Booklet 7 - Flow Measurement Methods in Open Channels

Usually, in a calculation that involves multiplying or dividing with figures from observations, the result shouldcontain the same number of figures as the value that has the least number of figures.Application of this rule implies that the last figure does not necessarily represent a value but rather a moreprobable value.1.12.10. Repeatability and reliabilityRepeatability, for a measuring instrument, is the ability to always produce the same result when used under thesame conditions.Reliability is the ability of an instrument to operate without failure, under controlled conditions and for aspecific period of time.It is important to ensure that information gathered is accurate and that fluctuations in measurement are theresult of an industrial process, not the instrument. This criterion is particularly important because inspection ofinstruments is often centralized and an operator cannot immediately determine if an instrument is operatingcorrectly. The period between a visual inspection and the date a reading is logged often raises questions whenresults are interpreted.1.12.11. Installation detailsManufacturing a layout that complies with code is recommended, rather than trying to estimate the effects ofconditions that do not comply with regulations and trying to correct values obtained under these conditions.Primary measuring devices should be located in a straight section of flow and should be perfectly aligned withthe direction of flow.Flumes and weirs must be placed level longitudinally and transversely, in other words, in the direction of flowand perpendicular to flow.In the approach section, there should be no branch lines, curves, heads of water or abrupt changes in thechannel bottom.To avoid generating a submerged flow due to backwater, the channel should have enough capacity to allowwater to discharge immediately at the highest flowrate.Installation of primary measuring devices equipped with stilling wells is recommended. Stilling wells areconnected to the measuring device through small conduits. They must be positioned in locations where waterdepth measurements are taken in the flume. The dimensions of wells and conduits must correspond to thestandards prescribed in Table 4.Stilling wells should be wide enough to allow a flowmeter level detector to be installed and operated and thewell to be cleaned.A staff gauge (ruler) must be permanently placed at the measuring point.Page 32 of 223Booklet 7 - Flow Measurement Methods in Open Channels