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Name: Theresa
Status: other
Grade: other
Country: USA
Date: Summer 2013

If you are filling an above ground pool - does the water come out of a hose faster if the hose it placed on top or near the top of the water versus at the bottom of the pool? I was filling my pool yesterday and I had the hose closer to the bottom of the pool (pool is 52" deep), and my brother told me that the water would come out of the hose quicker if I moved the hose closer to the top of the water because the pressure of the water was closing the water flow down.

The hydrostatic pressure of 52" of water is about 2 psi. If the water is coming out of the hose at X psi, then there would be a reduction of 2 psi when the hose is immersed at the 52" depth. The flow rate change can be calculated as ( delta pressure*2/density)^0.5

Sri R Narayan

Hi Theresa,

Thanks for the question. The water will come out of a hose faster if the hose is placed on top of the water and not submerged. If the hose is submerged, there will be more pressure (due to the water in the pool above the hose) and this pressure will inhibit the flow of water. The extent of the inhibition is not that great, but it could mean that filling the pool may take an hour or so longer.

I hope this helps. Please let me know if you have more questions. Thanks Jeff Grell

Hi, Theresa

Thank you for submitting an excellent question.

If we suppose for a moment that the pool were empty so that the hose end is not in water: The higher you locate the end of the hose, the higher the water must be lifted before it exits the hose. This added height requires increased pressure to push the water out of the hose and would reduce the fill rate.

But now if there is water in the pool so that the hose end is immersed at either height, the pool water will exert increasing pressure back on the hose end as you lower the end of the hose. The increased pressure per meter of depth in the pool exactly balances the decreased pressure per meter of height that would be required in the hose if you lowered the end of the hose if you had no water in the pool. So the net result is that so long as the end of the hose is in the water, raising or lowering the end of the hose should not affect the fill rate. You will get the fastest filling when the pool is empty and the hose end is at the bottom. The filling will slow somewhat as the height of the water raises in the pool.

If the depth of the water in the pool was high enough the water would stop flowing. This is theoretically possible but is not likely!

The principle here is similar to that of a siphon.

Best regards, Bob Zwicker

Yes, water comes out of a hose at a rate that depends on the ambient pressure at the output. (Actually, the rate depends on the difference between the input pressure and the output pressure, but you are only varying the output pressure.)

Tim Mooney

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