Campbell CR800 Series - Resistance Measurements - Details

Section 8. Operation
332
• position of the on-board reference thermistor in the wiring panel is not
optimal.
The absence of these design features causes significant error in the reference
junction temperature measurement.
If the CR800 must be used for thermocouple measurements, and those
measurements must be better than roughly 5 degrees in accuracy, an external
reference junction, such as a multiplexer
(p. 562), should be used. In addition, you
should carefully evaluate relevant parts of the Thermocouple Measurements
section of the CR1000 Datalogger Operator's Manual, which is available at
www.campbellsci.com/manuals.
On the other hand, the CR800 can make amazing thermocouple measurements if
you understands these limitations and work around them. Suggestions are:
• Use an external charge regulator instead of the internal one.
• Thermally insulate the enclosure and prevent air currents from changing
the wiring panel temperature
• Use only one differential measurements and multiplex these through an
AM16/32B multiplexer, terminating the thermocouples on the
multiplexer and getting the reference temperature at the wiring panel of
the well insulated multiplexer.
8.1.2.3 Resistance Measurements — Details
Related Topics:
• Resistance Measurements — Specifications
• Resistance Measurements — Overview
(p. 69)
• Resistance Measurements — Details (p. 332)
• Measurement: RTD, PRT, PT100, PT1000 (p. 258)
By supplying a precise and known voltage to a resistive-bridge circuit and
measuring the returning voltage, resistance can be calculated.
CRBasic instructions for measuring resistance include:
BrHalf() — half-bridge
BrHalf3W() — three-wire half-bridge
BrHalf4W() — four-wire half-bridge
BrFull() — four-wire full-bridge
BrFull6W() — six-wire full-bridge
Read More Available resistive-bridge completion modules are listed in
Signal Conditioners — List (p. 563).
The CR800 has five CRBasic bridge-measurement instructions. Table Resistive-
Bridge Circuits with Voltage Excitation
(p. 333) shows ideal circuits and related