Specifications

Section 7. Grounding

7-5

7.4.2 External Signal Conditioner

External signal conditioners, e.g. an infrared gas analyzer (IRGA), are

frequently used to make measurements and send analog information to the

CR1000. These instruments are often powered by the same AC line source as

the CR1000. Despite being tied to the same ground, differences in current

drain and lead resistance result in different ground potential at the two

instruments. For this reason, a differential measurement should be made on the

analog output from the external signal conditioner.

7.5 Ground Looping in Ionic 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 CR1000 ground, and can be represented by the model

diagrammed in FIGURE 7.5-1.

FIGURE 7.5-1. Model of Resistive Sensor with Ground Loop

In Equation 14.5-1, 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

CR1000 earth ground. With R

in the network, the measured signal is:

()

Gfsfs

R/RRRR

[14.5-1]

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

CR1000 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