Environmental Engineering Reference
In-Depth Information
Figure 9.19
Diffused air volume indicator (from Fredlund, 1975c).
indicator. Two O-rings are placed around the top of the
burette to form a seal with the Lucite cylinder. The base
of the burette is sealed against the Lucite cylinder using
a threaded brass sleeve tightened against an O-ring. The
exit tube is connected to the burette to form a U-tube. A
constant air pressure
u
ab
is supplied to the Lucite cylinder
to establish a pressure gradient relative to the controlled
water pressure below the ceramic disk.
The burette is first filled with water to any desired level.
A few drops of the commercial cleaner Fantastik should be
added to the water in the burette to reduce surface tension
and promote the upward movement of air bubbles in the
burette.
The valve connecting the base plate and the diffused air
volume indicator is only opened when making a diffused
air volume measurement. The water pressure below the disk
is controlled at a constant pressure
u
c
.
A backpressure
u
ab
(i.e.,
u
ab
<u
c
) can be applied to the air in the Lucite cylinder
in order to establish a suitable pressure gradient across the
base plate. The pressure gradient between the water pres-
sure in the base plate and the air pressure in the diffused
air volume indicator is the driving force for flushing water
containing air bubbles from the base plate into the burette.
The pressure gradient should not exceed 70 kPa to avoid
causing bubbles to form in the water due to the sponta-
neous release of dissolved air from the water below the
base plate.
9.8.3 Procedure for Measuring Diffused Air Volume
The initial water level reading in the burette,
h
a
1
, can be
taken when the air pressure in the Lucite cylinder is at atmo-
spheric pressure or at the applied backpressure (i.e., at the
air pressure
u
ab
) (Fig. 9.20). The connecting valve is then
opened momentarily to apply a pressure gradient across the
base plate. The pressure gradient flushes the diffused air
into the burette. The flushed diffused air displaces the water
in the burette while water flows out of the exit tube. After
Figure 9.20
Computation of volume of diffused air under isother-
mal conditions.
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