Environmental Engineering Reference
In-Depth Information
(a)
(b)
Figure 20.18.
(a) Bishop and (b) USBR type hydraulic piezometers (ANCOLD, 1983).
Some organisations use flow meters on the inlet and outlet lines as an aid to flushing. A
flow of 300 ml/min is maintained to prevent air becoming trapped in high spots in the
lines. It is also good practice to flush before all readings, since equal pressures on the lines
can occur with air in both lines.
Eagles (1987) reports that if the entry to the piezometer is smaller than the tubing (e.g.
1.5 mm to 2 mm compared to 5 mm), crystal growth from chemicals dissolved from the
tubing can cause blockage and it is important that the entry be the same diameter as the
tubing.
The tip pressure is measured using Bourdon gauges, measuring manometers, or pres-
sure transducers in the terminal structure. Most installations use Bourdon pressure gauges
- either one gauge for each piezometer, with a master gauge to check, or a master gauge
with a switching system. Typical layouts are given in ANCOLD (1983) and US Department
of Interior (1981) and USBR (1987). When using a master gauge, it is necessary to estimate
 
 
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