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 am not an engineer 
, but I suppose that over time some algae buildup or some dirt partially blocks the opening of your dripper and slows down the drip rate. The problem could be fixed if you use clean water and anti algae tablets.
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.
Don't know the answer but have you tried a higher pressure (higher head of water)
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
If your float controls the feed of water to vessel then surely you'd have a difference of volume in the tank? If so the this would affect the dripper, the old header tanks in a household would lose some flow as you released the water through a tap, as the supply is trying to catch up with the flow
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.
maybe the same answer will help my Guitar strings stay in tune.
Is your dripper a ball valve type, a gate valve type, or a needle seat type??????
at any rate if there is a thread involved you may simply have to adjust it is the same direction. ie always adjust towards less flow or always adjust towards more flow.
Like guitar strings you always tune up
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.
It's a very basic needle seat construction.
I also thought my guitar goes out of tune because the wood and strings expand and contract at different rates.
The loop to catch sediment sounds like a good idea. I'll need to think how to implement it.
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.
Select to expand quotePhoney said..
Wait, why do you have a garden dripper system on your kite?
lol - moved to the general forum...
Is the tank sealed?
If so then after using a bit of water you might be introducing a partial vacuum in there, reducing the flow. Putting a vent in would solve the issue.
Of course the float valve should be replacing the water that gets removed, but nothing is perfect and that float valve might take a minute or so to catch up.
Select to expand quoteoldbones 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...
I have been frustrated with all existing drippers, for irrigating my garden while using well filtrated water from pond.
All failed later or sooner ( Bunings and other suppliers) .
I did setup fully automatic dripper irrigation system , expecting that no further intaraction is required. No amount of filtering or pressure changes helped. All due sendiment clogging ( as somebody mentioned above)
Eventually I come up with very simple mechanical device design, that is constantly moving,scrapping, removing eventual sedimentation. Additionally mine is self regulating , adjusting to pressure differences as my block is sloppy and some drippers have more pressure then others on same line but you want them all deliver same or adjusted amount over time.
But I doubt you will be able to buy one of those already in mass production. 
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.
Select to expand quoteGalahOnTheBay said..
Phoney said..
Wait, why do you have a garden dripper system on your kite?
lol - moved to the general forum...
Yep , should be moved to heavy weather once device is working and dropping rain on our heads while guy is kiting ,
I always was a bit suspicious where feel rain drops in clear sunny day and only few kites around.
I have nothing to value add to this thread except curiosity as to what the rig is for?
Select to expand quoteChris6791 said..
I have nothing to value add to this thread except curiosity as to what the rig is for?
Hydro
I wouldn't worry about it too much, pot plants don't really mind if the watering isn't totally constant.
Select to expand quoteFreddofrog said..
Which drips are slowing down? the ones coming out of the float valve or the ones coming out of the adjustable dripper?
You are not using the dripper as it is designed to be used. Basically you have it almost on zero PSI or whatever you blokes use to measure water pressure. These are not designed to work at the full pressure you would find in your home's plumbing system either but rather at a reduced pressure, they sell a special pressure reducing fitting for this because full pressure tends to blow off the little tubing from the barb fittings. Your setup would be called an open system, it needs to be a closed system. It's possible if you kept a large enough head or volume of water in the tank it could achieve the correct pressure but the tank would need to be very large.
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.
Select to expand quoteDARTH said..Chris6791 said..
I have nothing to value add to this thread except curiosity as to what the rig is for?
Hydro
So his liquid fertiliser is clogging the dripper?
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 !
Select to expand quoteChris6791 said..DARTH said..Chris6791 said..
I have nothing to value add to this thread except curiosity as to what the rig is for?
Hydro
So his liquid fertiliser is clogging the dripper?
That is an issue, after time it builds up in the lines and gets into drippers and sprayers.
Select to expand quoteLostDog 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.
What are you trying to control
Level in tank , flow rate in to the tank or flow out of the tank??
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 ?
