Campbell CR23X, Руководство оператора

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SECTION 13. CR23X MEASUREMENTS
13-25
INFLUENCE OF GROUND LOOP ON
MEASUREMENTS
When measuring soil moisture blocks or water
conductivity, the potential exists for a ground
loop which can adversely affect the
measurement. This ground loop arises
because the soil and water provide an alternate
path for the excitation to return to CR23X
ground, and can be represented by the model
diagrammed in Figure 13.6-2.
In Figure 13.6-2, 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 CR23X earth ground. With R
G
in
the network, the measured signal is:
V V
R
R R R R R
x
s
s f s f G
1
=
+ +
( ) /
[13.6-1]
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 CR23X 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.
13.7 CALIBRATION PROCESS
The CR23X 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 15 bit successive 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).
The CR23X 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 CR23X. Calibration is executed
under four conditions:
1. When the CR23X 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 CR23X is constantly calibrated
without user programming. The CR23X
defaults to automatic calibration when
Instruction 24 is not contained in a program
table.
The CR23X performs an automatic self-
calibration in background to compensate for
device changes due to aging along with more
rapid changes due to fluctuations in
environmental temperature.
The CR23X only calibrates those parameters that
are necessary for the users datalogger program.
As a result, the time required for a complete self-
calibration in background depends upon the
number of ranges selected by the user. The
CR23X performs a segment of the background
calibration every 4 seconds, and each unique
voltage input range (13 total) consists of four
segments. The background self-calibration
always contains at least nine segments because
the
±
5000 mV input range is calibrated and then
used for a background internal and panel
temperature measure. As an example, if the
±
5000 mV,
±
200 mV, and
±
50 mV input ranges
are selected in a program, then the total self-
calibration would consist of 9 + 8 = 17 segments