![]() This implies that it is possible to create almost any characteristic equation we might like just by carefully shaping the weir’s notch in some custom form.Ī good example of this is the so-called proportional or Sutro weir, which is designed to have a linear relationship between head and flow rate: Q = Volumetric flow rate (cubic feet per second – CFS)Īs you can see from a comparison of characteristic flow equations between these three types of weirs, the shape of the weir’s notch has a dramatic effect on the mathematical relationship between flow rate and head (liquid level upstream of the weir, measured above the crest height). This dependence of notch area on flow rate creates a very different relationship between flow rate and liquid height (measured above the crest) than the relationship between flow rate and differential pressure in an orifice plate: ![]() As one might expect, the mathematical functions describing each of these flowmeter types is unique! Weirs are variable-pressure, variable-area flowmeters. Rotameters are constant-pressure, variable-area flowmeters. Note : Orifice plates are variable-pressure, constant-area flowmeters. This effectively increases the cross-sectional area of the weir’s “throat” as a taller stream of liquid exits the notch of the weir (Note). As liquid flow is increased even more, a greater pressure (head) will be generated upstream of the weir, forcing the liquid level to rise. The height of liquid above the crest of a weir is analogous to the pressure differential generated by an orifice plate. This height of liquid upstream of the weir represents a hydrostatic pressure, much the same as liquid heights in piezometer tubes represent pressures in a liquid flowstream through an enclosed pipe. In order for this to happen, the level of the liquid upstream of the weir must rise above the weir’s crest height. As liquid begins to flow through the channel, it must spill over the crest of the weir in order to get past the weir and continue downstream in the channel. The following photograph shows water flowing through a Cippoletti weir made of 1/4 inch steel plate:Īt a condition of zero flow through the channel, the liquid level will be at or below the crest (lowest point on the opening) of the weir. A V-notch weir has a triangular notch, customarily measuring either 60 or 90 degrees. A Cippoletti weir is much like a rectangular weir, except that the vertical sides of the notch have a 4:1 slope (rise of 4, run of 1 approximately a 14 degree angle from vertical). Three styles of weir are shown in the following illustration the rectangular, Cippoletti, and V-notch:Ī rectangular weir has a notch of simple rectangular shape, as the name implies. ![]() The first form of open-channel flowmeter is the weir, which is nothing more than a dam obstructing passage of liquid through the channel. By measuring this liquid level rise, it is possible to infer the rate of liquid flow past the obstruction. If an obstruction is placed within a channel, any liquid flowing through the channel must rise on the upstream side of the obstruction. A very different style of variable-area flow meter is used extensively to measure flow rate through open channels, such as irrigation ditches.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |