Ok, totally left field problem.

Who can explain why my dripper slows down???

In the picture you'll see I have attached a garden retic dripper beneath a container. Initial rate set to 40 drips per minute. A float valve keeps the water level constant +/- 10mm.

But for some unknown reason, the drip rate always slows down over time even though the water level is more or less constant.

Why does it do this? I need a constant rate +/- 5%

Any suggestion people?

I should have mentioned the reduction is evident within 30mins so I doubt algae but I'll give filtered water a go and see if that makes a difference.

It has been a while since i studied fluid mechanics. But here are some ideas.
Is it a reliable adjustable orifice/valve?
How deep is the water in the tank (head)? If it's only a shallow tank you might have to reduce the high-low limit in the head tank to reduce the gravity effect of water on the orifice. Saying that... 10mm difference won't be of great effect.
Is the temperature stable and not heated by a pump? The kinematic viscosity of water is reasonably stable
Is there any vibration due to a pump or float valve fluctuating?

Guess the first thing you could try fully open the adjustable valve and attach a different type or fixed orifice to the output. Also think about valves made of different materials that could be less effected by temperature. A simple plastic stop cocktail or needle valve might be better than a brass one.
At this stage don't worry how many drips per min... just that it's constant. The drip rate can be adjusted once the problem is sorted

The orifice is just a garden dripper so not exactly a precise instrument.
The high/low limit is just governed by the drip rate so it's not set as such. As it drips, level drops, float value open and brings level back up. Only changes by <10mm or approx 5% so minimal impact.
Water pressure is maintained by mains scheme water pressure, ie no pump, so temp and hence viscosity should remain relatively constant.

I think I need to source a better dripper or a needle valve. I wonder if the plastic is swelling a brass one might be better. No sure.

The limiting rate is definitely the dripper not the float value. Even if I removed the dripper, the float value would be able to keep up with the volume.

Gotta be sediment blocking dripper I reckon. Maybe take dripper supply from point above tank bottom. Perhaps even upside down U bend take off so sediment can play no part.

we have a tap that has a gasket made of some material that expands after the tap is opened, meaning the flow rate through it reduces and we have to re-adjust the tap to suit. Perhaps something like that is happening?

On the guitar subject, tuning before playing is like pumping a kite before kiting - it just has to be done and it's part of the ritual.

Phoney said..
Wait, why do you have a garden dripper system on your kite?

lol - moved to the general forum...

oldbones said..
Gotta be sediment blocking dripper I reckon. Maybe take dripper supply from point above tank bottom. Perhaps even upside down U bend take off so sediment can play no part.

^ I was going to say something like this...

Just curious, if you have mains pressure, why not regulate just from that (without the header tank)?

Maybe a pressure regulator then straight into you flow control valve...

Small hole regulators are notoriously unreliable if you require constant flow rates. Specially at low pressures.
Microscopic sediments build up around the regulator hole and block it up, eventually completely.

A more reliable and constant rate can be achieved using a long capillary tube of much greater diameter than the single jet.
The restriction is then supplied by friction in the length of the tube.

The flow rate will still diminish over a long time due to sediment and algae growth on the inside of the tube but it usually takes weeks or months or years even if the fluid is clean.

I have nothing to value add to this thread except curiosity as to what the rig is for?

I wouldn't worry about it too much, pot plants don't really mind if the watering isn't totally constant.

Freddofrog said..

Which drips are slowing down? the ones coming out of the float valve or the ones coming out of the adjustable dripper?

There both Variable. The Adjustable dripper being gravity . And the float valve being How much the float is opening up the ports/port in the demand valve.

Most intriguing post ! May I ask, FreddoFrog...(1). why 40 drips/minute ? (2). You actually counted them ? (3). Is it for your garden ?

I am curious because I am running an automatic reticulation in my garden. Without mains water, I can only rely on gravity. Unfortunately, the water level in my rain water tank varies depending on the amount of rain I received, hence the water pressure. Your system resembles a toilet flushing unit, perhaps I can learn a few things from your project?

To prevent dirt getting into the system, I have an "In-line" filter from Bunnings which does a good job to prevent larger particles from entering the drips. I still get very fine silt and dust getting through. A good flushing yearly was needed. My main problem are ants. They found their way into the drip heads searching for water in hot days, and can cause a total blockage at times !

LostDog said..
we have a tap that has a gasket made of some material that expands after the tap is opened, meaning the flow rate through it reduces and we have to re-adjust the tap to suit. Perhaps something like that is happening?

On the guitar subject, tuning before playing is like pumping a kite before kiting - it just has to be done and it's part of the ritual.

My shower taps are like this, I don't like wasting water so I adjust hot and cold for a fairly low flow. But after a few minutes, the cold flow reduces, so I have to add more cold, then the hot flow reduces, so I add more hot. It takes several minutes for the flow to stabilise. My wife on the other hand likes a generous shower and doesn't have the same problem.

I wonder why drip rate is so important. The best way to ascertain the flow rate is to measure the amount of fluid within a set time. Droplets are determined by surface tension of the fluid. I wonder if the temperature difference affects the drips ?