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

Setting water level recorder using a pond Place temporary dams upstream of stilling-well pipe and downstream of flume Fill pond so that water level is at least 2 inches above crest, or at most common water level for flume operation Install recorder and all related equipment in position to record Observe recorder output to be sure pond is watertight Read head above crest at control section using a ruler –Control section is 1/4 to 1/3 L from downstream edge of crest Adjust recorder to match measured head Repeat preceding steps at a different water level

Setting recorder in an empty canal

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

Setting Recorder in a Flowing Canal

To sensor To funnel on crest

Setting Recorder in Flowing Canal Install water level recorder and attach point gage and a funnel or cup to rigid support that spans control section Attach transparent hose to the perforated sensing pipe and locate sidewall holes of sensing pipe at the gaging station Take point gage reading on the weir crest at critical section Raise pointer and swing funnel or cup beneath pointer Lower 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