Humboldt H-4146 Compaction Permeameters - Manual (Page 4)

and the valve opened to remove all air from the system, he valve is closed after
air removal. If a small pressure head is sufficient, the elevation of the water in
a pipette may be taken as the headwater elevation. If a large pressure head is
required, constant air pressure may be introduced at the top of the pipette and
this air pressure head added to the pipette head. When a very high pressure
head is used it is usually sufficiently accurate to average the start and finish
heads and to calculate the permeability, as in the constant head test. For tests in
which it is desired to maintain a high hydrostatic pressure, but a small difference
in pressure head, i.e. simulating underground low depth pressures, two pressure
tanks on at each end may be used. One would become the head pressure and
the other the tail pressure.