Booklet 7 - Flow Measurement Methods in Open Channels

1.12.13. CalibrationA primary device may require calibration in the following instances:−−the system does not comply with standards specified in this guide;a serious deterioration of the system is noted.Calibration consists of determining, for varying degrees of flow, a relationship between the flow and waterdepth measured in the primary device. On the basis of this, a new empirical depth/flow relationship can beestablished for the flow of liquid that may be measured by the device.Even in systems have been installed under exacting standard, measuring devices produce an estimated 5 %deviation, it may be difficult to find a method that is precise and accurate enough to perform calibration.Technically, the method that is used should have no error. In practice, however, it is realistic to tolerate a 1 %to 2 % margin of error. Precision (which can improve through repeated measurements) should beapproximately 95 %. Section 5 discusses the precision and accuracy of methods. It is the responsibility of theuser to demonstrate that the method(s) employed complies with the aforementioned accuracy and precisionobjectives or other objectives prescribed by a public authority or in accordance with regulation.Generally, the volumetric method is used only if the following conditions can be respected (5) ;−−−A regular shape tank is available and its capacity can be measured at different levels with a precision of99 %.For each test, water depth in the primary device at the measuring point is stable. The allowable deviationlimits are detailed in the description of each primary device.It takes longer than 90 seconds to fill the measuring tank.The dilution method, which uses salt or a chemical tracer, if:−−−the primary measurement device is too large for use of the volumetric method;the water depth in the primary device at the measuring point is stable, for each test. The allowabledeviation limits are indicated in the description of each primary device;a regular shape tank is unavailable, the capacity of which can be calibrated precisely at different levels.During calibration of the primary device, information detailing its installation and status must be taken down.The calibration procedure and method of measuring water depth and flow must be described in detail. Thisinformation must be kept as a reference document, during the entire period the primary device is in use.Page 34 of 223Booklet 7 - Flow Measurement Methods in Open Channels