SECTION 13. CR510 MEASUREMENTS
13-17
In Figure 13.5-2, V
x
is the excitation voltage, R
f
is a fixed resistor, R
s
is the sensor resistance,
and R
G
is the resistance between the excited
electrode and CR510 earth ground. With R
G
in
the network, the measured signal is:
R
s
V
1
= V
x
__________________
[13.6-1]
(R
s
+R
f
) + R
s
R
f
/R
G
R
s
R
f
/R
G
is the source of error due to the ground
loop. When R
G
is large the equation reduces to
the ideal. The geometry of the electrodes has a
great effect on the magnitude of this error. The
Delmhorst gypsum block used in the 227 probe
has two concentric cylindrical electrodes. The
center electrode is used for excitation; because
it is encircled by the ground electrode, the path
for a ground loop through the soil is greatly
reduced. Moisture blocks which consist of two
parallel plate electrodes are particularly
susceptible to ground loop problems. Similar
considerations apply to the geometry of the
electrodes in water conductivity sensors.
The ground electrode of the conductivity or soil
moisture probe and the CR510 earth ground
form a galvanic cell, with the water/soil solution
acting as the electrolyte. If current was allowed
to flow, the resulting oxidation or reduction
would soon damage the electrode, just as if DC
excitation was used to make the measurement.
Campbell Scientific probes are built with series
capacitors in the leads to block this DC current.
In addition to preventing sensor deterioration,
the capacitors block any DC component from
affecting the measurement.
13.6 CALIBRATION PROCESS
The CR510 makes voltage measurements by
integrating the input signal for a fixed time and
then holding the integrated value for the analog
to digital (A/D) conversion. The A/D conversion
is made by a 13 bit approximation using a digital
to analog converter (DAC). The result from the
approximation is DAC counts, which are
multiplied by coefficients to obtain millivolts
(mV). There are 10 calibration coefficients, one
for each of the 5 gain ranges for the fast and
slow integration times.
The CR510 has an internal calibration function
that feeds positive and negative voltages
through the amplifiers and integrator and
calculates new calibration coefficients. By
adjusting the calibration coefficients the
accuracy of the voltage measurements is
maintained over the -25 to +50
°
C operating
range of the CR510. Calibration is executed
under four conditions:
1.
When the CR510 is powered up.
2.
Automatically when Instruction 24 is not
contained in a program table.
3.
When the watchdog resets the processor.
4.
When the calibration instruction, Instruction
24, is executed.
AUTOMATIC CALIBRATION SEQUENCE
The primary advantage of automatic calibration
is that the CR510 is constantly calibrated
without user programming. The CR510 defaults
to automatic calibration when Instruction 24 is
not contained in a program table.
Every 8 seconds one part of a 22 part
calibration sequence is performed. Program
execution is interrupted (5.4 - 21.4 ms), when
necessary, for each part of the calibration.
Every 2.9 minutes (8 seconds * 22) ten
calibration coefficients are calculated. The
calculated coefficients are multiplied by 1/5, and
then added to 4/5 times the existing
coefficients. Averaging is done as a safeguard
against coefficients calculated from a noisy
measurement.
The above weighting of the newly calculated
coefficients results in a 15 minute time constant
(see Instruction 58) in the response of the
calibration to step changes affecting the
calibration coefficients (primarily temperature).
For most environmental applications a 15
minute time constant is acceptable. The
automatic calibration may result in the
calibration coefficients not being optimum for
applications that subject the CR510 to extreme
temperature gradients.
Automatic calibration extends the processing
time 5.4 to 21.4 ms when it is executed (every 8
seconds). If the processing time exceeds the
execution interval the CR510 finishes
processing the table and awaits the next
occurrence of the execution interval before
initiating the table. At the fastest execution
interval of 1/64 (0.0156) second the program
Содержание CR510
Страница 1: ...CR510 DATALOGGER OPERATOR S MANUAL REVISION 2 03 COPYRIGHT c 1986 2003 CAMPBELL SCIENTIFIC INC ...
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Страница 44: ...SECTION 1 FUNCTIONAL MODES 1 14 This is a blank page ...
Страница 56: ...SECTION 3 INSTRUCTION SET BASICS 3 8 ...
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Страница 128: ...SECTION 10 PROCESSING INSTRUCTIONS 10 8 This is a blank page ...
Страница 144: ...SECTION 12 PROGRAM CONTROL INSTRUCTIONS 12 10 This is a blank page ...
Страница 172: ...SECTION 14 INSTALLATION AND MAINTENANCE 14 10 This is a blank page ...
Страница 176: ...APPENDIX A GLOSSARY A 4 This is a blank page ...
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Страница 188: ...APPENDIX D DATALOGGER INITIATED COMMUNICATIONS D 4 This is a blank page ...
Страница 196: ...APPENDIX F MODBUS ON THE CR10 X AND CR510 F 4 This is a blank page ...
Страница 197: ...APPENDIX G TD OPERATING SYSTEM ADDENDUM FOR CR510 CR10X AND CR23X MANUALS ...
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