Campbell Scientific CR7 Measurement and Control System - Manual (Page 149)

SECTION 14. INSTALLATION
14-5
3. When the CR7 is to be located in a gas-
tight enclosure or used in a gas-tight mode
with the standard ENVIRONMENTALLY
SEALED FIBERGLASS CASE, the internal
lead acid batteries SHOULD BE REMOVED
and an external battery substituted.
14.3 HUMIDITY EFFECTS AND
CONTROL
The CR7 system is designed to operate reliably
under environmental conditions where the
relative humidity inside its enclosure does not
exceed 90% (noncondensing). Situations where
the humidity tolerances are exceeded may
result in damage to IC chips, microprocessor
failure and/or measurement inaccuracies due to
condensation on the various PC board runners.
Effective humidity control is the responsibility of
the user and is particularly important in
operational environments where the CR7 is
exposed to salty air.
Several precautionary methods are available for
protecting the CR7 against excessive humidity
and subsequent component damage. Selection
of the appropriate method or combination of
methods will depend on the environmental
characteristics prevailing at a specific data
acquisition site. Humidity control methods
include:
1. the use of desiccant
2. sealing the CR7 enclosure gas-tight
3. the nitrogen purging technique
14.3.1 DESICCANT
As a minimal precaution, the packets of HUMI-
SORB desiccant shipped with the CR7 should
be placed inside the Control Module. These
packets should be routinely removed from the
CR7 and reactivated by warming them in an
oven for about 16 hours at a temperature of 120
oC (250 oF). The recommended time between
reactivations varies from one location to
another. Obviously, the desiccant requires more
frequent attention in environments where the
relative humidity is high.
14.3.2 GAS-TIGHT ENVIRONMENT
Another method for controlling humidity involves
the proper selection of connectors for use with
the sealed fiberglass enclosure such that the
CR7 operates in a gas-tight environment.
Various connector options and associated
details are described in Section 12. Desiccant
is still required with this method.
14.3.3 NITROGEN PURGING
Several CSI customers have had success in
preventing humidity-related equipment
malfunctions in harsh environments by allowing
nitrogen gas to slowly bleed into the datalogger
enclosure. The sensor leads, power cables,
etc., are routed to the terminal blocks of the
datalogger through simple, inexpensive conduit
elbows which are left unplugged. A nitrogen
bottle is then left at the field site with its
regulator valve slightly open so that nitrogen is
allowed to escape slowly through a rubber tube
which is routed along with the sensor leads
through the conduit elbows into the CR7
enclosure. The tube vent should be positioned
underneath the Control Module.
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 site visitations. Where
practical, larger nitrogen bottles should be used
to reduce cost and refilling frequency.
14.4 RECOMMENDED GROUNDING
PRACTICES
14.4.1 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 that are rated to
10,000 amps. The spark gaps are connected
directly to the heavy copper grounding bar on
each input card with no more than 2 inches of