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Calculating a meniscus height
http://byap.backyardmagazines.com/forum/viewtopic.php?f=11&t=6913		 Page 1 of 1
Author:  AaronJ [ Jan 28th, '10, 13:47 ]
Post subject:  Calculating a meniscus height
Hi Team,

For those of you with Physics/Math/Eng stills (or better Google search skills than me) I have a question. It s not a purely AP one either, but may have some relevance.

What is the formula for calculating the high of a meniscus for water flowing over an edge:

E.g. I have tank with a standpipe. The standpipe is 50mm so the linear edge (circumference) is approximately 157mm (C=2[pi]r or C=2x3.142x25mm). Assume say a water flow rate of 5000lph through the standpipe, what would the height of the meniscus be?

Cheers,
Aaron
Author:  chillidude [ Jan 28th, '10, 13:53 ]
Post subject:  Re: Calculating a meniscus height
If I understand your question right, Aaron, I can't see how it would be of significance - it's be tiny, say 1-2mm.

Are you sure you're not getting mixed up with the capillary effect ?
Author:  RupertofOZ [ Jan 28th, '10, 14:00 ]
Post subject:  Re: Calculating a meniscus height
Got no idea... but I'm curious as to why you ask... and what does it matter???
Author:  Ian [ Jan 28th, '10, 14:15 ]
Post subject:  Re: Calculating a meniscus height
This link will get you started.
http://www.jfccivilengineer.com/broad_c ... eirs_2.htm

Then look for "Bell Mouth" weirs to more closely apropximate your open top stand pipe.
It's not really a menicus but a "dynamic head".
You won't have to worry about the flow you have described building up too much for your standpipe to fill, about 3.8mm by my calcs. It depends on the actual condition of the rim of the weir; sharp or flat or radiused. Smooth or rough, etc to get your flow co-efficient for calcs.

Cheers IanK
Author:  AaronJ [ Jan 28th, '10, 14:21 ]
Post subject:  Re: Calculating a meniscus height
Ok,

Hopefully I have used the right term and not thrown you guys off the scent….

I'm about to install a 50mm standpipe on my new 600L display tank.

The final height of the standpipe will determine the water height in the tank.

I want the water height to be a couple of millimetres under a horizontal tank brace.

Therefore, I need to get the standpipe height right, taking into considering that the tank water level will be higher than the standpipe and determined by the flow rate (possible around 5,000lph).

I could just set, forget and deal with whatever resulting water height I get, but there is a hydro calculation out there somewhere. So, pedantic as it may be, I’d like to have a little calculating behind the final standpipe height.
Author:  AaronJ [ Jan 28th, '10, 14:22 ]
Post subject:  Re: Calculating a meniscus height
Ah... good one Ian.

Will check it out a bit later. Thanks.
Author:  AaronJ [ Jan 28th, '10, 17:06 ]
Post subject:  Re: Calculating a meniscus height
Hmmmm Nope.... no idea how to apply that theory to what I want to know.
Author:  Dufflight [ Jan 28th, '10, 17:53 ]
Post subject:  Re: Calculating a meniscus height
In my 650ltr tank the water overflows down to a sump. It goes through a sponge and a 20mm bit the sponge didn't cover. 2000lpr pump and it does not have a problem with the 50mm hole that restricks flow to the sump. The spillway when I remove the sponge is about 20cms. The water surface is 5mm ish above the spillway but the flow through to the sump didn't change when I removed the sponge. Just remember its more about how much water can gravity through 50mm per hr. And that the pumps head will affect the total lph that the pump can move.

PS watch out for how you pump water into the tank. I like the flow to suck in air to create bubbles. This also breaks the siphon if the power goes out. The sump will also have to deal with that 5mm of water over the surface of the tank. And if the siphon does not break the tank will drain till the water reaches the inlet pipe. Worked that out the messy way. :mrgreen:
Author:  DéjàVoodoo [ Jan 28th, '10, 22:26 ]
Post subject:  Re: Calculating a meniscus height
perhaps dryfit a 50mm coupling on the very top that could be moved up or down to fine tune the overflow.

Mark
Author:  Ian [ Jan 29th, '10, 08:57 ]
Post subject:  Re: Calculating a meniscus height
Try this one,
For the technically minded the formula for determining the head (h) over a
Bellmouth Inlet lip of the design advised above is: -

h=177.65(DxV)to the power of 0.67

Where; h = head over the B/M lip - mm
D = O/F outlet diameter - mm
V = O/F pipe flow velocity - m/s
This is for an outflow diameter pipe which is half the diameter of the bell mouth lip diameter. Say 50mm bell to a 25 mm outlet.

Cheers IanK
Author:  gemmell [ Jan 29th, '10, 10:59 ]
Post subject:  Re: Calculating a meniscus height
AaronJ wrote:
Therefore, I need to get the standpipe height right, taking into considering that the tank water level will be higher than the standpipe and determined by the flow rate (possible around 5,000lph).

I could just set, forget and deal with whatever resulting water height I get, but there is a hydro calculation out there somewhere. So, pedantic as it may be, I’d like to have a little calculating behind the final standpipe height.


OR

Use a height adjustable stand pipe.
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Author:  Delgrade [ Jan 29th, '10, 20:09 ]
Post subject:  Re: Calculating a meniscus height
DéjàVoodoo wrote:
perhaps dryfit a 50mm coupling on the very top that could be moved up or down to fine tune the overflow.

Mark


id try this one start with it just tight then if you champer the edge of the pipe you should be able to push it down a few extra mm if needed, should seal up nicely but any leaks would be into the drain anyways

Cheers
pete
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