Types of Temperature Sensors

A large distinction can be made among temperature sensor types. From one perspective they can be simply classified into two groups, contact and non-contact. The two links below take you to descriptive pages on each type with a breakdown by more specific, detailed types. There are also vendors of each sensor type and some vendors sell more than one type. Start your search either for a specific type or go to the vendor page index and you can get to the vendors of specific types from there. Both contact and non-contact sensors require some assumptions and inferences in use to measure temperature. Many, many well-known uses of these sensors are very straightforward and few, if any, assumptions are required. Other uses require some careful analysis to determine the controlling aspects of influencing factors that can make the apparent temperature quite different from the indicated temperature.

Both contact and non-contact sensors require some assumptions and inferences in use to measure temperature. Many, many well-known uses of these sensors are very straightforward and few, if any, assumptions are required.

Remember the truism that all sensor have errors in their readings- all the time. One key secret to high quality measurement results is to have confidence in the error estimates. Neglecting to make a careful error analysis can result in error much larger than the assumed values.

It is worth noting that all competent error analyses start with the uncertainties assigned to the traceable calibration of the sensor itself. Without traceable calibration, one is forced to make assumptions. (You know what the word ass|u|me means, we hope.)

1. Thermocouples
   Thermocouples are among the easiest temperature sensors to use. They are widely applied in science and industry. They are based on the Seebeck effect that occurs in electrical conductors that experience a temperature gradient along their length.

2. Thermistors
   Thermistors are tiny bits of inexpensive semiconductor materials with highly temperature sensitive electrical resistance. They are used in many applications where they are never seen because they are buried inside something else, There are also a special group of very precise thermistors that are used as the sensors in Electronic thermometers for taking the temperature of people.

3. Liquid-In-Glass Thermometers
   The thermometer that checked your fever when you were young was a specialized version of this oldest and most familiar temperature sensor.

4. Resistance Temperature Detectors (RTDs)
   RTDs are among the most precise temperature sensors commercially used. They are based on the positive temperature coefficient of electrical resistance.

5. Filled System Thermometers
   In the USA, most home thermal cooking ovens are controlled by little temperature sensors that look like small metal tubes with bulges on the end-filled system thermometers-much like liquid-in-glass-but different.

6. Bimetallic Thermometers
   The simple mechanical sensor that works in most "old-fashioned" thermostats based on the fact that two metals expand at different rates as a function of temperature.

7. Semiconductor Temperature Sensors
   Commercial temperature sensors have been made from semiconductors for a number of years now. Working over a limited temperature range, they are simple, linear, accurate and low cost devices with many uses.

8. Labels, Crayons, Paints, Tabs (Phase Change Devices)
   What can people use to do a quick, inexpensive check on a process or experimental temperature?. Often the answer is some simple phase change device.

9. Other Temperature Sensors
   Temperature measurement occasions often seem to stretch the capabilities of existing sensors and inventive minds continue to create new and/or better ways to measure those temperatures. There's quite a list of them, the "Other" devices, already and it's sure to grow.
   

