However, they may be “fooled” by foam or debris floating on top of the liquid, as well as waves on the liquid surface. Ultrasonic level sensors are completely non-contact, which means they cannot become fouled by the process liquid (or debris in the process liquid). Perhaps the most common technology for weir/flume level sensing is ultrasonic. Once a weir or flume has been installed in an open channel to measure the flow of liquid, some method must be employed to sense upstream liquid level and translate this level measurement into a flow measurement. In such applications, flumes are often the more practical flow element for the task (and more accurate over the long term as well, since even the finest weir will not register accurately once fouled by debris).

If the liquid stream being measured is drainage- or waste water, a substantial amount of solid debris may be present in the flow that could cause repeated clogging problems for weirs.

Q = Volumetric flow rate (cubic feet per second – CFS)įlumes are generally less accurate than weirs, but they do enjoy the advantage of being inherently self-cleaning. Q = 4LH 1.53 1-foot to 8-foot wide throat Parshall flume Q = 3.07H 1.53 9-inch wide throat Parshall flume Q = 2.06H 1.58 6-inch wide throat Parshall flume Q = 0.992H 1.547 3-inch wide throat Parshall flume The following formulae relate head (upstream liquid height) to flow rate for free-flowing Parshall flumes: Parshall when it was developed in the year 1920. One of the most common flume design is the Parshall flume, named after its inventor R.L. Like weirs, flumes generate upstream liquid level height changes indicative of flow rate. As one might expect, the mathematical functions describing each of these flowmeter types are 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 flow stream 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 the 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. At 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.

Flume water