Long cables

Digital data transfer eliminates offset and noise errors due to cable lengths. However, digital communications can break down when cables are too long, resulting in either no response from the sensor or corrupted readings. The original SDI-12 standard specifies the maximum total cable length of 60 meters (200 ft). Low power SDI-12 sensors, such as the HygroVue 10, have been used with longer cables up to 300 m (1000 ft). This model of sensor is normally only supplied with cables up to 10 m in length. If the cable is to be extended, follow these guidelines:

• Use low capacitance, low resistance, screened cable (as fitted by Campbell Scientific) to reach distances of several hundred meters.

• Ensure the power ground cable has low resistance and is connected to the same ground reference as the data logger control ports.

• Be aware that “daisy-chaining” or connecting sensors in parallel to the same SDI-12 port reduces the total cable length roughly in proportion to the number of sensors connected. For example, while it may be possible to use one cable run of 300 m (1000 ft) to a single sensor, the cable length would be limited to around 100 m (300 ft) per sensor if three sensors are connected to one port.