Application and Technical Data (cont.)
Temperature Coefficient (Alpha)Temperature coefficient, or Alpha, is the term given to the average resistance/temperature relationship of an RTD over the temperature span of 0100°C and is expressed as ohm/ohm/0°C. The formula for determining Alpha is:
Typical Temperature Coefficients are:
Note: SAMA Standard RC2141966, has a temperature coefficient of .003923. The SAMA 100 ohm (Nominal) element only has a resistance of 98.129 ohms at 0°C. This element is in common use and should not be confused with the more commonly used elements having a 100 ohm resistance at 0°C.
Platinum is by far the most commonly used material in RTD probes. The other materials are used where the higher resistance change or the nonlinearity of their curves are advantageous. It becomes obvious that since Alpha is a value developed using the variables of R(0°C) & R(100°C) that the Alphas noted above are not absolutes.They do however commonly serve as adequate specifications for the standard elements. Alpha serves as an integral component in developing resistance versus temperature tables. The R/T table for platinum sensors published at the back of this catalog were developed using the CallendarVan Dusen equation which corrects for the departure from linearity at temperatures other than 0100°C which is stated by Alpha. CallendarVan Dusen Equation Where: α = Constant (see formula above) R_{T} = R_{O} (1 + AT + BT^{2}) R_{T} = Resistance at temperature T Accuracy  InterchangeabilityThe terms accuracy and interchangeability are used jointly when considering the accuracy of an RTD. The factors affecting the accuracy of an RTD measurement excluding the accuracy of the readout instrument, is the relationship of the "as built" sensor to the resistance vs. temperature curve when compared to the assumed curve (the resistance vs. temperature table), and any aging or other environment effects on the sensor. It is impossible to manufacture on a production basis sensors that will adhere to the calculated value of their resistance vs. temperature tables. There are three terms to be considered to understand why. They are: Proportional Error  Adjustment Error  Intrinsic Error 
Note: Not all sensors are usable at all the temperatures shown. The tolerances are applicable up to the maximum temperature of a given sensor.
