2 cr9058e isolation module measurements – Campbell Scientific CR9000X Measurement and Control System User Manual

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Section 3. CR9000X Measurement Details

In Figure 3.1.7-1, V

is the excitation voltage, R

is a fixed resistor, R

is the

sensor resistance, and R

is the resistance between the excited electrode and

CR9000X earth ground. With R

in the network, the measured signal is:

(

)

R R R

s f

[3.1.7-1]

is the source of error due to the ground loop. When R

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
CR9000X 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.

3.2 CR9058E Isolation Module Measurements

Each CR9058E input channel has its own 24 bit sigma delta analog to digital
converter taking approximately 10,417 measurements per second, or one
measurement sample per 96 microseconds. The effective resolution at this
sample rate is 18.7 bits, or +/- 10 microvolts when using the +/- 2 Volt range,
because of the inherent noise of the A/D converter and noise from other
sources. The effective resolution can be dramatically improved through
filtering, and/or integrating, multiple measurements. Thus, noise reduction and
measurement speed can be traded off using the Integration parameter. Noise is
reduced by approximately the square root of the number of samples within the
integration time. Thus, if the integration time is set to 9600 versus 96
microseconds, noise should be reduced approximately by a factor of ten . This
approximation assumes that the noise is white noise, which is not

entirely true

because some of the noise is due to interference from sources at fixed
frequencies. Noise reduction by filtering can go just so far, and the best the
CR9058E can achieve is approximately 21 bits of resolution (+/- 2 micro-volts
on the 2 Volt range).

The CR9058E isolated input module is similar in operation to the CR9050
analog input module except for:

• The CR9058E has ten differential input channels instead of 14 differential

/ 28 single-ended inputs.

• The CR9058E has different voltage ranges: +/- 60 Volts DC, +/- 20 Volts

DC, and +/- 2 Volts DC.

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