All critical-flow devices determine flow rate by measuring one or more water levels and computing flow from a calibration equation.Water Level Sensors and Their Importance

Typical Rating EquationQ = K1(h1+K2)UEven if h1 can be measured perfectly, there is still some uncertainty in the computed dischargeDifferences in approach flow conditionsUncertainties in theory or calibration methodsVariations in construction, etc.2% for long-throated flumes and broad-crested weirs, 3 to 5% for most lab-calibrated devices operating in recommended ranges

Effect of SystematicHead-Measurement ErrorsIf h1 is consistently too high or low (systematic error), then Q is also too high or too lowPercentage error is magnified by exponent U

Source of error can be the sensor itself, or the zeroing of the sensor relative to the crestAccumulated flow volumes will be in error by a similar percentage

Effect of RandomHead-Measurement ErrorsIf there is random uncertainty in the measurement of h1, it adds to the total random uncertainty in QEffect is magnified by the exponent UErrors in accumulated flow volume over time will be reduced by averaging effect

Effects of Flume DesignMagnitude of systematic or random errors in head measurement is often constant, independent of water level.So, a flume or weir that creates a larger head for a given flow rate leads to a lower percentage error in head and lower percentage error in measured flow

Sensor OptionsDirect-reading devicesStaff gages, point gages, dipsticks, etc.FloatsPosition of float measured by potentiometer, encoder, etc.Pressure transducers and bubblersMeasure hydrostatic pressure to determine depthAcoustic sensorsMeasure water surface by its ability to reflect soundAffected by speed of sound travel through airCapacitive/resistive sensors

Improving Head-Measurement AccuracyStilling wellsReduce waves, introduce lag, can become pluggedProtect sensor, provide more stable environmentWave suppressorsReduce waves, add head lossReduce approach velocity & Froude numberReduces wavesUse better sensors

Zero-Referencing of Water Level Sensors and Gages#1 source of systematic errors in flume measurementsImportant at small flows because even small errors are large on a percentage basisImportant at large flows because Q is proportional to h1U where U is 1.5 to 2.5

Setting water level recorder using a pondPlace temporary dams upstream of stilling-well pipe and downstream of flumeFill pond so that water level is at least 2 inches above crest, or at most common water level for flume operationInstall recorder and all related equipment in position to recordObserve recorder output to be sure pond is watertightRead head above crest at control section using a rulerControl section is 1/4 to 1/3 L from downstream edge of crestAdjust recorder to match measured headRepeat preceding steps at a different water level

Setting recorder in an empty canal

Setting Recorder in an Empty CanalSimilar to the pond procedure, but funnel, tubing and stilling well take place of pondMeasure head at control section using two readings from a point gage mounted on a stiff support

Setting Recorder in a Flowing Canal

Setting Recorder in Flowing CanalInstall water level recorder and attach point gage and a funnel or cup to rigid support that spans control sectionAttach transparent hose to the perforated sensing pipe and locate sidewall holes of sensing pipe at the gaging stationTake point gage reading on the weir crest at critical sectionRaise pointer and swing funnel or cup beneath pointerLower transparent hose below water level at crest until air is purged from hose. Lower cup beneath water and attach hose to cup underwater. Raise back above water and place beneath pointer. Allow water level in cup to stabilize.Read level of water in cup using point gage. Difference in readings is sill-referenced head.Repeat after flow has changed