices like thermocouples and RTDs, but all too often these devices are inaccurate, too slow, difficult to use, or require frequent replacement, creating process downtime and reducing productivity. For many applications, infrared pyrometers are the perfect solution because they can accurately and reliably measure a target’s temperature without contact. This capability is ideal for applications involving:
- High Temperatures
- Moving or Inaccessible Targets
- Hostile or Hazardous Environments
- Fast Response Times
With the Silver, Gold, and Pro Series products, Williamson offers the optimal pyrometer for a wide range of applications.
How Infrared Pyrometers Work
Every object emits infrared energy proportional to its temperature. Hotter objects emit more energy; cooler objects emit less energy. Infrared pyrometers collect the infrared energy emitted by an object and convert it into a temperature value. The amount of energy collected by a sensor is influenced by the emissivity characteristics of the target and the transmission characteristics of any intervening optical obstructions between the sensor and the measured target. The influence of these factors varies significantly at different infrared wavelengths. Selecting a pyrometer filtered at an appropriate wavelength makes all the difference in achieving accurate readings. Emissivity is a term used to quantify a material’s tendency to emit infrared energy. It is related to the reflective and transmissive characteristics of the material and is measured on a scale of 0.0 to 1.0. In practical terms, emissivity is the opposite of reflectivity. For example, a highly reflective surface like aluminum has a low emissivity of 0.1, while a dull surface like refractory brick has a higher emissivity of 0.9. Intervening optical obstructions such as steam, water vapor, flames, or combustion gasses have the potential to interfere with the amount of energy that is measured by the sensor.
Williamson – Where Wavelength Matters
The most important Williamson difference is our particular emphasis on wavelength. By carefully selecting the wavelengths in our pyrometers, we can view through intervening optical obstructions, reduce emissivity variation and provide more stable and accurate temperature measurements. For over 60 years, Williamson has been making the most accurate pyrometers for demanding industrial applications. With a history of engineering customized solutions, our philosophy is that there is a specific pyrometer for every application, not a handful of pyrometers to fit every application.
Why Wavelength Matters
For temperature measurements in ideal laboratory settings, all that matters is the calibration accuracy of the pyrometer. However, most industrial applications involve less than ideal operating conditions with a number of interferences and factors that contribute to inaccurate readings. Thoughtful wavelength selection can dramatically reduce or even eliminate errors due to optical obstructions, emissivity variation, background reflections, and misalignment. Most pyrometer manufacturers focus on calibration accuracy, optics, and temperature range but not wavelength selection. At Williamson we emphasize thoughtful wavelength selection to ensure our pyrometers provide the most accurate temperature measurement under any operating condition.
Williamson offers 6 different infrared technologies with a variety of wavelength options, multiple optical configurations, temperature spans, and accessories to ensure that each pyrometer can be optimally configured for each application.
Errors are relatively small for moderate emissivity variation, optical obstruction, and misalignment, particularly at lower temperatures. Certain models can view through common interferences.
Low-cost pyrometers ideal for general purpose applications measuring temperatures below 100˚C / 200˚F.
Used when the target is least reflective and most opaque at a specific wavelength or when optical obstructions are most transparent at a specific wavelength.
Advanced Infrared Technologies
- Ratio pyrometers designed to compensate for emissivity variation and modest optical obstruction or misalignment.
- Ratio pyrometers designed to measure the hottest temperature viewed.
- Select wavelength sets tolerate water, steam, flames, plasma, and laser energy.
- More tolerant of scale, misalignment, and optical obstructions than Two-Color.
- Used for non-greybody materials such as aluminum, copper, stainless steel, and zinc.
- Application specific algorithms adjust for complex emissivity characteristics.
Single-wavelength pyrometers are preferred when appropriate due to simpler, lower-cost technology. For most applications, select the shortest wavelength compatible with the measurement conditions and desired temperature span. Specialty wavelengths may be necessary depending on the optical and emissivity properties of the target.
Williamson places a strong emphasis on short-wavelength single-wavelength pyrometers because of their ability to better tolerate emissivity variation and optical obstruction. As a result, these short-wavelength sensors are able to provide superior performance over a wide range of real world operating conditions.
Short-Wavelengths Reduce Error From Emissivity Variations
For most applications, selecting the shortest practical wavelength is recommended. As indicated in the chart, shorter wavelengths result in smaller errors. In fact, short-wavelength sensors can be 4-20 times less sensitive to emissivity variation compared to long-wavelength sensors.
Short-Wavelengths Can View Through Optical Obstructions
Wavelength selection is a critical factor in Williamson’s short-wavelength technology. By choosing the correct wavelength span, you can view through water, steam, flames, combustion gasses, plasmas, and other common industrial interferences.
These pyrometers tend to be lower in cost, but when measuring temperatures above 100˚C / 200˚F, errors can be large due to optical obstructions, misalignment, and emissivity variations. These are general purpose sensors used for many low-temperature or near ambient measurements and high emissivity materials.
