Temperature affects all processes but it’s always difficult to measure. Here, Bill Earlie of electrical test and measurement specialists Cropico looks at one way of achieving greater accuracy in temperature measurement - platinum resistance thermometers.
Platinum resistance thermometers are much more accurate than thermocouples but they have some drawbacks, notably a more limited temperature range, a higher thermal mass and usually are more expensive. These resistance thermometers have a linear and repeatable resistance against temperature: The two common types in use are Pt100 which has a resistance of 100Ω at 0°C and Pt25 which has a resistance of 25Ω at 0°C.
Platinum is used because it has a stable temperature coefficient and being a noble metal is not very susceptible to contamination. Pt100 (PRT) is the most commonly used and has a temperature coefficient of ? = 0.00385 (European standard) which corresponds to an average resistance change, over the temperature range 0 to 100°C of 0.385Ω per °C.
Both the absolute resistance value and the change in resistance per °C are both relatively small and give rise to measurement problems, especially when the resistance of the connection leads are taken into consideration. There are other standards also in use – for instance, the US standard for pt100 has an alpha of 0.00392.
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