Campbell Scientific CR7 Measurement and Control System - Manual (Page 94)

SECTION 9. INPUT/OUTPUT INSTRUCTIONS
9-4
*** 6 FULL BRIDGE WITH SINGLE ***
DIFFERENTIAL MEASUREMENT
FUNCTION
This Instruction is used to apply an excitation
voltage to a full bridge (Figure 13.5-1), make a
differential voltage measurement of the bridge
output, reverse the excitation voltage, then
repeat the measurement. The resulting value is
1000 times the ratio of the measurement to the
excitation voltage.
PAR. DATA
NO. TYPE DESCRIPTION
01: 2 Repetitions (95 max)
02: 2 Range code (Table 9-1)
03: 2 Analog card number for first
measurement
04: 2 Differential channel number
for first measurement
05: 2 Excitation card for first
measurement
06: 2 Excitation channel number
for first measurement
07: 2 Number of measurements
per excitation channel
08: 4 Excitation voltage (millivolts)
09: 4 Input location number for first
measurement
10: FP Multiplier
11: FP Offset
Input locations altered: 1 per repetition
*** 7 THREE WIRE HALF BRIDGE ***
FUNCTION
This Instruction is used to determine the ratio of
the sensor resistance to a known resistance
using a separate voltage sensing wire from the
sensor to compensate for lead wire resistance.
The measurement sequence is to apply an
excitation voltage, make two voltage
measurements on two adjacent single ended
channels, the first on the reference resistor and
the second on the voltage sensing wire from the
sensor (Figure 13.5-1), then reverse the
excitation voltage and repeat the
measurements. The two measurements are
used to calculate the resulting value which is
the ratio of the voltage across the sensor to the
voltage across the reference resistor.
PAR. DATA
NO. TYPE DESCRIPTION
01: 2 Repetitions (95 max)
02: 2 Range code for both
measurements (Table 9-1)
03: 2 Analog card number for first
measurement
04: 2 Single-ended channel number
for first measurement
05: 2 Excitation card for first
measurement
06: 2 Excitation channel number
for first measurement
07: 2 Number of measurements
per excitation channel
08: 4 Excitation voltage (millivolts)
09: 4 Input location number for first
measurement
10: FP Multiplier
11: FP Offset
Input locations altered: 1 per repetition
*** 9 FULL BRIDGE WITH EXCITATION ***
COMPENSATION
FUNCTION
This Instruction is used to apply an excitation
voltage and make two differential voltage
measurements, then reverse the polarity of the
excitation and repeat the measurements. The
measurements are made on sequential
channels. The result is the voltage measured
on the second channel (V2) divided by the
voltage measured on the first (V1). If V1 is
measured on the 5V range (code 8 or 18 in
Parameter 2), then the result is 1000 times
V2/V1. A 1 before the excitation channel
number (1X) causes the excitation channel to
be incremented with each repetition.
When used as a 6 wire full bridge (Figure 13.5-
1), the connections are made so that V1 is the
measurement of the voltage drop across the full
bridge, and V2 is the measurement of the
bridge output. Because the excitation voltage
for a full bridge measurement is usually in the
5V range, the output is usually 1000 V2/V1 or
millivolts output per volt excitation. When used
to measure a 4 wire half bridge, the connections
are made so that V1 is the voltage drop across
the fixed resistor (Rf), and V2 is the drop across
the sensor (Rs). As long as V1 is not measured
on the 5V range, the result is V2/V1 which
equals Rs/Rf.