| Infrared Temperature Measurement
Temperature is a commonly measured industrial process parameter. Most temperature measurements are made using contact devices like thermocouples or RTDs, but for many applications involving moving or batch-heated products, these contact devices can only be used to provide an indirect measure of product temperature, while infrared thermometers can be used to provide a direct measurement of the product’s surface temperature. In many heating operations, direct temperature measurement is a critical control parameter for the optimization of product quality and process productivity.
Infrared thermometers use an optical system to collect infrared energy from a measured target area. This energy is used to calculate the target surface temperature. The measured temperature can be displayed on a digital meter, or it can be in the form of an analog output signal that varies linearly with temperature. These temperature output signals can be input into a computer, a controller, or other device for process monitoring and control.
There are three different types of infrared thermometers that offer unique capabilities for specific application requirements. These include sensors with single wavelength, dual wavelength [ratio], and multi-wavelength designs. While there are many factors that affect the selection of a sensor, one of the most important considerations is the emissivity characteristics of the measured surface. Emissivity is a technical term used to quantify the amount of energy emitted from a surface relative to its theoretical maximum for a given temperature. In general, most applications can use a single-wavelength sensor because most materials exhibit a high and constant emissivity. However, for accurate and reliable measurements on most metal materials, where the emissivity is low or can vary, then the dual and the multi-wavelength sensors are recommended. The sensor selection guide describes these choices in more detail.
Williamson temperature sensors use a fixed optical system to 'focus' on the desired target area. Consequently, the selection of a Sensor’s Optical Resolution or Field of View (FOV) is important because it defines the measured target area that is used by the sensor to determine the object's temperature. As illustrated in the figure below, the sensor's Field of View (FOV) is a cone shaped optical pattern that is defined by the diameter of the target area (d) and the working distance (D) of the sensor to the measured target. This figure also illustrates that depending on the size of the target and the geometry of the sensor installation, the target will either fill or partially fill the measured target area of the sensor. The sensor selection guide describes the options that are available to meet the different application alignment requirements.
Sensors offer a variety of sighting capabilities that include:
The Sensor’s Temperature Range is the optimal range for the sensor's detector, filter, and optics. It can be specified in either a Fahrenheit or Celsius scale. Using the sensor’s text based menu system, it is possible to configure the temperature range to a narrower span and to convert all of the sensor’s temperature based parameters between the Fahrenheit or Celsius scales.
The Sensor’s Spectral Response defines the wavelength of the energy measured by the sensor. Each sensor's filters are defined in a narrow band that operates in an atmospheric window that eliminates distance sensitivity. For most general-purpose applications, it is recommended that the shortest wavelengths of 1 or 2 microns be used to minimize the effects of changes in emissivity. However, for some special applications like the measurement of thin plastics or measurements through flames, the spectral response selection can provide unique capabilities.
The sensor provides both analog and digital outputs in both a stand-alone and system configuration with an optional interface module. All interface module outputs can be used simultaneously. The analog outputs can be configured for either a 4-20mA or 0-20mA output and the digital outputs include both RS485 and RS232.
The PRO Series text based menu system offers access to a wide range signal conditioning and diagnostics capabilities . Specifically, they include adjustable settings for emissivity compensation, averaging time, peak hold functions, as well as options for on/off control and alarm capabilities.
To simplify and enhance the installation of the sensors, there are a variety of Options and Accessories that can be added to the sensor. This includes mounting brackets and flanges, water cooling options, air purges, and many other items.
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