Popular Long-Wavelength Applications
- Building Materials
- Glass Surface
- General Purpose Measurements
- Building Materials
- Glass Surface
- General Purpose Measurements
Specialty-wavelength pyrometers are used when the target is least reflective and most opaque at a specific wavelength or when optical obstructions are most transparent at a specific wavelength.
At 7.9µm, polyester film is opaque. Pyrometers filtered at this special wavelength are appropriate for making this measurement.
Ratio Pyrometer Technology
Ratio pyrometers are different from single-wavelength pyrometers in that they measure infrared energy at two wavelengths instead of one. The ratio of energy between the two measured wavelengths is then converted into a temperature value. This method of measurement allows ratio pyrometers to compensate for emissivity variation, partially filled fields of view, and optical obstructions.
Two-Color (TC) and Dual-Wavelength (DW)
There are two types of ratio pyrometer technologies, and Williamson is the only company to offer both. Two-Color technology uses a sandwich detector and a fixed set of wavelengths. Dual-Wavelength technology uses a single detector with two unique and selectable wavelengths which allows for all of the benefits of a Two-Color pyrometer plus some significant added capabilities.
- General purpose wavelength set
- Compensate for variable emissivity and modest opitcal obstruction or misalingment
- Used when there is a clear optical path between the pyrometer and target
- Measure temperatures above 1100°F / 600°C
- Ideal for scale-free and uniformly heated ferrous metals
- Carefully selected wavelength set
- Compensate for variable emissivity, temperature gradient, severe optical obstruction, and misalignment
- Wavelength set may be selected to view through water, steam, flames, plasma, etc/
- Measure temperatures above 200°F / 95°C
- Better tolerates scale, temperature gradients, and non-grey interference (20x smaller errors) due to greater separation between wavelengths
Optical Transmission Through Water by Wavelength
For applications involving water, a dual-wavelength pyrometer is ideal because it uses wavelengths that can clearly view through water. Water interferes with two color sensors due to its fixed wavelength set.
Ratio pyrometers help reduce total measurement error by automatically compensating for emissivity variation and misalignment.
With the appropriate wavelength selection, Dual-Wavelength pyrometers can eliminate interference from common industrial obstructions like steam, water, flames, combustion gases, plasma, and laser energy. Two-Color pyrometers, because of their fixed wavelength set, will still experience some error whenever these interferences are encountered.
- Steel Mills
- Casting, Forming, Joining, and Heat Treating of Metals
- Induction, Resistance, Friction, Flame, and Laser Heating
- Forging Plants: Billet, Die, Heat Treat
- Wire, Rod, and Bar Mills
- Rotary Kilns , Thermal Reactors and Solid Fuel Power Boilers
- Engineered Materials: Silicon Crystals, CVD Diamonds, Carbon Densification, High Temperature Ceramics
Certain materials can be difficult or near impossible to measure with precision using single-wavelength or ratio pyrometers because of their complex emissivity characteristics. These types of materials are called non-greybody materials and their emissivity varies with wavelength.
Typical Non-Greybody Materials Include:
- Stainless Steel
Williamson’s Multi-Wavelength Advantage
With over four decades of refinement and improvement since our first multi-wavelength pyrometer, Williamson has a long history of making accurate temperature measurements for the most challenging and difficult applications. To compensate for the unique emissivity characteristics of non-greybody materials, Williamson has developed a series of multi-wavelength pyrometers that incorporate application specific algorithms.
How Multi-Wavelength Pyrometers Work
Multi-wavelength pyrometers use application-specific algorithms to characterize infrared energy and emissivity across the measured wavelengths to accurately calculate both the actual temperature and emissivity of these complex non-greybody materials. These algorithms have been developed and refined from extensive data collected from off-line simulations and on-line trials. Each multi-wavelength sensor can hold up to eight selectable algorithms, so that the same pyrometer can be used for multiple applications.
How Does a Multi-Wavelength Compare to a Ratio and Single Wavelength Pyrometer?
A popular multi-wavelength application is for a continuous steel annealing line. As the graph below illustrates, single-wavelength and ratio pyrometers can produce significant errors as the surface emissivity varies. The multi-wavelength technology is able to accurately correct for these variations which are due to:
- Changes in alloy, surface texture, or surface oxidation
- Abnormal operating conditions such as a furnace leak, bad roll, or a reheated coil
With a multi-wavelength pyrometer, consistent and accurate readings are achieved under a wide range of operating conditions, with no sensor adjustments.
Aluminum & Copper
- Extruded Surface
- Rolled Surface
- Cast Surface
- Sheared Surface
- Forged Surface
- Brazing Operations
- Coating Preheat
- Dies and Molds
Steel & Zinc
- Cold Rolled Steel
- High Alloy Steels
- Electrical Steel
- Zinc-Coated Steel
- Shot-Blasted Pipe
- High Strength Bearings
- Motor Rotors
- Glass Molds and Plungers
- Magnesium Strip
- All other non-greybody materials previously listed
Consult With One of Williamson’s Temperature Experts
We would love to discuss your temperature measurement application with you.