Campbell CR3000 Micrologger, Руководство оператора

Страница: 300 / 644

Section 7. Installation
300
7.7.17.8 PRT Callendar-Van Dusen Coefficients
As shown in the preceding PRT measurement examples, use the PRTCalc()
instruction in the CRBasic program to process PRT resistance measurements.
NOTE PRT() (not PRTCalc()) is obsolete.
PRTCalc() uses the following inverse Callendar-Van Dusen equations to
calculate temperature from resistance.
For temperatures <0 °C:
T = g • K • j + K
2
• i + K
3
• h + K
4
, where K = R
S
/R
0
– 1 (Eq. 1)
For temperatures ≥0 °C:
T =
(sqrt(d • R
S
/R
0
+ e) – a) / f (Eq. 2)
Eq.1 conforms to US ASTM E1137-04 standard for conversion of resistance to
temperature. For temperatures 0 to 650 °C, it introduces <±0.0005 °C error to
the measurement. The source of the error is rounding errors in CR3000 math.
Eq. 2 is derived from US ASTM E1137-04 and conforms to other industry
standards. For temperatures –200 to 0 °C, it introduces < ±0.003 °C error to the
measurement.
Eq. 1 and Eq. 2 yield approximations of the true linearity of a PRT. The
approximation error can be as high as several hundredths of a degree Celsius at
different points in the temperature range, and it varies from sensor to sensor.
Individual sensors also have errors relative to the ASTM E1137-04 standard.
These errors can be as much as ±0.3 °C at 0 °C and increasing away from 0 °C.
Purchasing high quality PRTs will minimize this error.
The best accuracy comes from calibrated sensors over the range of use.
Calibration factors are applied to one or more of the following PRTCalc()
parameters:
•
Source
•
Multiplier
•
Offset
See the calibration sections in the previous PRT procedures for more information.
The following tables show sets of a, d, e, f, g, h, i, and j coefficients that are used
in the Eqs. 1 and 2, depending on the PRTType code entered in PRTCalc().
Coefficients are rounded to the seventh significant digit to match CR3000 math
resolution.