CR1000 Manual - Campbell Scientific

Section 7. Installation
Table 50. PRTCalc() Type-Code-6 Sensor
Standard ITS-90 SPRT, alpha = 0.003926 (Reference: Minco /
Instrunet)
Constant
a
d
e
f
g
h
i
j
Coefficient
3.9848000E-03
-2.3480000E-06
1.8226630E-05
-1.1740000E-06
1.6319630E+00
-2.4709290E+00
8.8283240E+00
2.5091300E+02
7.8.18.2 Measuring PT100s (100-Ohm PRTs)
7.8.18.2.1 Self-Heating and Resolution
7.8.18.2.2 PT100 in Four-Wire Half-Bridge
PT100s (100-ohm PRTs) are readily available. The CR1000 can measure PT100s
in several configurations, each with its own advantages.
PRT measurements present a dichotomy. Excitation voltage should be maximized
to maximize the measurement resolution. Conversely, excitation voltage should
be minimized to minimize self-heating of the PRT.
If the voltage drop across the PRT is ≤ 25 mV, self-heating should be less than
0.001°C in still air. To maximize measurement resolution, optimize the excitation
voltage (Vx) such that the voltage drop spans, but does not exceed, the ±25-mV
input range.
Example Shows:
• How to measure a PRT in a four-wire half-bridge configuration
• How to compensate for long leads
Advantages:
• High accuracy with long leads
Example PRT Specifications:
• Alpha = 0.00385 (PRT Type 1)
A four-wire half-bridge, measured with BrHalf4W(), is the best configuration for
accuracy in cases where the PRT is separated from bridge resistors by a lead
length having more than a few thousandths of an ohm resistance. In this example,
the measurement range is -10 to 40°C. The length of the cable from the CR1000
and the bridge resistors to the PRT is 500 feet.
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