I'm confused. Wouldn't a depth gauge (Ie a level transmitter) be more appropriate? The whole point of a properly set up weir is that the flow (which is equivalent to velocity) is directly related to the height of the fluid level above the weir.
Since the height is proportional to the flow, a camera could be used to measure it. I don't know of any package right off hand that would have a specific setup for weir measurement. But given lighting conditions that let you find the edges of the flow as it exits, it would be a simple pixel counting measurement. With a commodity camera and frame grabber. I think you could get 1% resolution or better depending on the variation you have to cover. I'd bid this in a heartbeat:^).
> Since the height is proportional to the flow, a camera could be used to measure it
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It is not exactly proportional. The close approx equation is
Q= W*H*C_v *(2gH)^0.5 =A*C_v *(2gH)^0.5
Where
W= width of the weir
H= water level above weir
C_v = Coefficient of velocity (Dimensionless)
g = gravitational acceleration
C_v has to be determined for a few elevations before you can use this approximation. It value so much depends type of materials, surface finished and shape of the weirs.
Well, the height can be measured with good accuracy anyway:^)
I will assume the others are known or can be obtained by other means and used for calibration. A camera would be unique in providing instant verification or information on an unexpected reading.
> Why not just bite the bullet and buy one of the ultrasonic transmitters that are designed for just that?
Tel me more about this equipment if you know something, Mr. Roy. I plan to buy 4 pieces. I have two questions to ask you.
1. Can this equipment gives 2.5mm resolution for 2 m measurement range?
2. I have a very narrow slot to insert this equipment through. Typically about 2 cm "grating slot". I have 2 m range to measure. If I insert this equipment through the slot it will be located about 3cm above water level. Can I still get a good reading?
Siemens' OCM III has a resolution of 0.2mm but the minimum distance above the high water level is either 300mm (blanking zone) for the 0.3 to 1.2m range or 600mm for the 0.6 to 3m range.
It cannot shoot through a 2cm slot. It has to have a clear, unobstructed shot over its range (as do others)
It (and others like it) does the weir flow calculation, but it has to be mounted for a clear shot.
An alternative for raw level is a submersible head pressure sensor. Siemens P-MPS has a model with a 2mH2O range, which sounds ideal for you. It's IP68 and goes under the water and measures the head pressure above it. Conventional 4-20mA output. You'll have to crank through the weir flow calculation.
