Upstream conditions should promote laminar flow conditions at the
inlet of the flume. Channel turns, tees, elevation drops and other disturbance creating
situations immediately upstream (25 pipe diameters) of the flume should be avoided.
Excessive slope in the upstream piping will result in poor accuracy due to turbulence at
the measuring point. Small lines to have a 2% maximum slope, larger sizes to have less
slope.
The crest of the flume (floor of the throat section) must be level both longitudinally
& transversely.
Downstream piping should not permit submerged flow conditions to occur. A fall
immediately downstream of the metering station can eliminate the possibility of submerged
flow conditions. Long, narrow, flat or undersized channels can result in a backwater
effect at the flume and should be avoided.
Flume must be installed at an elevation such that at zero flow conditions the throat of
the flume is dry. A high standing water level in a line at zero flow conditions will
result in inaccurate flow metering.
In a Palmer-Bowlus flume, depth measurements to determine the flow rate are made at a
distance D/2 upstream of the beginning of the ramp to the crest.
Palmer-Bowlus flumes are designed with a "wet zero". In a zero flow condition
water pools upstream of the throat of the flume. The top of the pool will be even with the
floor of the throat. The water surface is the "wet zero" level of the
Palmer-Bowlus flume.
Measurements to determine the flow rate through the flume should be taken using this
level as zero. The distance of the "wet zero" above the bottom of the flume is
equal to D/6. Depth gauges and flow transmitters should be calibrated to indicate zero
when the water level is equal to the "wet zero".
