PCWA-L 467.pdf - PCWA Middle Fork American River Project ...

4" STAGE MEASUREMENT
ACCURACY OF BUBBLE..GAGE RECORDERS
This section ofthe report discusses the inaccuracies inherent in any
bubble-gage system for sensing stage; the discussion is not concerned
with such sources of error as datum corrections, sediment deposition
on the bubble orifice, and leaks in the system, which were discussed
in the preceding sections. The principal sources oferror inherent in a
bubble-gage recorder are variation in gas friction, variation in required
bubble-feed rate with rate of increase in stage, and variation
in weight of gas column with stage.
Variation in gas friction.-Friction created by the flow of gas
through the bubble tubing results in the pressure at the manometer
being slightly higher than that at the orifice. If the bubble-feed rate
could be kept co'nstant and temperature did not vary, the friction
would remain constant and the accuracy of the gage would be unaffected,
because the manometer is always set to agree with the watersurface
elevation. However, changes in gas-feed rates cause variation
in the friction of the gas flowing through the tube, and where long
bubble tubes are used, the variation in friction can produce
significant error in recorded gage height.
Inaccuracies due to variation in gas friction can be eliminated by
using two gas tubes-one to feed gas to the bubble orifice, the other to
aet as a static pressure tube to transmit pressure from a point at or
near the orifice back to the manometer.
As a conservative criterion for determining when the use of dual
tubing is desirable, it is suggested that variations in gas friction be
limited to 0.01 ft (0"003 m). If, for a given length of orifice line, an
error no greater than 0.01 ft results from a 100 percent increase in
bubble rate, a single bubble tube will be satisfactory. Figure 36, based
on laboratory tests using the standard U.S" Geological Survey bubble
gage shows the relation between the length of bubble tubing and a
100-percent variation in bubble rate for a gas-friction error ofO"Ol ft.
To illustrate the use offigure 36, assume that a 100-percent variation
in bubble rate represents a change from 40 bubbles per minute to 80
bubbles per minute. The corresponding length of bubble tubing indicated
by the diagram is 370 ft (113 m). Thus any lengtb of single
tubing up to 370 ft could be used in tbis particular case without
introducing a friction error in excess ofO"Ol ft. Ifmore than 370 ft of
tubing is required at this particular site, and a gage-height error
greater than 0.01 ft cannot be tolerated, two gas tubes should be used"
When two tubes are used, they should bejoined at a T-connector, from
whose vertical leg a single tube extends to the orifice. The
T-connector should be located above the normal high-water elevation
to prevent the entry of water into both legs of the system if pressure
71