Two Types: Contact and Non-contact

Significant differences exist between different temperature sensors or temperature measurement device types. Using one perspective, how they work, they can be simply divided into two groups, contact and non-contact.

The other pages here will take you to descriptive pages on each type with a breakdown by more specific, detailed types under that first, simple classification.

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.

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.

So, it is easier to seek a solution or application that someone else has already pioneered for you. Some of them are archived here, also.

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.

Tell your new product and application story and I will consider adding it here with your byline! You can use the comment form below to contact me about your ideas.

Remember the truism that: all sensor have errors in their readings – all the time. A key secret to high quality measurement results is to have confidence in the error estimates and a high confidence that likely errors are acceptable in your case.

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.

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.

Then obtain 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.

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 temperatures sensors infer the temperature of an object from which the radiant power is collected and assumed to be emitted (some may be reflected rather than emitted).

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 object is semitransparent within the measuring wavelength passband of the sensor).

Thermal Imaging
The special world of infrared thermal Imaging (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 who use thermal imaging. It has some unique uses in other fields as well.

Some 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, nearly all non-contact temperature devices and thermal imagers work because of the same Law of Physics, called Planck’s Law.

Applications collections can lead you to many well-known solutions or examples, possibly similar to the one you are trying to solve. We have a few and will be adding them soon. So, too, do most makers of non-contact and thermal imagers.

But there are many more helpful hints and examples of successful measurements out there.

Why re-invent the wheel?
Two excellent reference by Baker et. al. are worth reading to get an idea of the complexities that can arise in attempting to make a temperature measurement. They also get into how to test for errors 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. I have not seen any recent analyses with as much detail!

This page has been edited from the original of 2009 and was published on February 27, 2019.

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