CR300-Series Datalogger
30
TABLE 6-1. Resistive-Bridge Circuits with Voltage Excitation
Resistive-Bridge Type and
Circuit Diagram
CRBasic Instruction and
Fundamental Relationship
Six-Wire Full-Bridge
1
CRBasic Instruction: BrFull6W()
Fundamental Relationship
2
:
= 1000
= 1000
+
+
1
Key: V
x
= excitation voltage; V
1
, V
2
= sensor return voltages; R
f
= “fixed,” “bridge,” or “completion” resistor; R
s
=
“variable” or “sensing” resistor.
2
Where X = result of the CRBasic bridge measurement instruction with a multiplier of 1 and an offset of 0.
3
Campbell Scientific offers resistive-bridge terminal input modules to facilitate this measurement.
6.1.2.1 Voltage Excitation
VX terminals supply precise voltage in the range of 150 to 5000 mV. The two
VX terminals source up to 50 mA total when used concurrently, or 50 mA each
when used independently.
The CR300 is not capable of applying reverse excitation. Set the
RevEx parameter of all bridge measurement instructions to False.
6.1.2.2 Ratiometric Accuracy
See also: Section 7.2, Resistance Measurements – Specifications
(p. 44).
The accuracy specifications for ratiometric-resistance measurements are
summarized below:
0 to 40 °C: ±(0.05% of voltage measurement + offset)
–40 to 70 °C: ±(0.06% of voltage measurement+ offset)
These specifications are based on a calibrated CR300. They do not include
bridge resistor errors or sensor measurement noise, and they assume input
reversal for differential measurements (RevDiff).
6.1.3 Current Measurements – Details
Terminals SE1 and SE2 can be configured to make analog current
measurements. Each of the two terminals can independently measure 0 – 25
mA. When configured to measure current, terminals SE1 and SE2 each have
an internal resistance of 140 Ω which is placed in the current measurement
loop. The return path of the sensor must be connected directly to the analog
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