This is a paper written for Infrared Thermographers, presented at the IR/INFO Conference, January 2003, in Orlando Florida, USA.
It is intact in every way, except it has been reformatted slightly to make it easier to read online. There is also a PDF version that can be downloaded from a link at the end of the article, if you wish to keep a copy for further reading.
Abstract
The R&R (Repeatability and Reproducibility) of temperature-measuring devices applies to quantitative thermal imagers as well as the most precise temperature sensors used in standards calibration laboratories. Once you understand what’s involved with R&R and how it can affect the results of your measurements, you will think about real temperature and temperature difference measurements in a new way. The links to calibration and traceability are then relatively easy steps to take. The significance of calibration of a temperature-measuring thermal imager and the likely uncertainty of results in the field begin to make real sense. A better understanding of these measurement fundamentals can help you relate measurement results and their confidence limits.
Introduction
Measurement science, called metrology by some, is a very precise discipline. It is best known for its use in National Standards labs, like NIST (National Institute of Standards and Technology) in the USA, and NRC (National Research Council) in Canada. When using measurement science, people are usually pushing the limits of their available technology to get the smallest measurement uncertainties possible.
However, just because Thermographers are not, or don’t think they are, pushing the limits of available technology when measuring temperatures, it does not imply that they should be neglecting good measurement science practices in their work.
Measurements are measurements regardless of who makes them and they have value depending upon the understanding and necessary care taken when the measurements are made.
If you report measurements, you are in the measurements business and you should understand not only your equipment, and all the lore of thermography, but also about measurement science and the use of statistics.
Actually, with software and compact computers available today, the statistics are the easy part. The hard part is deciding to follow the established practices related to good measurement science practices.
The object of this paper is to review some simple measurement science concepts and how they can be used in making thermographic temperature measurements and what needs to be reported of the data taken and the people and equipment making the measurements.
An instrument reading of temperature needs to be well understood and sometimes challenged by the person responsible for the measurement or else the value and confidence in the measured values are greatly diminished.
Confidence is, after all, one of the keys to customer satisfaction. If they are confident that you are doing your job correctly then your relationship will grow. Similarly, confidence in measurement results is a key to self-assurance; further, it can be quantified, or not, as part of the measurement practices followed.
It is also critical to realize that better measurement practices are an integral part of ISO 9000 and all modern statistical process control and maintenance reliability practices.
The quality assurance wheel is still turning, even though it doesn’t make much press. Its impact will increase rather than decrease in the future, if, for no other reason, than increasing global competitiveness.
Basic Measurement Concepts
Measurement results can never be better than the basic measurement capability of a given measurement device. It is often overstated, by implication, in reported results by having too many significant figures in the results data.
If a result is an average of say six measurements that mathematically work out to 23.33 °C and that precise value is reported, it implies that we have a measurement capability of 0.03 °C!
We may be able to see a 0.1 °C temperature value, but certainly not 0.03 °C! So, common sense when reporting result values is important and should not imply that you have more capability than is true.
As an example, typical rulers used in carpentry are graded in 1/16th inch intervals. If one claimed a measurement capability of 1/64th of an inch with such a device, far better than its minimum measurement resolution, it would of course not be believed because it is impossible to achieve.
Furthermore, anything less than 1/16th is suspect because that value appears to be the basic calibration limit of the device.
We don’t usually have rulers certified and calibrated at the 1/16th inch level, but there are indeed gauge blocks and precision gauges used by machinists that are not only certified, but carry a correction in a certificate as a function of the block’s temperature, to correct for any expansion or contraction. Typically such blocks and gauges measure to within 1/10,000th of an inch or thereabouts.
However, someone who used them would not claim measurement capability to within 1/100,000th of an inch.
What about thermal imager temperature resolution? We can see usually 1 °C or, on some units, 0.1 °C resolution.
Does that imply a measurement capability? Some manufacturers, by implication, suggest that you can, when in fact you cannot.
Most thermal imagers have, as a minimum, about a 2% accuracy specification, or something closer to about 2 or 3 °C calibration uncertainty.
Clearly, such devices are different, as measurement devices, than common rulers. They have a calibration limit that is larger than the temperature resolution capability of the device. So, what would be the minimum believable temperature resolution?
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