4230 Flow Meter
Section 3 Installation
3-14
Note
The 4230 cannot accurately measure liquid levels that are
even with or below the bubble line outlet.
If you need to measure the liquid level down to the actual
“zero” level of the primary device, Isco recommends placing
the bubble line outlet at least 1 to 2 inches (2.5 to 5.1 cm)
below the primary device zero level to avoid measurement
failures when the liquid level is even with the outlet. Since the
flow meter can display negative measurements, you can com-
pensate when you set the level in the flow meter.
Figure 3-4 Positioning the Bubble Line in the Flow Stream
3.5.8 High-Velocity Flow
Streams
The normal position of the bubble line in the flow stream is at a
right angle to the flow. However, studies have indicated that at
relatively high flow stream velocities (greater than five feet per
second [1.5 meters per second]), this location may lead to a mea-
sured level lower than actual. This negative level offset is due to
localized areas of low pressure near the bubble line outlet; the
size of the offset depends upon the velocity of the flow stream and
the configuration of the flow stream channel. You can, of course,
adjust the level at the flow meter to compensate for the error.
However, this is only effective if the flow rate and velocity are
essentially constant. If the flow rate drops and the velocity
decreases, the level adjustment you made earlier will be
incorrect for the new, lower velocity and flow rate.
If the outlet is above the zero
level, the flow meter cannot
measure this head range.
Attach bubble line to the side of
the flow stream.
Bubble
Line
End View
Side View
Distance as Prescribed for
Primary Device
Bubble line
Zero Liquid Level
of Weir or Flume
Vertical location of outlet end of bubble line ranges
from ½” (1.3 cm) above. To 6” below the zero level.
(The preferred location is at least 1” to 2” [2.5 to 5 cm]
below the zero level.)
Flow
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