Constant drip speed

[s060340]

Hi all,

I want to make a dripper with a constant drip speed in the range of ~1Hz
In order to have the drip speed constant, one needs to make sure the water pressure stays constant, so I made two designs that do that. Does any of you know either of these might work to give a constant dripping speed? Thanks in advance!

[tiny-tim]

Hi s060340! Welcome to PF! :wink:

They look the same to me …

both should work, for exactly the same reason: your design keeps the water surface at a constant height, so the pressure will always be the same. :smile:

[Danger]

Agreed that they look identical other than the level of the transfer pipe on #2, which will limit the level (I think) in the tank. Anything that maintains, more or less, the level should work fine. A toilet tank float is a reasonable analogy.

[Molydood]

What happens to the overflow water? Does it flow back to a reservoir where the water is pumped from? It seems a bit wasteful if it isn't recirculated, but then I suppose you could have a tap on your input pipe and regulate it until the overflow rate was minimised (ie when the input flow rate was approx the same as the output drip rate)

[s060340]

yes the water flows back to a reservoir that is pumped up again.

I tried design 1 once but that resulted in a constand flow, rather than dripping. I thought maybe the chamber in design 2 would create a vacuüm while water flows out, so the flow stops and continues in dripping.. would that be right? or does the pressure change 'commutes' to the left chamber which effectively yields design 1?

[reasonableman]

You are right that designs 1 and 2 will behave the same (except when there is a high level of water). The pressure change in propagates at the speed of sound so will be transferred very quickly.

I think you are looking at the wrong part of the system here. The speed of the dripping is an interplay between the surface tension and flowrate. Yes changing the pressure changes the flow rate but the size of the hole is also important.

As surface tension is also a consideration the shape of the hole also matters.

I think this is a pretty complex problem to model so experimentation is probably the best way to go.

[Molydood]

yes the water flows back to a reservoir that is pumped up again.

I tried design 1 once but that resulted in a constand flow, rather than dripping. I thought maybe the chamber in design 2 would create a vacuüm while water flows out, so the flow stops and continues in dripping.. would that be right? or does the pressure change 'commutes' to the left chamber which effectively yields design 1?

for design 2 there will be no vacuum because the water level remains approx. as it is in the open section. There will actually be a higher than ambient pressure in that sealed air space from when the water first entered. Anyway, design 2 is not adding much from what I can see, other than it separates the output section from the potentially turbulent water in the open section, but I don't think that is going to have any effect in the real world and as reasonableman says, this is more to do with the size of the hole once you have the water pressure constant.