Campbell CR23X, Operator'S Manual

Page: 226 / 374

SECTION 13. CR23X MEASUREMENTS
13-26
and would complete in background every (17
segments) * (4 sec/segment) = 68 seconds. If
a user selects all possible input range codes, with
the exception of either 60 Hz or 50 Hz rejection,
but not both, along with period averaging, the
background calibration would consist of 44 total
segments and would require (44 segments) * (4
sec / segment) = 176 seconds (2.9 minutes) to
complete in background. If this rate of update for
self-calibration is too slow, because of possible
rapid thermal fluctuations, then a user can force
a self-calibration to be done every scan with the
P24 (calibrate) instruction.
INSTRUCTION 24 CALIBRATION
The alternative to automatic calibration is the
use of Instruction 24, the calibration instruction.
Instruction 24 implements a complete calibration
which occurs ONLY when EXECUTED by a
program table. Instruction 24 calibration is the
average of 10 calibrations, and takes
approximately 2.8 seconds to complete.
Automatic calibration is disabled when a
program is compiled that contains Instruction 24.
Instruction 24 calibration, as opposed to
automatic calibration, may be advantageous in
applications where: 1) the CR23X is exposed to
extreme thermal gradients, or 2) automatic
calibration would interfere with the desired
sampling rate, and the ambient temperature is
stable enough to allow calibration at specific
points during program execution.
Calibration coefficients are replaced each time
that Instruction 24 is executed. Unlike
automatic calibration, there is no time constant
for the coefficients to respond in changes to
calibration. Instruction 24 calibration ensures
that the coefficients are optimum at the time
that the instruction is executed. For example,
consider a CR23X mounted under the dash of
an automobile, where temperature could easily
change 50 degrees. Temperature changes
affect the measurement circuitry which must be
compensated for by calculating new
coefficients. Each time Instruction 24 is
executed a new set of calibration coefficients is
calculated based on the measurements made
at that time.
Calibration at a certain point during program
execution may be advantageous for some
applications. For example, suppose Table 2 has
an execution time of 15.6 ms, but only executes
when flag 1 is set. Table 1 has a 5 minute
execution time which makes a temperature
measurement, and sets flag 1 if the temperature
exceeds a fixed value. To prevent overrun errors
which would occur in Table 2 if the automatic
calibration was used, Instruction 24 could be
executed before the temperature measurement
was made by Table 1.
Instruction 24 also has an option to store the
results of the automatic calibration in Input
Storage. This can be used to detect hardware
problems. If -99999 appears in any of the input
locations reporting calibration values, the
CR23X has a hardware problem or needs
factory calibration.