BS ISO 4360:2020 pdf download.Hydrometry一Open channel flow measurement using triangular profile weirs.
7 Maintenance
Maintenance of the measuring structure and the approach channel is important to secure accurate continuous measurements.
The approach channel shall be kept free of silt, vegetation and obstructions which might have deleterious effects on flow conditions specified for the standard installation.
Where used, gauge wells and their entry from the channel shall also be kept clean and free from deposits. The downstream channel shall be kept free of obstructions which could result in non-modular flow.
The weir structure shall be kept clean and free from clinging debris and care shall be taken in the process of cleaning to avoid damage to the weir crest.
Head-measurement piezometers, connecting conduits and gauge wells shall be cleaned and checked for leakage. The device used to measure head shall be checked periodically to ensure accuracy.
If a flow straightener or similar is used in the approach channel, it shall be kept clean.
8 Measurement of head(s)
8.1 General
Where spot measurements are required, the heads can be measured by staff gauges, hooks, points, wires or tape gauges.
Where continuous records are required, recording gauges shall be used.
Devices for the measurement of head are described in ISO 4373141.
Periodic checks on the measurement of the head in the approach channel shall be made.
The accuracy of the head measuring device shall be considered when considering the uncertainty of the flow measurement (see Clause 10).
NOTE As the size of the weir and head reduces, small discrepancies in construction and in the zero setting and reading of the head measuring device become of greater relative importance.
8.2 Location of head measurement(s)
8.2.1 Modular (free) flow
Flow is modular when it is independent of variations in tailwater level. This requirement is met when the tailwater total head is equal to or less than 75 % of the upstream total head.
The location for the measurement of head on the weir should be at a sufficient distance upstream from the weir to avoid the region of surface drawdown. On the other hand, it should be close enough to the weir to ensure that the energy loss between the section of measurement and the control section on the weir shall be negligible. Taking these considerations into account, the head-measurement section shall be located at a distance between 2 and 4 times the maximum head (2 hmax to 4 hmax) upstream of the crest.
8.2.2 Non-modular (drowned) flow
A significant error in the calculated discharge will develop if the tailwater total head above crest level exceeds 75 % of the upstream total head, unless a crest tapping or downstream (tailwater) measurement is provided and two independent head measurements are made.BS ISO 4360 pdf download.