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2. SYSTEM COMPONENTS
The Device consists of a drum on which is wound a length of 1/16 inch, nylon-jacketed, stainless
steel aircraft cable. As movement occurs, the cable reels off the drum, and the drum turns. (The
tension on the cable is maintained by a constant force spring inside the drum). The drum is
connected to a lead-screw in such a way that the rotation of the drum is converted into a linear
motion of the lead screw. The lead-screw is connected to a Model 4450 Vibrating Wire
Displacement Transducer, which measures the linear motion. In this way a one-meter movement
of the aircraft cable is converted into roughly 25 mm movement of the Transducer. The whole
mechanism is enclosed within a rainproof enclosure.
A thermistor is included with the transducer so that temperature changes can be monitored.
The enclosure has a gasketed, hinged cover and is mounted on a three-inch threaded PVC pipe
flange, which will mate with a three-inch pipe designed to be installed and grouted inside a
borehole drilled perpendicular to the slope. This standpipe can be provided by the installer or is
available at Geokon.
Also included with the enclosure is a “weak link” for attachment between the tensioned aircraft
cable inside the sensor enclosure and the extension cable, which stretches between the two points
being monitored.
Figure 2 - The Weak Link
Experience has shown that unless this extension cable is fenced off there is a danger of large
animals or pieces of equipment blundering into it; this can seriously damage the internal sensor
mechanism. Should this happen, the weak link is designed to break at a relatively low cable
tension, thus preventing damage to the mechanism from over-ranging. Also supplied is a
compression spring which absorbs the shock of the recoil of the cable if the weak link is broken.