Campbell CR3000 Micrologger, Operator'S Manual

Page: 421 / 644

Section 8. Operation
421
8.1.8 Cabling Effects — Details
Related Topics:
• Cabling Effects — Overview
(p. 79)
• Cabling Effects — Details
(p. 421)
Sensor cabling can have significant effects on sensor response and accuracy. This
is usually only a concern with sensors acquired from manufacturers other than
Campbell Scientific. Campbell Scientific sensors are engineered for optimal
performance with factory-installed cables.
8.1.8.1 Analog Sensor Cabling
Cable length in analog sensors is most likely to affect the signal settling time. For
more information, see Signal Settling Time
(p. 342).
8.1.8.2 Current Excitation Cabling
When relatively large resistances are measured (> 1000 ohms), or relatively long
cable lengths are used (> 50 feet) with sensors requiring current excitation, a 0.1
µf feed -forward capacitor should be placed between the IX and IXR to prevent
excessive ringing
(p. 553).
With this capacitor present, a minimum of 3 ms is
recommended for the SettlingTime parameter in the measurement instruction. The
capacitor simply connects between the IX terminal and the IXR terminal. The
capacitor has no polarity.
8.1.8.3 Pulse Sensor Cabling
Because of the long interval between switch closures in tipping-bucket rain gages,
appreciable capacitance can build up between wires in long cables. A built-up
charge can cause arcing when the switch closes and so shorten switch life. As
shown in figure Current-Limiting Resistor in a Rain Gage Circuit
(p. 421),
a 100 Ω
resistor is connected in series at the switch to prevent arcing. This resistor is
installed on all rain gages currently sold by Campbell Scientific.
FIGURE 102: Current-Limiting Resistor in a Rain Gage
Circuit