Williamson’s PRO 42, 43, and 52 Sensors use a unique auto null balancing design to enable more accurate and repeatable measurements on applications with low temperatures and varying emissivity. Typical applications that exhibit these characteristics are aluminum strip coating lines, steel strip coating lines, aluminum aging applications, low temperature induction heating operations, steel tempering applications, cold rolled steel operations, metal coating and laminating applications, as well as thermal spray and thermal expansion applications.

There are three types of infrared noncontact temperature sensors used for process monitoring and control. These are single-wavelength, dual-wavelength, and multi-wavelength pyrometers. The dual and multi-wavelength sensors are used in applications to compensate for emissivity variance of the measured target, or to compensate for any application interference such as a dirty lens or a partially filled field of view. The single-wavelength sensors are used when the measured target emissivity is relatively constant, or the dual and multi-wavelength technologies are not appropriate. Some examples of applications where single-wavelength sensors are used in place of dual or multi-wavelength sensors are when:

• The emissivity varies and the temperature is below the lower limit of 300 ° F (150 ° C) for a dual wavelength sensor.
• There is a hot background and the target is reflective which causes the dual wavelength sensor to read high when it measures the hot reflections off of the target from the background.
• The cost of a dual or multi-wavelength sensor is prohibitive.

The remainder of this section focuses on these difficult to measure single-wavelength applications where dual and multi-wavelength sensors are not appropriate. It explains the critical issues with varying emissivity at low temperatures and how Williamson's patented Auto Null Balancing design resolves these issues.

Minimizing Emissivity Error with Single Wavelength Sensors

For most applications, any single-wavelength sensor may be selected to properly measure temperatures when:

• the target emissivity, thought of as the opposite of reflectivity, is constant.
• the sensor's optical path to the target is unobstructed.

In general, the sensor wavelength selection, or spectral response, is not critical. However, in order to minimize the measurement sensitivity to changes in emissivity or surface structure, it is recommended to select the sensor with the shortest possible wavelengths for a given application. An example of an application with varying emissivity is with metals where emissivity, or reflectivity, varies with changes in surface texture, degree of oxidation, microstructure, material or alloy, and with changes of surface coating or contamination. For these difficult applications, short wavelength, single-wavelength sensors are recommended because they are less sensitive to emissivity, or reflectivity, variance.

To illustrate the benefits of short wavelength sensors, the graph below shows how a sensor filtered in the 1 or 2 micron range is 4 to 8 times less sensitive to emissivity variance than a sensor filtered at 8-14 microns. This sensitivity difference is related to the fundamental infrared energy distribution curve which shows that emissivity values are higher at shorter wavelengths. Consequently, for a given change in emissivity, these higher emissivity values reduce the percentage change in emissivity which leads to a smaller error in temperature measurements.

Most short-wavelength sensors which measure temperatures above 500 ° F (260 ° C) are filtered in the 1 micron range and use an infrared detector called a photovoltaic. Most sensors used to measure temperatures below 500 ° F (260 ° C) are filtered at longer wavelengths of 8 to 14 microns and use an infrared detector called a thermopile. Both of these types of infrared detectors are very stable with time and are easily characterized for ambient temperature drift.

However, for specific applications with varying emissivity below 500 ° F (260 ° C), the long wavelength (8-14 micron) sensors are ineffective.   For these applications the short-wavelength sensors using the photo-conductor detector (typically PbS or PbSe) must be used to minimize measurement errors due to changes in emissivity.  While these infrared detectors are naturally unstable, both with time and ambient temperature, they are extremely sensitive at short-wavelengths which makes them very effective for these low temperature applications with varying emissivity. Consequently, the issue with the photo-conductor detector is how to stabilize the long and short-term drifting which affects the sensor's calibration in an unpredictable fashion. For higher temperature applications with varying emissivity, these issues are not a problem because the short-wavelength sensors at higher temperatures use the stable photovoltaic detectors.

The most common technique employed to compensate for detector drift is a periodic calibration check utilizing an internal reference lamp. With this technique, a switch is used to activate a light bulb within the sensor housing. The infrared temperature sensor is then "calibrated" to a predetermined output level associated with the expected lamp intensity. Although this is useful for verifying the proper operation of the infrared temperature system, this technique should not be used as a means for precision calibration verification. This technique has four critical concerns which limit its effectiveness:

• The calibration verification is only performed periodically when the switch is activated.
• During the calibration verification procedure, the sensor is unable to measure temperatures.
• The calibration verification is only performed at one temperature.
• The internal light bulb is subject to aging effects which cause intensity and calibration variations.

Williamson Corporation has developed a patented Auto Null Balancing technique to compensate for the detector issues noted above. This technique utilizes very stable solid state circuitry for continuous (up to 80 times per second) self calibration of the infrared detector. This circuit emulates the energy emitted by the measured target regardless of detector sensitivity which eliminates the resultant calibration drift of the photo-conductor detector. By using this unique calibration stabilization technique, the Auto Null design eliminates the limitations of the "reference lamp" technique because:

• The Auto Null compensation is automatic and continuous.
• The sensor is able to continuously monitor process temperatures.
• The Auto Null compensation operates over the entire temperature range of the sensor.
• The Auto Null solid state reference source is inherently stable with time and ambient temperature.