When deciding how to measure the temperature of a process, one of the first questions is whether to make the measurement with a contact sensor or an optical sensor. In many applications, the answer is clear; in others, not so clear. When evaluating your process, consider the following questions and answers as well as how they apply to the specifics of your installation.
- Is it critical to measure the temperature of the object being processed? Or, is it acceptable to measure the environment instead?
If the object is being processed in a furnace operating at constant temperature for an extended period, then measuring the environment with a thermocouple works very well. If the object to be measured is heated inductively, or for some reason is not in thermal equilibrium with the environment, then the object being processed should be measured directly. A noncontact, optical sensor can provide direct measurement of the object rather than its environment.
- Is the object to be measured moving?
If the material or product to be measured is not moving, and if it can be touched, then a thermocouple likely can be used. Optical pyrometer instruments usually are the best solution when the target is rotating or otherwise moving.
- Is the surface of the part changing during processing?
Coatings and oxide layers applied during the process can affect the accuracy of optical pyrometers. Thermocouples usually are unaffected by the surface coating on the processed part and may be best for those objects. If the change is minimal and short wavelengths are used, however, the effect will be small and an optical sensor likely can be used.
- Is the object to be measured undergoing rapid changes in temperature?
Accurate contact measurements require the sensor to change temperature with the object. If the target is changing temperature rapidly, or the thermal mass of the contact sensor is too high, a contact sensor will not follow the process. In these situations, an optical pyrometer with good response may be better suited.
- Is there a bright light source in the environment emitting light at wavelengths the optical instrument will detect?
Emissions from a bright light source can cause the optical instrument to provide incorrect readings. While there are techniques for eliminating the effects of alternate light sources in the field of view of the optical sensor, thermocouples and other contact sensors will not be affected by such background light.
- Is the object being heated inductively, or are there other sources of electromagnetic interference (EMI) in the measurement area?
A direct contact thermocouple will not be affected by EMI. However, if the application is better suited to a noncontact temperature sensor, optical pyrometers can be configured in a way that eliminates EMI as a concern.
- How high is the temperature to be measured?
In general, optical techniques are better suited for use at high temperatures. Unless precious metal thermocouples are used, thermocouple measurements are more difficult and prone to drift at high temperatures.
- If an optical technique is used, will deposition, smoke or floating particulate be a problem?
Dirt, smoke and floating particulate can attenuate the measurement signal of the optical sensor. In dirty or smoke-filled environments, a contact sensor may provide a more accurate temperature reading.
- Is it necessary to measure temperature variation on the surface?
Optical techniques make scanning a surface easier.
This article is adapted from materials provided by BASF Catalysts LLC, Iselin, N.J., a global division of BASF AG, Ludwigshafen, Germany. BASF Catalysts offers contact and noncontact temperature sensors. For more information about its temperature sensors, call (732) 205-5000 or visit www.catalysts.basf.com.