10. Radiation Thermometers
    Includes Pyrometers, Infrared Thermal Imaging Cameras (with temperature measurement capability), line-measuring thermometers (most of the time they're called line scanners-but all don't scan) and infrared radiation thermometers, or, perhaps the most-misused term, spot radiometers (Note: radiometers are calibrated in units of power, such as microwatts, watts, kilowatts, temperature measurement devices are calibrated in units of temperature). The noncontact temperature sensors with many names and many shapes, sizes, prices and capabilities are well and flourishing. Based on Planck's Law of the thermal emission of electromagnetic radiation; many industries could not produce goods as efficiently or quickly were it not for them. More recently the medical world has adopted the IR ear thermometer (it has its own set of standards) that is at heart a single waveband radiation thermometer. The majority of devices in use are single waveband thermometers (they measure a portion of the received thermal radiation in a single waveband, or portion of the infrared part of the electromagnetic spectrum). However, the number of ratio thermometers (two color pyrometers) on the market has grown considerably in the past ten years or so. Single waveband radiation thermometers are usually designed to measure the true temperature when they recieve all the radiation from an object that has an emissivity effectively of 1.0, or under blackbody conditions. This occurs most often when the devices are being calibrated, since they are calibrated under simulated blackbody conditions. The accuracy of the simulation bears much on the uncertainty of the calibration of the device. When these devices are used under effectively blackbody conditions, and their emissivity correction is set at 1.0, they can measure very accurately, indeed. Few people seem to appreciate that blackbody conditions occur regularly in many process applications, such as in portions of furnaces that are close to thermal equilibrum, such as glass melters & forehearths, steel mill soaking furnace zones or when a radiation thermometer is correctly sighted into a closed isothermal cavity, such as a miniature cavity on the end of a sapphire light pipe or quartz fiber optic.
11. Thermal Imagers
    Quantitative thermal imagers are a special sub-class of these thermal imaging devices, they measure radiation temperature distributions as well as shown a false color thermal image. They are, at heart, single waveband radiation thermometers that measure a two dimensional space instead of just radiation from a single spot. These are used so widely that they are described in more detail in a seperate section of this site that is all about thermography or thermal imaging.
12. Emissivity
    The topic of emissivity is also a broad and complex one. One cannot mention radiation thermometry without mentioning emissivity. Some fundamental understanding of it is essential to successful use and application of any temperature measuring radiation thermometer. It might be limited to just the details of one specific application; that's enough in many cases. It is not magic, it is not unknowable, otherwise all advanced thermal processes in the world would be running at lower efficiencies than they are. There are many people who underatnd the subject and can explain it. This is our part in that educational direction.We started a section on this site devoted to helping people better understand some of the basics of the subject from an applications perspective. Pardon our cynicism, but the section was initiated after this site author attended a "Seminar" on Infrared Thermometery a few years ago. The topic of emissivity came up many times and it was clear that the company representative giving the presentation had little to no understanding of the subject, unless the purpose of the talk was to confuse matters. Most people came away, we believe, with a poorer understanding of the subject at the end than at the beginning. It's sad when those apparently helping do not do their job competently.

13. Ratio Thermometers
    The ratio pyrometer, ratio thermometer or two color pyrometers (or two colour thermometers, if you prefer) are unique devices, touted imprecisely by all too many vendor marketing people as being emissivity independent when they are nothing of the sort. They measure in two seperature wavebands and internally create the ratio of signals (usually that of the shorter waveband in the numerator to avoid the complication of dividing by zero-because usually the shorter waveband signal drops out as a function of received radiation, before the longer waveband signal). The ratio of radiances in two wavebands has been shown to be a function of temperature and a function of the ratio of the spectral emissivity in the two wavebands as well (So much for the emissivity independence, guys!) When measuring objects that have an emissivity ratio of 1.0, they can have their emissivity ratio correction set to 1.0, just like a single waveband thermometer does when measuring under blackbody conditions; in this latter case one is said to be measuring under graybody (greybody) conditions.

14. Optical Pyrometers
    The old and trusty Optical Pyrometer not only refuses to go away, there's even a new version on the market. Check out our page and learn about the two USA companies that still make these devices.(Just between us: These things are really just another variation of the Planck's Law-based Radiation Thermometers described above, albeit one of the tried and accepted versions..But these darn things garnered so much fame and fans over the years that some people just won't settle for anything else. No matter that the technology can and does produce better devices, but snake-oil salesmen who can't produce better results with their new devices foster this sort of conservatism on the part of an undereducated user community.)
    
    

15. Fiber Optic Temperature Sensors
    There's enough uses and varieties of fiber optic-related temperature sensors these days to require a separate hyper-link category for them, To complicate matters a little more, there really are two groups of them contact and noncontact fiber optic thermometers. They're all covered on this one page. One of the fabulous uses for these thermometers is to actually provide a temperature limit signal for operating jet engines in flying aircraft. It's not all that new, either. Rolls-Royce engines in some European military planes have been flying for about 20 years using this technology.

16. Other Temperature Sensors
    Temperature measurement occasions often seem to stretch the capabilities of existing sensors and inventive minds continue to create new and/or better ways to measure those temperatures. There's quite a list of them, the "Other" devices, beginning with line scanners, two wavelength radiation thermometers, hybrid systems and multiwavelength pyrometers, already and it's sure to grow.
    
    
    
    
    
    
    The information of this page is presented with the permission of the copyright holder,Temperatures.com, Inc. For more information on temperature sensors, visit About Temperature Sensors their definitive Web Guide to temperature sensors and their uses. For other measurement areas, their new website and newsblog is growing in popularity as both a Web Guide and hyper-handbook on Measurement Devices and measurement across all disciplines of science and industry.