SECTION 13. CR10 MEASUREMENTS

FIGURE 13.6-2. Model of Resistive Sensor with Ground Loop

In Figure 13.6-2, Vx is the excitation voltage, Rf is a fixed resistor, Rs is the sensor resistance, and RG is the resistance between the excited electrode and CR10 earth ground. With RG in the network, the measured signal is:

Rs

V1 = Vx __________________ [13.6-1]

(Rs+Rf) + RsRf/RG

RsRf/RG is the source of error due to the ground loop. When RG 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 CR10 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.

The CR10 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 CR10. Calibration is executed under four conditions:

1.When the CR10 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 CR10 is constantly calibrated without user programming. The CR10 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.

13.7 CALIBRATION PROCESS

The CR10 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.

13-22

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 CR10 to extreme temperature gradients.

Automatic calibration extends the processing time 5.4 to 21.4 ms when it is executed (every 8