Campbell Scientific CR23X Micrologger - Manual (Page 140)

SECTION 9. INPUT/OUTPUT INSTRUCTIONS
9-6
*** 5 AC HALF BRIDGE ***
FUNCTION
This instruction is used to apply an excitation
voltage to a half bridge (Figure 13.5-1), make a
single-ended voltage measurement of the
bridge output, reverse the excitation voltage,
then repeat the measurement. The difference
between the two measurements is used to
calculate the resulting value which is the ratio of
the measurement to the excitation voltage. A 1
before the excitation channel number (1X)
causes the channel to be incremented with
each repetition.
The excitation "on time" for each polarity is
exactly the same to insure that ionic sensors do
not polarize with repetitive measurements. The
range should be selected to be a fast
measurement (range 11-15), limiting the
excitation on time to less than 800
microseconds at each polarity. A slow
integration time should not be used with ionic
sensors because of polarization error.
PARAM. DATA
NUMBER TYPE DESCRIPTION
01: 2 Repetitions
02: 2 Range Code (Table 9-1)
03: 2 Single-ended channel
number
04: 2 Excitation channel
number (Table 9-2A)
05: 4 Excitation voltage
(millivolts)
06: 4 Input location number
for first measurement
07: FP Multiplier
08: FP Offset
Input locations altered: 1 per repetition
*** 6 FULL BRIDGE WITH SINGLE ***
DIFFERENTIAL MEASUREMENT
FUNCTION
This Instruction is used to apply an excitation
voltage to a full bridge and make a differential
voltage measurement of the bridge output. The
measurement is made with the polarity of the
excitation voltage both positive and negative
(Figure 13.5-1). The result is 1000 times the
ratio of the measurement to the excitation
voltage. A 1 before the excitation channel
number (1X) causes the channel to be
incremented with each repetition.
PARAM. DATA
NUMBER TYPE DESCRIPTION
01: 2 Repetitions
02: 2 Range code (Table 9-1)
03: 2 Differential channel
number for first
measurement
04: 2 Excitation channel
number (Table 9-2A)
05: 4 Excitation voltage
(millivolts)
06: 4 Input location number
for first measurement
07: FP Multiplier
08: 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 second voltage sensing wire from the
sensor to compensate for lead wire resistance.
The measurement sequence is to apply an
excitation voltage, make a single-ended
measurement on the first channel, then repeat
the measurement with an excitation of the
opposite polarity. The same sequence is then
applied to the second single-ended channel.
This sequence is used for offset voltage
removal. The combined results for the two
single-ended measurement channels are then
used to calculate the resulting value, which is the
ratio of the voltage across the sensor to the
voltage across the reference resistor (Figure
13.5-1). A 1 before the excitation channel
number (1X) causes the channel to be
incremented with each repetition.