The gas-purge system transmits the pressure head of water
in the stream to the manometer pressure transducer location.
A gas is fed through a tube and bubbled freely into the
stream through an orifice at a fixed elevation in the
stream. The gas pressure in the tube is equal to the
piezometric head on the bubble orifice at any gage height.
The standard bubble rate in the USGS bubble system is 60
bubbles per minute in a sight glass. This low rate conserves
gas supplies and keeps friction in the polyethylene tubing
to less than an equivalent 0.01 ft. of water in feed lines
as long as 250 feet.
