Editing Pressure measurement (section)

===Thermal conductivity===
Generally, as a [[real gas]] increases in density -which may indicate an increase in [[pressure]]- its ability to conduct heat increases. In this type of gauge, a wire [[Electrical filament|filament]] is heated by running current through it. A [[thermocouple]] or [[resistance thermometer]] (RTD) can then be used to measure the temperature of the filament. This temperature is dependent on the rate at which the filament loses heat to the surrounding gas, and therefore on the [[thermal conductivity]]. A common variant is the [[Pirani gauge]], which uses a single platinum filament as both the heated element and RTD. These gauges are accurate from 10<sup>&minus;3</sup>&nbsp;Torr to 10 [[Torr]], but their calibration is sensitive to the chemical composition of the gases being measured.

====Pirani (one wire)====
[[File:Pirani 02.jpg|thumb|Pirani vacuum gauge (open)]]
{{Main|Pirani gauge}}
A [[Pirani gauge]] consists of a metal wire open to the pressure being measured.  The wire is [[Joule heating|heated]] by a current flowing through it and cooled by the gas surrounding it.  If the gas pressure is reduced, the cooling effect will decrease, hence the equilibrium temperature of the wire will increase.  The [[Electrical resistance|resistance]] of the wire is a [[Electrical resistance#Temperature dependence|function of its temperature]]: by measuring the [[volt]]age across the wire and the [[Electric current|current]] flowing through it, the resistance (and so the gas pressure) can be determined. This type of gauge was invented by [[Marcello Pirani]].

====Two-wire====
In two-wire gauges, one wire coil is used as a heater, and the other is used to measure temperature due to [[convection]]. '''Thermocouple gauges''' and '''thermistor gauges''' work in this manner using a [[thermocouple]] or [[thermistor]], respectively, to measure the temperature of the heated wire.