Big differences exist between different temperature sensor or temperature measurement device types. Using 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 under that simple, first breakout.
There are also vendors of each sensor type, some vendors sell more than one type and some sell nearly all types, but not always all brands. There are differences between brands and the differences are most evident among those device types for which there are few if any recognized standards. Start your search either for a specific temperature measurement device type or go to the vendor page index and you can access 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.
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.)
Without traceable measurements, the numerical values of results will always be questionable and hardly worth the effort, and cost. It most often pays to get started on the right path to technically sound measurements by beginning with some understanding of the options involved in selecting a temperature measurement device and then in obtaining one that meets the expected conditions and standards, is calibrated and that the calibration is traceable to either a fundamental standard (e.g. the triple point of water) or a national standard. See our calibration and standards pages for more details on each aspect of sound measurement practice.

Contact Sensors

Contact temperature sensors measure their own temperature.
One infers the temperature of the object to which the sensor is in contact by assuming or knowing that the two are in thermal equilibrium, that is, there is no heat flow between them.

Non contact Sensors

Most commercial and scientific non contact temperature sensors measure the thermal radiant power of the Infrared or Optical radiation that they receive from a known or calculated area on its surface, or a known or calculated volume within it (in those cases where the obect is semitransparent within the measuring wavelength passbad of the sensor).
One then infers the temperature of an object from which the radiant power is assumed to be emitted (some may be reflected rather than emitted). Sometimes the inference requires a correction for the spectral emissivity (NB: the two words, spectral & emissivity, are used together in correcting IR Thermometer readings -the “emissivity”, unspecified, is a big trap which even some of the suppliers of devices and calibration equipment fall into unwittingly for a variety of reason about which one can only speculate ) of the object being measured.
Knowing how and when to apply a spectral emissivity correction is part of the inference, too, and can introduce significant errors if not done correctly. See our Trip down the Emissivity Trail to help you understand that aspect a little better.

Dew Point Temperature

— Humidity–

Although this area is in reality just an application of temperature sensors and other sensors, it grew out of temperature measurements.
Remember the old style humidity indicators that consisted of two little glass thermometers, the wet and dry bulb thermometers with a little look up table that told you the humidity, both absolute and relative? Have a look, it’s a very important area in terms of human comfort, food safety and energy conservation and efficiency in thermal processes.

Thermal Imaging

The special world of thermography and thermal images includes the temperature-measuring kind of thermal imagers called “Radiomatic”, by those in the business, and “Quantitative” by those mostly in R&D with thermal imaging. Then, too, there are those who call it “Thermology” when it applies to measurements made on the human body and “Medical Thermography” by still others, some even in the same business.
Users of infrared thermal imaging have many options in cameras both with and without temperature scales or temperature indication.
It seems really odd to have all these different names kicking about, when they all refer to the same basic technology. The names seem to differ only by application area. In reality, they all work because of the same Law of Physics, called Planck’s Law.
That’s the same law that describes how IR thermometers, optical pyrometers, radiation thermometers and infrared intrusion or people detectors work (note the common trait of multiple names).
The only thing that an IR thermal imager of any denomination really does is take the output from an infrared detector, or plethora of detectors, and presents a 2-D scan of the infrared intensity distribution in the field of view of an optical system. These devices could be called by one common name. The devices that provide temperature information, probably more than any other type of device should be called Infrared Imagers, or Thermal Infrared Imagers or, simply, Thermal Imagers.
Go to our thermal imaging section by clicking the above underlined link and learn more than you ever thought you would want to know about the subject.
The Applications page can lead you to many well-known solutions or examples, possibly similar to the one you are trying to solve. Why re-invent the wheel?
Two excellent reference by Baker et al. are listed in the References page and worth reading to get an idea of the complexities that can arise, how to test and get around them. They are older books and while the technology of the equipment has changed, especially the electronics, the measurement fundamentals have not. Heat flow is heat flow and thermal radiation physics was unified theoretically by Max Planck more than 100 years ago!
A great many temperature measurement problems are solved through a good understanding of the heat flow involved in a specific measurement situation.
Surface temperature problems with contact sensors are often best solved in many cases through the use of non-contact sensor. They are in use in many industrial plants worldwide in great numbers. The above reference texts provide interesting analyses of the likely errors making contact temperature measurements of surfaces, both stationary and moving surfaces. We have not seen any recent analyses with as much detail!
Original Source: