April 6, 1998 Design News
From the regional editors
Flowmeter combines flume and Venturi
measurements under full or partially full pipe flows
by David J. Bak, International Editor
Austin, TX--Municipalities and metropolitan
areas require flow data when a) billing industries for
wastewater disposal, b) estimating sewage treatment
plant capacity, and c) performing inflow and infiltration
studies. A patented flow tube, designed for all flow
conditions, is said to yield more accurate flow measurement
data than presently used level-only or velocity-area
Because level-only meters such as flumes and weirs
cannot be used for full-pipe flow, their use is limited
to open channel applications. Velocity-area meters (flow
= velocity x area) handle full-pipe flows. Accuracy,
however, depends on thorough velocity profiling--a lengthy,
unpleasant, and sometimes dangerous task performed at
the bottom of a manhole.
The DataGatorr flowmeter, developed with grants from
the U.S. Department of Agriculture, overcomes the shortcomings
of competing designs. Three parts comprise the whole:
flowtube, electronics, and software for communication
between the electronics and a PC. Flowtube design is
a combination flume and Venturi tube.
Under open-channel conditions, the flow tube functions
as a Venturi flume, i.e., a constriction in an open
channel so proportioned as to cause the occurrence of
flow at critical depth. Under full-pipe conditions,
the flowtube functions as a Venturi tube, i.e., a constriction
in a closed conduit designed to increase the velocity
of flow and decrease pressure.
A predictable relationship between flow depth in the
flowtube throat and flow depth immediately upstream,
says Marappan Natarajan, president and engineering director
at Renaissance Instruments, permits simultaneous filling
of the throat and upstream pipe. "This feature,"
he claims, "eliminates inaccuracies during the
transition of flow from open-channel to full-pipe conditions."
As a result, the DataGator system is good for both partially
full and full-pipe flow metering.
Three pressure transducers--inlet, throat, and outlet--measure
hydrostatic pressures. Flowtube symmetry accommodates
reverseflow with accuracy equal to forward flow. In
addition, profile independence and fixed flowtube dimensions
for a given pipe size allow precalibration at the factory.
This, in turn, reduces
installation to approximately ten minutes--a welcome
change for operators used to spending hours inside a
manhole with velocity-area meters.
Additional details...Contact Renaissance Instruments,
2013 Wells Branch Parkway, Suite 117, Austin, TX 78728-6904,
Tel: (512) 251-6647.
Clean water flow