Campbell CR850, Operator'S Manual

Page: 258 / 566

Section 7. Installation
258
Table 50. PRTCalc() Type-Code-6 Sensor
Standard ITS-90 SPRT, alpha = 0.003926 (Reference: Minco /
Instrunet)
Constant Coefficient
a 3.9848000E-03
d -2.3480000E-06
e 1.8226630E-05
f -1.1740000E-06
g 1.600
h -2.400
i 8.800
j 2.502
7.8.18.2 Measuring PT100s (100-Ohm PRTs)
PT100s (100-ohm PRTs) are readily available. The CR800 can measure PT100s
in several configurations, each with its own advantages.
7.8.18.2.1 Self-Heating and Resolution
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.
7.8.18.2.2 PT100 in Four-Wire Half-Bridge
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 CR800
and the bridge resistors to the PRT is 500 feet.