weir flow measurement

[cvg]

I am trying to specify a flume or weir, plus automatic recorder for measuring water flow in a toe drain. Client is asking for a product that can be installed in a manhole and fiberglass with automatic recorder. Flow rate is limited to gravity flow through a 4 inch drain pipe. I had specified a steel v-notch weir. Would a Palmer-Bowlus or Parshall Flume have any benefits over a v-notch? As far as recording flow, is ultrasonic the recommended choice? What manufacturers would you recommend?

 

[ccor]

A flowrat from Rocky Mountain Instruments can do both the measuring and recording just by placing it in the pipe. Its been a while since I have spec'd one and to my surprise I couldn't easily find info online. Worth checking into though.

Not that familiar with the palmer-bowlus but a Parshall flume is used where you can't have a severe backwater condition. I doubt backwater is much of a concern on such a small pipe.

 

[Ussuri]

I notice in your post you mention 'drain' and 'manhole', is it possible you can get debris of sorts in the flow? If so I would avoid a V notch as they tend to catch and store gunk.

I have never heard of a 'flowrat' but I have often used magflow meters on pipes.

The most common flume I have used is a rectangular throated flume, which is similar to a Parshall Flume. they are very simple and the flow is calculated and logged using an ultrasonic and data logger.

 

[bimr]

Give Plastifab a call. They should be able to help you over the phone. I believe the v notch that you selected is recommended for accuracy with lower flows. The parshall flume would be recommended if you have higher flows and suspended matter in the stream.

http://www.interfels.com/English/Pdf/V-Notch Weir.pdf

http://www.plasti-fab.com/flumes/parshall/parshall.html

 

[cvg]

hopefully, debris won't be a problem as this will be subsurface flow collected by a french drain. weir/flume in a covered, locked manhole. I would suspect that flow rates will be in the normal range of 1 - 2 gpm, but possibly up to 10 - 20 gpm

 

[Ussuri]

Ok, a magflow wont work in your situation. My mistake.

 

[bltseattle]

In my opinion 1-2 gpm is too low of flow to measure accurately using a weir or flume. I've looked into this issue quite a bit for a green roof monitoring project we're conducting in Seattle (low intensity rainfall with a greatly dampened runoff response). Your depth measurements must be highly accurate or else your error in the flow estimate can easily exceed 50%. You can see this for yourself by using the weir or flume equation: back-calculate the delta-H that corresponds to 1 or 2 gpm, then adjust that depth by the tolerance of your stage measurement device. Recompute the flow using the revised depth and you will see a substantial error (at low flows).

Example: Triangular notch weir, 28.07 deg notch
At head = 0.10 ft, the flow is 1.05 gpm using standard weir equation (Reference: ASTM D5640-95, D5242-92). At head - 0.11 ft, flow estimate is 1.32 gpm. So a depth measurement error of 0.01 ft would lead to about a 25% error in the computed flow.

IMO it is going to be quite difficult to achieve 0.01 ft or better accuracy in a system with moving water.

So if quantifying the 1-2 gpm range of flows is critical you should consider another system (e.g. tipping bucket system). If you are really after the higher flows, say 10 gpm, the flume or weir could be an okay choice. The same calculation for a weir at 10 gpm, with a 0.01 depth measurement error, only throws off the flow estimate about 10%.

 

[cvg]

Current situation is a 90deg v-notch with a gauge mounted on the weir box. The gauge is read manually every few months when somebody makes it out to this remote spot. And then, only if they venture into the wetland / marsh at the toe of the dam. With an ultrasonic level sensor, flow meter and data logger, I will now have daily or perhaps hourly data on the seepage through the dam. I am not particulary concerned with accurately measuring the low flows of a few gpm. I am more concerned with tracking the flows and detecting any large increases. For instance, if flow increases, especially if it is not in relation to a rising water level in the lake, then I can investigate to determine if there are any seepage related safety problems with the dam.

 

[sam74]

A product called Flo-Dar could provide what you are looking for with out the use of a weir. Check out this website

http://www.marsh-mcbirney.net/articles/Understanding-the-Flo-Dar-Flow-Measuring-System.pdf

 

[hydrae]

I would suggest a 60 degree V flume, 1 gpm at 0.08 ft, 30 gpm at 0.3 ft. It is self clearing unlike the weir.
For automatic monitoring I would use a Vibrating wire weir monitor by GeoKon. I have found it gives reliable results without drift unlike a low range pressure transducer or ultrasonic devices.
To resolve BLT's issues on moving water, use a stilling well and stilling chambers inside the well.
Hydrae