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Old 03-02-2006, 12:37 AM   #1
josh_simonson
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Cheap DIY liquid doser


Yesterday I came up with a DIY liquid doser for fertilizing the tank and constructed a prototype. Here's how it works:

A section of tubing is submerged in a jar of the liquid to be dosed and connected to an airpump. A small hole (I used a hot needle to poke a hole in a 'T' connector) allows the fluid to enter the tube and air to bubble out when under pressure. The airpump is then connected to a timer so that it runs for about 10 minutes/day.

What happens is that when the pump is off the fluid level inside the tubing equalizes with that inside the jar. When the pump turns on, the fluid is pushed up the tubing and into the tank by the pressurized air (it burbles like an espresso machine). The pressure in the tubing also bubbles out the inlet hole and prevents more fluid from entering the tube. Once the fluid has been pumped into the tank, air flows through the tubing until the pump turns off.

The amount of fluid pumped is equal to the internal volume of the submerged tubing. Standard airline tubing is 2"/ml, and mini airline tubing is 8"/ml. I found mini tubing works best for the coil and tube running to the tank.

A tiny hole is needed in the lid to let air in as the fluid is pumped out. We want both holes to be small so that the pressure in the jar is enough to push the liquid up to the tank. A check valve in the lid may work extremely well for this. If the doser can be mounted behind the tank at tank-water level, it's very easy to get enough pressure to pump the fluid. To pump from under the tank required some trial and error, but two small needle holes worked well for me.

As the fluid level drops, the volume of submerged tubing decreases and the size of your dose gradually drops. I compensate for this by coiling the tubing at the bottom of the jar and keeping the inlet and outlet tubes vertical to minimize the volume loss. This reduces the effect and a short, wide, pickle jar would work best.

I ran a bunch of tests into a test tube, with about 8" of mini tubing and 3" of regular tubing, I got 2.5ml with about 0.1ml accuracy.


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Old 03-02-2006, 01:00 AM   #2
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josh - welcome to PT! And what a brilliant first post! Affordable, reliable, autodosing would be a real boon to this hobby IMO, and I think this is a great contribution!

As you have probably seen, a number of people are working on this problem here. But I'm not sure if they have the accuracy you are talking about. It would seem that your only drawback is the reduction in fluid pumped as the holding tank level drops.

Have you thought about drilling a hole in the holding tank, then using some timed mechanism to top it off daily, so the level stays constant? I know that adds complexity and cost. Just my 2 cents.

But either way, welcome! And thanks for a great first post!
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Old 03-02-2006, 03:18 AM   #3
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I had to read this a couple times to figure out what exactly is going on here... But it does make sense and sounds like a good idea. One idea I have to alleviate the problem of the reduction of the fluid - How about using a plastic container, and drill the side of it near the bottom of the container for one of those airline bulkheads? that way, ALL of the tubing is submerged, and it wouldn't matter till the container was near empty.

May have to try something like this when I get the 125 set up, since I won't be dosing only Flourish in that tank....


EDIT: Crap, now that I think about it, I'm not sure that would work. Seeing as how any tube that is uder the water level will get filled, that's a no go. Unless of course you used a 2nd air pump in a similar fashion on the other side running at all times that the pumping air pump isn't on just to keep enough pressure to keep the liquid back...the remainder of the ir could go wherever you please....ah hell, I'm just rambling now....
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Old 03-02-2006, 04:49 AM   #4
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I think this might be the simplest auto dosing setup I have seen
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Old 03-02-2006, 04:52 AM   #5
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One could always make the vertical tubes with very thin tubing. Perhaps get 1mm ID tubing and use that for the vertical parts, then use wider tubing for the coil at the bottom. That'd get down to the point where the difference in output due to depth was negligible.

There's not much bother in topping it off once and a while. Once a month sure beats once a day! Also by topping it off, you won't shock the tank with a change in your pmdd recipe.

Otherwise you could always do this:

It's another container, but DIY plant geeks already have a PHD in glueing tubes to the caps of bottles.
I should point out that the bottle needs to be able to withstand negative pressure, thick plastic or glass - preferably clear or translucent.
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Old 03-02-2006, 11:07 AM   #6
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Josh - I know the extra jar on top complicates things, and makes it harder to make a physically stable and sturdy unit, but IMO that is an ingenious solution. You could be onto a highly accurate, consistent flowing, inexpensive auto dosing setup.

And while turning jars of ferts upside down, setting them into other jars, sounds a bit messy, you wouldn't have to do it often. I've been auto dosing for about a year and a half, and you would be surprised how long a liter or so of macro ferts goes. A liter of micros would need replenishment so infrequently it's ridiculous.

Great work! Looking forward to progress.
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Old 03-02-2006, 04:19 PM   #7
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Ok, I read this-Slowly-about 3 times Yesterday and still couldn't quite get it....

I think I get it now-Finally! If so, this really is Ingenious!

