Campbell CR3000 Micrologger, Руководство оператора

Страница: 275 / 644

Section 7. Installation
275
PRT Measurement Circuit Overview
Configuration Features Note
•
Voltage Excitation
Four-wire half-bridge (p. 277)
•
High accuracy over long leads
•
More input terminals: four per sensor
•
Slower: four differential sub
measurements per measurement
Best voltage excitation
configuration
Three-wire half-bridge (p. 281)
•
Good accuracy over long leads.
•
Fewer input terminals: two per sensor
•
Faster: two single-ended sub
measurements per measurement
Costs less to build
Four-wire full-bridge (p. 285)
•
High resolution response to change
•
More complicated to build
•
Two input terminals per sensor
•
Two differential sub measurements
per measurement
•
Best over short leads.
•
Best resolution since the
bridge balances at the
temperature-range midpoint.
•
Current excitation
Basic four-wire circuit
•
Minimizes errors due to voltage drop
with long leads.
•
Uses off-the-shelf PRTs
•
More tolerant of lower-grade wiring
•
Two input terminals per sensor
Simplest to use
Full-bridge circuit
•
Minimizes errors due to voltage drop
over long leads.
•
More tolerant of lower-grade wiring
•
Two input terminals per sensor
Best overall
7.7.17.2 General Procedure (PRT)
Following is a general procedure for using a PRT:
1. Build circuit.
2. Wire circuit to the CR3000.
3. Calculate excitation voltage or current.
4. Calibrate PRT.
5. Measure PRT and convert output to temperature.
Several procedures follow that step you through use of common resistive-bridge
configurations to measure
a 100 Ω PRT (a.k.a, PT100). Use the following data to
help you understand the examples:
Procedure Data
•
Units used in examples: mV (millivolts), mA (milliamperes), and mΩ
(milliohms)