Campbell CR9000, Instruction Manual

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Section 1. Installation
1-9
allowed to escape slowly through a rubber tube which is routed along with the
sensor leads through the conduit elbows into the CR9000 enclosure.
Equipment required for this method of humidity control generally can be
obtained from any local welding supply shop and includes a nitrogen bottle,
regulator with tube adapter (content gauge, optional), hose clamp and a
suitable length of small diameter rubber tubing. Nitrogen bottles are available
in various sizes and capacities. The size of the nitrogen bottle used depends on
the transport facilities available to and from the field site and on the time
interval between visiting the site. Where practical, larger nitrogen bottles
should be used to reduce cost and refilling frequency.
1.4 Recommended Grounding Practices
1.4.1 Protection from Lightning
Primary lightning strikes are those where the lightning hits the datalogger or
sensors. Secondary strikes occur when the lightning strikes somewhere near
the lead in wires and induces a voltage in the wires. All input and output
connections in the I/O Module are protected using spark gaps. This transient
protection is useless if there is not a good connection between the CR9000 and
earth ground.
All dataloggers in use in the field should be grounded. A 12 AWG or larger
wire should be run from the grounding terminal on the right side of the I/O
Module case to a grounding rod driven far enough into the soil to provide a
good earth ground.
A modem/phone line connection to the CR9000 provides another pathway for
transients to enter and damage the datalogger. The phone lines should have
proper spark gap protection at or just before the modem at the CR9000. The
phone line spark gaps should also have a solid connection to earth ground.
1.4.2 Effect on Measurements: Common Mode Range
A difference in ground potential between a sensor or signal conditioner and the
CR9000 can offset the measurement. A differential voltage measurement gets
rid of offset caused by a difference in ground potential. However, in order to
make a differential measurement, the inputs must be within the CR9000
common mode range of
±
5V (+15/-5 for the CR9052E module,
±
50V for the
9055 module, or
±
60V for the CR9058E module).
The common mode range is the voltage range, relative to CR9000 ground,
within which both inputs of a differential measurement must lie, in order for
the differential measurement to be made. For example, if the high side of a
differential input is at 4V and the low side is at 3V relative to CR9000 ground,
there is no problem, a measurement made on the
±
1.5V range would indicate a
signal of 1V. However, if the high input is at 5.8V and the low input is at
4.8V, the measurement cannot be made because the high input is outside of the
CR9000 common mode range.