So let me be sure I'm understanding this:

Quote:
What happens is that when the pump is off the fluid level inside the tubing equalizes with that inside the jar. When the pump turns on, the fluid is pushed up the tubing and into the tank by the pressurized air (it burbles like an espresso machine). The pressure in the tubing also bubbles out the inlet hole and prevents more fluid from entering the tube. Once the fluid has been pumped into the tank, air flows through the tubing until the pump turns off.
Basically, the perforation in the Tee is to allow the tubing to "Refill" after an injection, and also creates outward air pressure--keeping any more fluid from entering the tubing until after the injection cycle is complete. And the perforation in the Top is to prevent air-pressure build up inside the container which might force fluid into the tubing during injection. Is that Correct?

That's Really Cool......

This could be built out of 3"-4" PVC and the "Bottle" on top also-Capped with a screw on drain plug for access.
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Old 03-02-2006, 05:19 PM   #8
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>And the perforation in the Top is to prevent air-pressure build up inside the container which might force fluid into the tubing during injection.

The hole on the lid also allows air in to replace the liquid that's pumped out. Without it the tube won't refil for the next injection. We want some pressure to build so the liquid can rise up the tubing, but we also want the pressure in the tube to be greater than that in the jar.

The only challenging part of building this thing was getting enough flow resistance in the two perforations that the pump could generate the pressure to push the fluid to the tank. My 2.5ml fills up 20" of mini tubing, so I need to supply more than the pressure of 20" of water to make it go up into the tank.

The worst failure mode for this thing would be if the hose comes off the T. That'd pump air straight into the jar, which would push all of the fluid up the tube and into the tank. Mounting it in the cabinet means that you'd have to generate enough pressure to go from jar level to tank level, about 4' in my case. My pump can't produce that pressure even with no air outlet in the jar, so there's no way it can dump the whole jar of ferts into the tank. If the jar were mounted near tank-water level this would be possible (though unlikely the tubing would come off), and if it were above tank level the jar could potentially syphon out if somehow the pump only turned on for a couple seconds and primed the output tube to syphon.

The other likely failure modes involve plugging of one or both of the holes, which stops dosing, or of course failure of the timer or pump, which also stops dosing. You'd notice a deficiency and be able to fix it before it was serious.
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Old 03-02-2006, 07:29 PM   #9
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Welcome, Josh! Agreed with Steve, this is a great little invention.

I'm not sure the second "holding tank" is really all that necessary . . . . the ratio of volume lost due to lower levels is quite small, especially if you have lots of coils in the bottom! you could have as many coils as you wanted, just make a less potent solution. . . .

Also, it seems that an airline gang valve may be really useful to adjust he amount of pressure when dosing: http://www.aquaticeco.com/index.cfm/.../9904/cid/2348

I really hope to see this idea catch on, I just love the concept of using the air pump rather than a powerhead or submersable!

PS do you have a picture of the prototype?
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Old 03-02-2006, 08:05 PM   #10
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I figure just about every planted tank owner has airpumps that they've retired when they went to CO2.

My prototype doesn't have the second tank to stabilize the fluid level, and I'm not sure I'll bother to put it in considering it's already installed on the tank. I do have some 1mm diameter teflon tubing (for insulating wire) I'll try that first on prototype #2. It may choke off the airflow too much to be able to push the fluid up the tube - most of it goes in one whoosh, but the last quarter ml or so is left as dropplets in the tube that air can get around. If you care enough about precision to want the 1mm tubing, you'd probably want to be sure those droplets are blown out.
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Old 03-03-2006, 12:55 PM   #11
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Great idea. Not sure, but the preferated part probably also agitates - excelent.

Love the KISS principal. Only improvement I can think is a modification to allow for substantially larger dosing.
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Old 03-03-2006, 02:25 PM   #12
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I've been thinking about this, and another advantage to the upper resevoir is re-filling: you wouldn't have to unscrew the near-airtight lower container.
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Old 03-03-2006, 06:25 PM   #13
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Quote:
Originally Posted by esarkipato
I've been thinking about this, and another advantage to the upper resevoir is re-filling: you wouldn't have to unscrew the near-airtight lower container.
If you put airline valves between the upper and lower containers, and a valve at the top of the upper container, you could add more liquid by closing the lower valves, opening the top valve and filling, then closing the top valve and opening the bottom valves again.
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Old 04-24-2007, 04:35 AM   #14
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Status update: my original device has been working fine for over a year now in my planted tank. I've refilled the nutes twice when it got to be about half empty. I've been taking effortless growth for granted now, occasional pruning, CO2/nute refills or bulb replacement is all they need to keep things churning along.

Considering the device cost a jar and $3 of tubing, I couldn't be happier with it.
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Old 04-24-2007, 04:17 PM   #15
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So the trick here is to find a hole/slit/perforation size that is small enough to resist a little bit of air pressure, but large enough to allow liquid passing through without plugging up over time.

What would happen if instead of the perforation in the cover the container would just be left open?
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