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The Infrared Thermocouple
Over the past decade or two, there has been a mushroom growth in the small, application-specific designs of infrared thermocouples. These contain a sophisticated optical system and electronic circuitry that belie the simplicity of their external, tube-like appearance. They use a special proprietary design of thermopile which develops enough emf to be connected directly to a conventional thermocouple type potentiometer or transmitter for all types of indication, recording, and control.
  A wide variety of these devices are commercially available, covering temperature ranges from -50 to 5000°F (-45 to 2760°C) with up to 0.01°C precision. The range of models includes:
• Standard units, simulating the thermocouple outputs J, K, T, E, R, and S and offering 12 different fields of view from 1:2 to 100:1. Minimum spot size is 1 mm and focused spot sizes available range from 4 mm to 12 mm.
• Handheld scanning models for such applications as accurate scanning of electrical equipment and NIST traceable surface temperature calibration--a must for ISO 9001, 9002, 9003 programs.
• Thermal switches that act like photocells for use in production line quality inspection of thermal processes with line speeds of up to 1000 feet per minute.

Figure 4-4: IR Thermocouple Output

  All infrared thermocouples are self-powered, using only the incoming infrared radiation to produce an mv output signal through thermoelectric effects. The signal thus follows the rules of radiation thermal physics and produces a curve as shown in Figure 4-4.
  Over a specific, relatively narrow temperature range, the output is sufficiently linear to produce an mv output that can be closely matched to the mv vs. temperature curve of a given thermocouple type (Figure 4-4). What's more, the designer can match the two curves to be within a degree of tolerance such as ±2%, as specified by the buyer.
  Each model is specifically designed for optimum performance in the region of best linear fit with the thermocouple's mv vs. temperature curve. The sensor can be used outside that range, however, by simply calibrating the readout device appropriately. Once so calibrated, the output signal is smooth and continuous over the entire range of the thermocouple, and will maintain a 1% repeatability over the entire range.
  The user can select a model to provide, say, a 2% accuracy, by referring in the supplier's literature to a Range Chart which provides a vertical list of "Range Codes" with a corresponding Temperature Range over which 2% accuracy is to be expected. The user also specifies the type of thermocouple (J, K, etc).
  A typical infrared thermocouple comprises a solid, hermetically-sealed, fully-potted system. As such, even during severe service, it does not change either mechanically or metallurgically. It contains no active electronic components and no power source other than the thermocouple itself. Thus, suppliers rate its long-term repeatability, conservatively, at 1%.
  Long term accuracy is influenced by the same factors that affect reliability. In comparison to the application of conventional thermocouples, the infrared thermocouple is well protected inside a rigid, stainless steel housing. Along with the solid, fully-potted construction, this design essentially eliminates the classical drift problems of conventional thermocouples. Double annealing at temperatures above 212°F (100°C) helps ensure long term stability.

Installation Guidelines
Like all radiation-based sensing systems, the infrared thermocouple must be calibrated for specific surface properties of the object being measured, including amount of heat radiated from the target surface and environmental heat reflections.
  The calibration is performed by measuring the target surface temperature with a reliable independent surface-temperature probe. One such device is a handheld infrared thermometer with a built-in automatic emissivity compensation system.
  The following procedure is recommended:
  (1) Install the infrared thermocouple as close as practical to view the target to be measured.
  (2) Connect the infrared thermocouple to the supervisory controller or data acquisition system in standard fashion (including shield). As with conventional thermocouples, the red wire is always negative.
  (3) Bring the process up to normal operating temperature and use the hand-held radiation thermometer to measure the actual target temperature.
  (4) Make the proper adjustments on the readout instrument so that its calibration matches the reading of the handheld device.

  References and Further Reading
  Handbook of Temperature Measurement & Control, Omega Press, 1997.
  New Horizons in Temperature Measurement & Control, Omega Press, 1996.
  The Infrared Temperature Handbook, Omega Engineering, 1994.
  Handbook of Non-Contact Temperature Sensors, The IRt/c Book, Third Edition, Exergen Corp., 1996.
  Instrument Engineers' Handbook, Third Edition, B. Liptak, Chilton Book Co. (CRC Press), 1995.
  Process/Industrial Instruments and Controls Handbook, Fourth Edition, D. M. Considine, ed., McGraw-Hill, 1993.
  "Some Basic Concepts of Thermoelectric Pyrometry," C.C. Roberts and C.A. Vogelsang, Instrumentation, Vol. 4, No.1, 1949.
  Temperature Measurement in Engineering, H. Dean Baker, E. A. Ryder, and N. H. Baker, Omega Press, 1975.
  

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