Temperature is the physical property most widely measured by sensing technology. There are many temperature sensors out there, but you don't want to choose a product based only on its measurement range and its price. The results will not be good. So what's your best course?
These examples of noncontact temperature sensors are available from Raytek Corp. (www.raytek.com), left, and FLIR Systems (www.flir.com)
The great dividing line in temperature sensing—and in many other sensing technologies as well—is this: Can you touch the object or the process fluid of interest or not? Now that you've answered that question as it pertains to your own application, you can move on and evaluate the various devices competing for your approval.
When Should You Use Contact Sensors?
- 1. Whenever you can make good thermal contact with the object or fluid
- 2. If the expected temperature is below ~1700°C (3400°F) or above about –40°C (–40°F)
Good thermal contact means the sensor and the object or fluid are at, or very close to, the same temperature. (This discussion pertains both to process fluids and discrete objects. For the sake of convenience, we will use the word object throughout.) This is usually the case when the sensor's dimensions and mass are small compared to what's being measured. You should maintain this physical contact by welding, soldering, clamping, gluing, or using some other reliable method of affixing the sensor to the object.
The contact-type temperature sensors shown here are by RdF Corp (www.rdfcorp.com), left, and Minco, with the sensing elements displayed below and the packagd products above (www.minco.com)
The two temperature limits are somewhat arbitrary. At 1700°C, platinum alloy thermocouples begin to lose calibration rapidly and the wires and common insulating materials may begin to soften. Specialized devices such as Type B platinum and tungsten-rhenium thermocouples can be used at higher temperatures, but only by those who are experts in high-temperature measurements. At the lower limit of –40°C you begin to encounter cryogenics issues. Many contact-type sensors work well below that temperature but, again, such applications are best left to the experts.
The most common contact temperature sensors are liquid-in-glass thermometers, thermocouples, RTDs, and thermistors. They are typically enclosed in a protective metal or ceramic sheath, called a thermowell, so that they can penetrate a process barrier and also be easily pulled out for calibration or repair without exposing the process and/or maintenance personnel to undesirable conditions. When you need to select a contact-type temperature sensor, Figure 1 should help with your initial ranking of the candidate devices.
Figure 1. Contact-Type Temperature Sensors
When Should You Use Noncontact Sensors?
First, bear in mind that even though noncontact temperature sensors are available in a myriad of styles and types, and are known by an assortment of names, they are all radiation thermometers if they operate according to Planck's law of thermal radiation. They are called radiation pyrometers, IR pyrometers, optical pyrometers, IR thermometers, thermal imagers, and so on. They can be battery-powered portables, fixed-mount, or online process-monitoring devices.
You will see that the list of application areas is a bit longer than that for contact temperature sensors because noncontact sensors do not have to be at the same temperature as the object. Choose and use this type when:
- 1. The object is moving
- 2. Contact would damage either the object or the sensor (e.g., extremely hot, corrosive, abrasive, etc.)
- 3. Contact with the object would change its temperature significantly
- 4. A large, observable area measurement is desirable
- 5. The object is too far away or very difficult to access, such as inside a special atmosphere or in outer space, (e.g., stars and galaxies)
A comparison of noncontact temperature sensors is difficult because there are only three worldwide standards that describe the properties of radiation thermometers. An ISO project is well under way that draws on most of the terms available in a German VDI/VDU recommended practices document. That said, Figure 2 might prove helpful when your application calls for noncontact temperature sensing.
Figure 2. Noncontact-Type Temperature Sensors
If you are about to bet your job on selecting a reliable temperature sensor, be sure you know what you are doing. This brief introduction to contact and noncontact temperature sensors should give you a starting point for finding the one that best suits your particular application. Depending on how much more you need to know, you could visit the manufacturers' sites on the Web or take a shortcut by clicking on www.temperatures.com and www.tempsensor.net. Some—or all—of the sources given here might help you out as well.
For Further Reading
American Society for Testing and Materials. ASTM Standards on Disc, Vol. 14.03, West Conshohocken, PA, 2005.
Idem. Manual on the Use of Thermocouples in Temperature Measurement, MNL 12, West Conshohocken, PA, 1993.
Baker, H.D. et al. Temperature Measurement in Engineering, Vol I., John Wiley & Sons, NY, NY, 1953. (An old but still good reference work, available from www.omega.com)
Idem. Vol. II. 1961. (Also old but valuable and also available from www.omega.com)
Chrzanowski, K. Noncontact Thermometry Measurement Errors, SPIE Polish Chapter, Warsaw, Poland, 2001, ISBN 83-904273-5-5.
Kaplan, H. Practical Application of Infrared Thermal Sensing and Thermal Imaging Equipment, 2nd Ed., SPIE Optical Engineering Press, Bellingham, WA, 1999.
Kerlin, T.W. Practical Thermocouple Thermometry, Instrument Society of America, Research Triangle Park, NC, 1999.
Kerlin, T.W. and R.L. Shepard. Industrial Temperature Measurement, Instrument Society of America, Research Triangle Park, NC, 1982.
Nicholas, J.V. and D.R. White. Traceable Temperatures, 2nd ed., John Wiley & Sons, London, 2001.
Richmond, J.C. and D.P. Dewitt, Eds. Applications of Radiation Thermometry, ASTM Special Technical Publication 895, American Society for Testing and Materials, Philadelphia, PA, 1985.
Taylor, B.N. and C.E. Kuyatt. Guidelines for Evaluating and Expressing the Uncertainty of NIST Meas-urement Results, NIST Technical Note 1297, 1994. (Available for download at www.nist.gov)
Verein Deutscher Ingenieur/Verband Deutscher Elektrotechniker (Germany), Guideline VDI/VDE 3511, Part 4.1, Temperature Measurement in Industry, Specification for Radiation Thermometers, available in English at www.vdi.de/vdi/presse/mitteilungen_details/index.php?ID=16341.