Campbell-scientific CR10X Measurement and Control System Uživatelský manuál stáhnout pdf (Strana 213)

SECTION 13. CR10X MEASUREMENTS

13-21

13.6 RESISTANCE MEASUREMENTS

REQUIRING AC EXCITATION

Some resistive sensors require AC excitation.

These include the 207 Relative Humidity Probe,

soil moisture blocks, water conductivity sensors,

and wetness sensing grids. The use of DC

excitation with these sensors can result in

polarization, which will cause an erroneous

measurement, and may shift the calibration of

the sensor and/or lead to its rapid decay.

The AC half bridge Instruction 5 (incorporated

into the 207 relative humidity measurement

Instruction 12) reverses excitation polarity to

provide ion depolarization and, in order to

minimize the time excitation is on, grounds the

excitation as soon as the signal is integrated

(Figure 13.6-1). The slow integration time

should never be used with a sensor requiring

AC excitation because it results in the excitation

lasting about 1.5 times as long, allowing

polarization to affect the measurement.

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 CR10X

ground, and can be represented by the model

diagrammed in Figure 13.6-2.

FIGURE 13.6-1. AC Excitation and Measurement Sequence for AC Half Bridge

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

1 2 ... 208 209 210 211 212 213 214 215 216 217 218 ... 361 362

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Campbell-scientific CR10X Measurement and Control System Uživatelský manuál Strana 213