Campbell Scientific CR6 CR6 Measurement and Control System - Page 236
PRTCalc
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Section 7. Installation 236 PRT(): an obsolete instruction. It calculates temperature from RTD resistance using DIN standard 43760. It is superseded in probably all cases by PRTCalc(). PRTCalc(): calculates temperature from RTD resistance according to one of several supported standards. PRTCalc() supersedes PRT() in probably all cases. For industrial grade RTDs, the relationship between temperature and resistance is characterized by the Callendar-Van Dusen (CVD) equation. Coefficients for different sensor types are given in published standards or by the manufacturers for non-standard types. Measured temperatures are compared against the ITS-90 scale, a temperature instrumentation-calibration standard. PRTCalc() follows the principles and equations given in the US ASTM E1137-04 standard for conversion of resistance to temperature. For temperature range 0 to 650 °C, a direct solution to the CVD equation results in errors < ±0.0005 °C (caused by rounding errors in CR6 math). For the range of -200 to 0 °C, a fourthorder polynomial is used to convert resistance to temperature resulting in errors of < ±0.003 °C. These errors are only the errors in approximating the relationships between temperature and resistance given in the relevant standards. The CVD equations and the tables published from them are only an approximation to the true linearity of an RTD, but are deemed adequate for industrial use. Errors in that approximation can be several hundredths of a degree Celsius at different points in the temperature range and vary from sensor to sensor. In addition, individual sensors have errors relative to the standard, which can be up to ±0.3 °C at 0 °C with increasing errors away from 0 °C, depending on the grade of sensor. Highest accuracy is usually achieved by calibrating individual sensors over the range of use and applying corrections to the RS/RO value input to the PRTCalc() instruction (by using the calibrated value of RO) and the multiplier and offset parameters. Refer to CRBasic Editor Help for specific PRTCalc() parameter entries. The following information is presented as detail beyond what is available in CRBasic Editor Help. The general form of the Callendar-Van Dusen (CVD) equation is: R/R0=1: T = (SQRT(d * (R/R0) + e) -a) / f Depending on the code entered for parameter Type, which specifies the platinumresistance sensor type, coefficients are assigned values according to the following tables. Note Coefficients are rounded to the seventh significant digit to match the CR6 math resolution. Note Alpha is defined as: α = (R100 - R0)/(100 • R0) α = (R100/R0 - 1)/100