Campbell CR800 Series - PRT Callendar-Van Dusen Coefficients

Section 7. Installation
275
Rs/R0, K, and temperature
Rs/R0 = –(R4/((R4*X3
CAL
)/(1–X3
CAL
)))*(Xp/(Xp – 1))
K = (Rs/R0)–1
T = (SQRT(d * (R/R0) + e) – a) / f (see PRT Calculation Standards for
coefficients)
or
T = g * K^4 + h * K^3 + I * K^2 + j * K (see PRT Calculation Standards for
coefficients)
Resistance of the PRT (R3):
R3 = (R4 • X3)/(1 – X3)
X3 = (X / 1000) + (R2/(R1
+ R2))
Measurement resolution:
There is a change of approximately 2 mV from the output at 40 °C to the
output at 51 °C, or 200 µV / °C. With a resolution of 0.33 µV on the ±25
mV range, this means that the temperature resolution is 0.0009 °C.
7.7.16.6 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NOTE PRT() (not PRTCalc()) is obsolete.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 CR800 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.