Campbell Scientific CR9000X Measurement and Control System - Manual (Page 90)

Section 3. CR9000X Measurement Details
not the case in the CR10, 21X and CR7 dataloggers where there is always a
setup time for each instruction.)
There are four parameters in the measurement instructions that may vary the
sequence and timing of the measurement. These are options to reverse the
polarity of the excitation voltage (RevEx), reverse the high and low
differential inputs (RevDiff), to set the time to wait between switching to a
channel and making a measurement (Delay), and the length of time to integrate
a measurement (Integ).
3.1.1.1 Reversing Excitation or the Differential Input
Reversing the excitation polarity or the differential input are techniques to
cancel voltage offsets that are not part of the signal. For example, if there is a
+5
μV offset, a 5 mV signal will be measured as 5.005 mV. When the input is
reversed, the measurement will be -4.995 mV. Subtracting the second
measurement from the first and dividing by 2 gives the correct answer: 5.005-
(-4.995)=10, 10/2=5. Most offsets are thermocouple effects caused by
temperature gradients in the measurement circuitry or wiring.
Reversing the excitation polarity cancels voltage offsets in the sensor, wiring,
and measurement circuitry. One measurement is made with the excitation
voltage with the polarity programmed and a second measurement is made with
the polarity reversed. The excitation "on time" for each polarity is exactly the
same to ensure that ionic sensors do not polarize with repetitive measurements.
Reversing the inputs of a differential measurement cancels offsets in the
CR9000X measurement circuitry. One measurement is made with the high
input referenced to the low input and a second with the low referenced to the
high.
3.1.1.2 Delay
When the CR9000X switches to a new channel or switches on the excitation
for a bridge measurement, there is a finite amount of time required for the
signal to reach its true value. Delaying between setting up a measurement
(switching to the channel, setting the excitation) and making the measurement
allows the signal to settle to the correct value. The default CR9000X delays,
10
μs for the 5000 and 1000 mV ranges and 20 μs for the 200 and 50 mV
ranges, are the minimum required for the CR9000X to settle to within its
accuracy specifications. Additional delay is necessary when working with
high sensor resistances or long lead lengths (higher capacitance). It is also
possible to shorten the delay on the 200 and 50 mV ranges to 10
μs when
speed and resolution is more important than high accuracy. Using a delay
increases the time required for each measurement.
When the CR9000X Reverses the differential input or the excitation polarity, it
delays the same time after the reversal as it does before the first measurement.
Thus there are two delays per channel when either RevDiff or RevEx is used.
If both RevDiff and RevEx are selected, there are four measurement segments,
positive and negative excitations with the inputs one way and positive and
negative excitations with the inputs reversed. The CR9000X switches to the
channel:
3-2