Editing Resistance thermometer (section)

==Function==
Resistance thermometers are constructed in a number of forms and offer greater stability, [[accuracy]] and [[repeatability]] in some cases than thermocouples. While thermocouples use the [[Seebeck effect]] to generate a voltage, resistance thermometers use [[electrical resistance]] and require a power source to operate. The resistance ideally varies nearly [[linear]]ly with temperature per the [[Callendar–Van Dusen equation]].

The platinum detecting wire needs to be kept free of contamination to remain stable. A platinum wire or film is supported on a former in such a way that it gets minimal differential expansion or other strains from its former, yet is reasonably resistant to vibration. RTD assemblies made from iron or copper are also used in some applications. Commercial platinum grades exhibit a [[temperature coefficient]] of resistance 0.00385/°C (0.385%/°C) (European Fundamental Interval).<ref>{{cite web |url=http://www.instrumentationservices.net/hand-held-thermometers.php |title=Hand Held Thermometers {{!}} Charnwood Instruments |website=www.instrumentationservices.net |url-status=dead |archive-url=https://web.archive.org/web/20090305232013/http://www.instrumentationservices.net/hand-held-thermometers.php |archive-date=2009-03-05}}</ref> The sensor is usually made to have a resistance of 100&nbsp;Ω at 0&nbsp;°C. This is defined in BS EN 60751:1996 (taken from IEC 60751:1995). The American Fundamental Interval is 0.00392/°C,<ref>{{Cite web|url=http://hyperphysics.phy-astr.gsu.edu/hbase/electric/restmp.html|title=Temperature Coefficient of Resistance}}</ref> based on using a purer grade of platinum than the European standard. The American standard is from the Scientific Apparatus Manufacturers Association (SAMA), who are no longer in this standards field. As a result, the "American standard" is hardly the standard even in the US.

Lead-wire resistance can also be a factor; adopting three- and four-wire, instead of two-wire, connections can eliminate connection-lead resistance effects from measurements (see [[#Wiring configurations|below]]); three-wire connection is sufficient for most purposes and is an almost universal industrial practice. Four-wire connections are used for the most precise applications.