4230 Flow Meter
Section 3 Installation
3-6
than does the
1
/16" (0.16 cm) ID bubble line. Likewise, increasing
the bubble rate from 1 to 2 bubbles per second causes a 2
1
/2 times
increase in power usage by the air pump.
The power the rest of the flow meter uses (except the printer) is
constant and cannot be changed, but you do select bubble rate
and bubble line size (as well as printer settings) and these items
do significantly affect power consumption. If your installation
must be battery-powered, consider the effect these factors have
very carefully before setting up the flow meter. One indication of
power consumption is the pump duty cycle. It should run less
than 20%. You can check this value in the STATUS menu of the
program.
Use caution when setting the bubble rate any higher than
one bubble per second. For greatest accuracy in small
channels, where you use either small ID tubing, or where the
bubble line is long, Isco does not recommend bubble rates over
one bubble per second. When measuring low water levels (a few
inches, typically) it is especially important to pay attention to the
bubble line inside diameter, length, and bubble rate.
Setting a faster bubble rate causes an increase in frictional back
pressure as a result of the tubing wall's resistance to increased
air flow. For example, consider a 25 foot (7.6 m)
1
/16" ID bubble
line; the back pressure at one bubble per second is approximately
equal to
1
/10" (0.25 cm) of water. If you increase the bubble rate to
2 bubbles per second, the back pressure rises to approximately
2
/10" (0.51 cm) of water, or double what it was at one bubble per
second.
Another problem that occurs with long, small-ID bubble lines set
at higher bubble rates, is that back pressure varies slightly
during the period between air pump cycles. This will show up as
a change in level on the display, and it will generally also show
up on the chart as a change in level. At the normal bubble rate
this variation is not noticeable, but it will become noticeable if
you set the bubble rate too high. For greatest accuracy in mea-
suring low levels, use short bubble lines with the flow meter set
at one bubble per second. If you need longer lines, use the
1
/8" ID
line rather than the
1
/16" ID line.
3.3.3 Purging a Clogged
Bubble Line
When you use the 4230 to measure levels in small channels, it is
best to keep the bubble rate at one-per-second and use shorter
bubble lines. Larger-ID bubble lines are less likely to clog than
small ones, but require more energy to supply the additional air
needed.
In applications where bubble line clogging is likely to be a
problem, using the 4230 Purge feature (increasing the frequency
of the automatic purge) may be a better solution than increasing
the bubble rate. However, turning on the Purge feature will also
cause an increase in battery consumption.
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