Editing Thermometer (section)

==Precision, accuracy, and reproducibility==
[[File:1913 Car-Nation Tourer RadiatorBoyce MotoMeter.jpg|thumb|upright|left|The "''Boyce MotoMeter''" radiator cap on a 1913 [[Car-Nation]] automobile, used to measure temperature of vapor in 1910s and 1920s cars.]]

[[File:Thermometer Separated Columns.jpg|thumb|Separated columns are often a problem in both [[alcohol thermometer|alcohol]] and [[mercury thermometers]], and they can make a temperature reading inaccurate.]]
The ''precision'' or ''resolution'' of a thermometer is simply to what fraction of a degree it is possible to make a reading. For high temperature work it may only be possible to measure to the nearest 10&nbsp;°C or more. Clinical thermometers and many electronic thermometers are usually readable to 0.1&nbsp;°C. Special instruments can give readings to one thousandth of a degree.<ref>{{cite journal |last1=Yoon |first1=Howard W. |last2=Khromchenko |first2=Vladimir |last3=Eppeldauer |first3=George P. |title=Improvements in the design of thermal-infrared radiation thermometers and sensors |journal=Optics Express |date=2 May 2019 |volume=27 |issue=10 |pages=14246–14259 |doi=10.1364/OE.27.014246 |pmid=31163876 |bibcode=2019OExpr..2714246Y |s2cid=155990906 |url=https://opg.optica.org/oe/fulltext.cfm?uri=oe-27-10-14246&id=412049 |access-date=7 March 2023|doi-access=free }}</ref> However, this precision does not mean the reading is true or accurate, it only means that very small changes can be observed.

A thermometer calibrated to a known fixed point is accurate (i.e. gives a true reading) at that point. The invention of the technology to measure temperature led to the creation of [[Scale of temperature|scales of temperature]].<ref>{{Cite magazine |title=The Strange History of the Invention of the Thermometer |url=https://time.com/6053214/thermometer-history/ |access-date=2022-12-21 |magazine=Time |language=en}}</ref> In between fixed calibration points, [[interpolation]] is used, usually linear.<ref name=Benedict11/> This may give significant differences between different types of thermometer at points far away from the fixed points. For example, the expansion of mercury in a glass thermometer is slightly different from the change in resistance of a [[platinum]] resistance thermometer, so these two will disagree slightly at around 50&nbsp;°C.<ref name=Duncan>T. Duncan (1973) Advanced Physics: Materials and Mechanics (John Murray, London) {{ISBN|0-7195-2844-5}}</ref> There may be other causes due to imperfections in the instrument, e.g. in a liquid-in-glass thermometer if the [[Capillary action|capillary tube]] varies in diameter.<ref name=Duncan/>

For many purposes reproducibility is important. That is, does the same thermometer give the same reading for the same temperature (or do replacement or multiple thermometers give the same reading)? Reproducible [[temperature measurement]] means that comparisons are valid in scientific experiments and industrial processes are consistent. Thus if the same type of thermometer is calibrated in the same way its readings will be valid even if it is slightly inaccurate compared to the absolute scale.

An example of a reference thermometer used to check others to industrial standards would be a platinum [[resistance thermometer]] with a digital display to 0.1&nbsp;°C (its precision) which has been calibrated at 5 points against national standards (−18, 0, 40, 70, 100&nbsp;°C) and which is certified to an accuracy of ±0.2&nbsp;°C.<ref name=Peak>[http://www.peaksensors.co.uk/acatalog/Reference_Thermometer.html Peak Sensors] {{Webarchive|url=https://web.archive.org/web/20110921101046/http://www.peaksensors.co.uk/acatalog/Reference_Thermometer.html |date=2011-09-21 }} Reference Thermometer</ref>

According to [[British Standards]], correctly calibrated, used and maintained liquid-in-glass thermometers can achieve a measurement uncertainty of ±0.01&nbsp;°C in the range 0 to 100&nbsp;°C, and a larger uncertainty outside this range: ±0.05&nbsp;°C up to 200 or down to −40&nbsp;°C, ±0.2&nbsp;°C up to 450 or down to −80&nbsp;°C.<ref name=BS1041>BS1041-2.1:1985 Temperature Measurement- Part 2: Expansion thermometers. Section 2.1 Guide to selection and use of liquid-in-glass thermometers</ref>