Campbell Scientific CR3000 Micrologger - Manual (Page 330)

Section 8. Operation
330
8.1.10.1 Analog Sensor Cables
Cable length in analog sensors is most likely to affect the signal settling time. For
more information, see Signal Settling Time
(p. 291).
8.1.10.2 Sensors Requiring Current Excitation
When relatively large resistances are measured (> 1000 ohms), or relatively long
cable lengths are used (> 50 foot) with sensors requiring current excitation, a 0.1-
µf feed-forward capacitor should be placed between IX and IXR to prevent
excessive ringing
(p. 465).
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 chosen IX output terminal and the IXR
return terminal. The capacitor has no polarity.
8.1.10.3 Pulse Sensors
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, shortening switch life. As shown
in figure Current Limiting Resistor in a Rain Gage Circuit
(p. 330),
a 100-ohm
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 107: Current limiting resistor in a rain gage circuit
8.1.10.4 RS-232 Sensors
RS-232 sensor cable lengths should be limited to 50 feet.
8.1.10.5 SDI-12 Sensors
The SDI-12 standard allows cable lengths of up to 200 feet. Campbell Scientific
does not recommend SDI-12 sensor lead lengths greater than 200 feet; however,
longer lead lengths can sometimes be accommodated by increasing the wire gage
or powering the sensor with a second 12-Vdc power supply placed near the
sensor.
8.1.11 Synchronizing Measurements
Timing of a measurement is usually controlled relative to the CR3000 clock.
When sensors in a sensor network are measured by a single CR3000,
measurement times are synchronized, often within a few milliseconds, depending
on sensor number and measurement type. Large numbers of sensors, cable length
restrictions, or long distances between measurement sites may require use of