Campbell Scientific CR9000X Measurement and Control System - Manual (Page 70)

Section 1. Installation
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 CR9000X
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 CR9000X 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 CR9000X. The
phone line spark gaps should also have a solid connection to earth ground.
1.4.2 Operational Input Voltage Limits: Effect on Measurements
A difference in ground potential between a sensor or signal conditioner and the
CR9000X 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 CR9000X's
operational input voltage range of
±5V (+15/-5 for the CR9052E module,
±50V for the 9055 module, or ±60V for the CR9058E module).
The operational input voltage limit is the voltage range, relative to CR9000X
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 4 V and the low side is at 3.1 V relative to
CR9000X ground, there is no problem, a measurement made on the
± 1000
mV range would indicate a signal of 1 V. However, if the high input is at 5.8
V and the low input is at 4.8 V, the measurement cannot be made because the
high input is outside of the CR9000X operational voltage range.
See Section 3.1.2 Single Ended and Differential Voltage Measurements for
more material about Input Limits and Common Mode voltage.
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