Owing to the strong interdependence of subglacial hydrological conditions and mechanical processes and properties at the glacier base, the analysis of data from borehole instruments cannot be complete without some means of measuring subglacial water pressures. Boreholes drilled to the bed are often assumed to act as manometers, allowing the water levels within them to be interpreted as subglacial water pressures. Most water-pressure transducers record the resistance of strain gauges bonded to a diaphragm as it flexes in response to the pressure differential imposed across it. The inner cavity of such transducers is commonly sealed at a fixed pressure (often termed 'absolute' or 'sealed-gauge' devices in contrast to 'relative' devices, in which a gland in the supply cable maintains the cavity at atmospheric pressure). Water-pressure transducers, which are typically 4-5 cm long (with the open pressure port at their base) and 2-3 cm in diameter, may be purchased as sealed units with cable attached (expensive but effective) or as open units that may be cast in resin, leaving only the pressure port exposed. Clear resin is recommended for home-potting, such that the cables and contacts may be viewed in the event of a problem. The most common problems encountered in using such sensors are shorting by water due to imperfect sealing, and damage to the conductor strain-gauge contacts within the body of the transducer. These strain-gauge contacts may be particularly fragile and damage may be caused by water ingress, build-up and discharge of static electricity, and rough handling.
We suspend our sensors on their cables in the borehole ca. 0.25-0.5 m above the bed. If a sensor is positioned too low, it may become packed with debris or sheared by glacier motion; if a sensor is positioned too high, the borehole water level might drop below it during periods of low subglacial water pressures.
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