Campbell CR800 Spezifikationen PDF herunterladen (Seite 96)

Section 3. CR800 Measurement Details

3-22

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

CR800 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 CR800 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 CR800 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 CR800 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.

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