RTDs are commonly categorized by their nominal resistance at 0 °C. Typical nominal resistance values for platinum thin-film RTDs include 100 Ω and 1000 Ω.
An approximation of the platinum RTD resistance change over temperature can be calculated by using the constant α. This constant is easily used to calculate the absolute resistance of the RTD at temperature T using equation 1.
Equation 1
Where:
RTD (T) is the resistance of the RTD element at temperature T ºC,
RTD0 is the specified resistance of the RTD element at 0ºC and,
T is the temperature of the RTD on the environment to be measured in degrees Celsius.
Figure 3. Resistance-Temperature Curve for a 100 Ω Platinum RTD, α = 0.00385
The most common RTD is the platinum thin-film with an α of 0.385%/°C and is specified per DIN EN 60751. The α value depends on the grade of platinum used, and also commonly include 0.3911%/°C and 0.3926%/°C. The α value defines the sensitivity of the metallic element, but is normally used to distinguish between resistance/temperature curves of various RTDs.
If the RTD element is excited with a current reference at a level that does not create an error due to self-heating, the accuracy can be ±4.3°C over its entire temperature range of -200°C to 800°C.
Wed, 2010-09-01 12:24
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