Specifications

Section 3. CR3000 Measurement Details
2. System background calibration. This automatically takes place in the
background while the user program is running. Multiple measurements
are not averaged, but a filter is applied to the new gain/offset values
obtained. The filter is used so that the calibration values change slowly.
The filter combines the newly measured value multiplied by 0.1 with the
previous calibration value by 0.9 to arrive at the new calibration value. A
piece of the background calibration is added to each fast scan in the user
program. The background calibration measurements will be repeated
every 4 seconds or the time it takes to complete them, whichever is
longer. If there is not enough time to do the background calibration, the
CR3000 will display: “Warning when Fast Scan X is running background
calibration will be disabled.” (X is the number of the fast scan where the
first scan entered in the program is 1, the next scan is 2, etc.)
3. Calibration under program control. When the calibrate instruction is
included in a program, the calibration is identical to the compile time
calibration. The calibration table values are replaced with those
calculated. The fast integrations have averaging as in the compile
calibrate. When a calibrate instruction is found in any scan the
background calibration will be disabled (even if the scan is not executed).
The calibrate instruction is described in Section 7.
The self calibration does not take place if there is not enough time to run it or if
the calibrate instruction is in the CR3000 program and never executed.
Without the self calibration the drift in accuracy with temperature is about a
factor of 10 worse. For example, over the extended temperature range (-40 to
85°C) the accuracy specification is approximately 0.1% of reading. If the self
calibration is disabled, the accuracy over the range is approximately 1% of
reading. Temperature is the main factor causing a calibration shift and the
need for the self calibration. If the temperature of the CR3000 remains the
same there will be little calibration drift with the self calibration disabled.
The time constant for the background calibration (at the 4 second rate) is
approximately 36 seconds. This allows the CR3000 to remain calibrated
during fairly rapid temperature changes. In cases of extreme temperature
change, such as bringing a vehicle from equilibrium in a chamber at -30°C out
into a hot Arizona day, it may be worthwhile to override the background
calibration by running the calibration instruction in the scan with the
measurements.
Another case where using the calibration instruction makes sense is where
there is not time for the background calibration in the normal scan but the
program can periodically stop making measurements and run the calibration
instruction in a separate scan.
3.9 Measurement Accuracy
Accuracy, which really means inaccuracy, defines how far from the truth a
measurement may be. The term “uncertainty” is preferred over accuracy in
metrology (the science of measurements), although accuracy is more widely used
in instrumentation. A typical datalogger accuracy specification includes only the
dataloggers contribution to measurement error, and not the error of attached
sensors. Since there is no universal method of specifying voltage measurement
accuracy several different methods exist. In the past Campbell Scientific, Inc. has
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