Booklet 7 - Flow Measurement Methods in Open Channels

1.12.7. PrecisionIt is important not to expect to obtain a very high degree of precision. Measurements where the margin forerror is less than 1 % may be unrealistic and often very expensive, whereas a margin for error of 3 % to 5 %, or10 % is quite acceptable in most cases (3) .Under normal conditions of use, where a device has been installed according to stringent codes and by skilledindividuals, most measurement methods produce a margin for error of less than 10 %.1.12.8. Range of measurementWhen selecting a method of measurement, it is important to examine the ratio between the minimum andmaximum discharge, in order to select an instrument that creates an optimum water depth differential.It is also important to ensure that the method chosen produces the same type of precision of measurements forthe range of flow values that are likely to occur.Table 3 shows a chart that can be used for a quick comparison of devices and to select the device best adaptedto the situation. A number of publications provide these types of charts (4) .1.12.9. Important figuresAlthough precision is required for the compilation of data collected, precision should not be estimated with aseries of figures in excess than what is allowed for the instrument with the least precision. This will result in aprecision that does not exist and can cause confusion (2) .Page 30 of 223Booklet 7 - Flow Measurement Methods in Open Channels