DAPL Operating System | Processing Command
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Module SENSORM :: RTD Convert resistance measurements to the corresponding temperature for an RTD device. Syntax RTD( RIN, VMODEL, TEMPOUT ) Parameters
Description The Though their temperature response is almost linear, RTD devices need a nonlinear conversion for full accuracy. The device model is a polynomial curve, with the number of terms required dependent on the material type used to fabricate the RTD. Devices have standardized conversion curves, but actual devices don't always match the standardized curves perfectly. For best accuracy, you can fit a polynomial curve to measured resistance and the corresponding temperatures, thus calibrating your own response curves. The following conversion characteristic for an RTD device is suitable for wide temperature ranges and a variety of RTD device types. R = R_{0} ( 1.0 + c_{1} T + c_{2} T^{2} + c_{3} T^{3} ...) The Some curve forms provided by manufacturers and standards bodies will not correspond to the general polynomial form. For these, you will need to adjust the coefficients. An important example is the platinum PT device series. It uses a conversion polynomial of the Callendar - Van Dusen form R = R_{0} ( 1.0 + a_{1} T + a_{2} T^{2} + a_{3}(T-100) T^{3} ) The appropriate conversions to the general polynomial form are: c1 = a1 c2 = a2 c3 = -100.0 a3 c4 = a3 There is an additional complication with standard
platinum RTD devices. The IEC751 standard specifies a different
polynomial for temperatures below 0 degrees C and above 0
degrees C. To allow for this, and also to allow you to calibrate
your own piecewise polynomial curves, the The following table provides coefficients that you can use for typical device types.
To summarize the layout of the device characteristic vector, the terms are specified in the following sequence.
When the operating temperature range is within interval T=0 to T=100 degrees Centigrade, and maximum accuracy is not required, the simplified "alpha coefficient" model is often used. The alpha coefficient is the slope of a straight line that matches the nonlinear conversion curve at the points T=0 and T=100. If you substitute the alpha coefficient for the c1 coefficient in the conversion data, and set all of the other coefficients to zero, the conversion is linear and fast, with maximum conversion error a fraction of a degree near the center of the range. This often is good enough. For wider temperature ranges, the conversion curves are not in the most useful form. An RTD curve yields resistance given the temperature, so calculating temperature requires evaluation of the inverse characteristic. The evaluation is almost perfect for low-order characteristics and moderate temperature ranges. For devices with significant nonlinearities at temperature extremes, the conversion error is typically better than 0.1 degrees C. Examples VECTOR vPT100POS FLOAT = ( 100.0, 3.9083e-3, -5.7750e-7, 0.0, 0.0, 0.0, 0.0 ) ... RTD(PRESIST,vPT100POS,TEMPR) A type PT100 RTD is used, but temperatures never go
below 0 degrees C. The characteristic terms for the sub-zero
piece of the standard conversion curve are omitted. Otherwise,
the conversion curve in vector VECTOR vPT100 FLOAT = ( 99.86, 3.9083e-3, -5.7750e-7, 4.1830e-10, -4.1830e-12, 0.0, 0.0, 0.000, 3.9083e-3, -5.7750e-7, 0.0, 0.0, 0.0, 0.0 ) ... RTD(PRESIST,vPT100,TEMPR) The same example, except that temperatures can range both below and above 0 degrees C. The resistance of the RTD device is calibrated by measuring accurately at 0 degrees C, and the observed value 99.86 ohms is specified as the base resistance. The first set of polynomial terms conform to the the IEC751 standard for temperatures below 0.000 degrees. For temperatures above 0.000 degrees, the conversion processing switches over to the second set of terms. See also: DIVIDER, BRIDGE |