Help on water height going over, overflow.
 

Ok, I need to calculate the height of the water over my overflow (not yet built). Overflow will be coast to coast 18".

The tank will have a total of 18" x 15.5" of area in which the water will be flowing into. There will be a total flow of 200gph into the tank from the sump.

This is what I figured it out as.

200gph = 0.05555gps rounded to 0.06gps (for safety)

gallons converted to cubic inches: 0.06g / 0.004cu in = 15 cu in

Volume: 18" x 15.5" x H = 15 cu in therefore the height above the overflow would be 0.05376" rounded to 0.06" for safety.

Does this all sound right?

You know, I just realized something.. all I really need to know is that there will be 0.06gal of water in the tank above the overflow at any given sec. So, really would that not be the amount of water flowing back to the sump? just 0.06 gals?

Correct............your going to put a valve on your return and your overflow plumbing also ...so you can adjust flows as needed right?

Well, I am doing the Herbie style overflow, just a single gate valve on the overflow end to adjust the water height so it is even with the flow from the return pump. I have spec'd the return pump to so it has a max of 200gph after factoring in head pressure. The tank is only a 34gal cube, I think 200gph should be plenty to service this size of tank. All the other flow will be from the closed loop I am putting in.

if your tank is 24" high the over flow should be around 22.5 high so 1.5 " from the top down if that helps so your coast to coast should be 1.5 " from the top down

why would it be necessary to have the overflow 1.5" from the top?

Theres' two places that can help you (here) RC drain calculator. You enter the gph and it gives the weir length with about a 1/4" over the weir lip. Other place is on RDO (download) for their Overflow calculator spreadsheet. It's for a slotted weir but if using a straight weir put number of opening as 1. Can easily play with the variables to match the height you want against flow and length.

It all seems to go back to Bazins formula Q = 0.66 x cB x (2g)0.66 x H1.5 http://www.aquatext.com/calcs/weir%20flow.htm and crank it out

No chad what I was saying is the top of you box would br around 1.5 " below the top of the tank.

I understand what you mean. I am just not sure why it needs to be 1.5" from the top of the tank. If there is such low flow going through the overflow and it being coast to coast, I would think I should be able to have it much higher..

I had figured to put it around 3/4" from the top, so I do not see the water line.