Section 8. Operation
365
3. Reverse the excitation, and then settles, and then
measure
4. Reverse the excitation, reverse the input terminals, settle,
measure
5. Reverse the excitation, settle,
measure
There are four delays per
measure
. The CR800 processes the four sub-
measurements into the reported measurement. In cases of excitation reversal,
excitation time for each polarity is exactly the same to ensure that ionic sensors do
not polarize with repetitive measurements.
Read More
A white paper entitled "The Benefits of Input Reversal and
Excitation Reversal for Voltage Measurements" is available at
www.campbellsci.com
.
Ground Reference Offset Voltage
When
MeasOff
is enabled (=
True
), the CR800 measures the offset voltage of the
ground reference prior to each
VoltSe()
or
TCSe()
measurement. This offset
voltage is subtracted from the subsequent measurement.
From Auto Self-Calibration
If
RevDiff
,
RevEx
, or
MeasOff
is disabled (=
False
), offset voltage compensation
is continues to be automatically performed, albeit less effectively, by using
measurements from the auto self-calibration. Disabling
RevDiff
,
RevEx
, or
MeasOff
speeds up measurement time; however, the increase in speed comes at
the cost of accuracy because of the following:
1
RevDiff
,
RevEx
, and
MeasOff
are more effective.
2 Auto self-calibrations are performed only periodically, so more time skew
occurs between the auto self-calibration offsets and the measurements to
which they are applied.
Note
When measurement duration must be minimal to maximize
measurement frequency, consider disabling
RevDiff
,
RevEx,
and
MeasOff
when CR800 module temperatures and return currents are slow
to change.
Time Skew Between Measurements
Time skew between consecutive voltage measurements is a function of settling
and integration times, A-to-D conversion, and the number entered into the
Reps
parameter of the
VoltDiff()
or
VoltSE()
instruction. A close approximation is:
time skew = settling time + integration
time + A-to-D conversion time + reps
where A-to-D conversion time equals
15
µs. If reps (repetitions) > 1 (multiple
measurements by a single instruction), no additional time is required. If reps = 1
in consecutive voltage instructions, add 15 µs per instruction.
Summary of Contents for CR800 Series
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Page 446: ...Section 8 Operation 446 8 11 2 Data Display FIGURE 100 CR1000KD Displaying Data ...
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