Wasserpests new airpump microdoser
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Old 10-08-2007, 02:10 AM   #1
Wasserpest
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Wasserpests new airpump microdoser


As you might know, I am slightly obsessed with autodosing. I have tried more designs than I care to remember. Sometimes an idea gets stuck in my head, and a few days later you find me going through scrap parts trying to assemble yet another monstrosity.

Some designs have worked out really well. Dosing N and K via the waterpump method, and P via the test tube method is extremely simple and reliable. There have been a lot of attempts for micro dosing that for different reasons did not work out long term. Dosing diluted solutions lead to undesired growth in the containers. Timer driven syringe dosers did not last. Peristaltic pumps are expensive, and one of the two I have tends to strip the plastic rotor and stops working.

There was another design which seemed to be promising. I spent many, many hours trying to make it work and never could. What would happen was that either the "refill hole" was too small and the tubing would not refill, or it was too large, which lead to little droplets entering when all the solution was pushed out, and when the pump stopped, the droplets would again prevent the tubing from refilling.

Still, this seemed like a real smart idea, so I kept playing around with it, and I think finally found a bit different design which seems to work better. The trick to making this work is to have a restriction ("small hole") while the pump is pushing the fertilizer out (which keeps the tubing free of more solution), and having open flow when the pump turns off, so the tubing refills readily.

After many hours of thinking I came up with a somewhat redneck, but simple and cheap solenoid valve. I used a piece of latex glove, wrapped around the end of some air tubing with a rubber band. That went into a plastic syringe. As the air pump pumps, some air is diverted to blow up the latex bubble, which blocks the flow through the syringe, therefore restricting the flow and keeping the tubing clean.

Okay -- here is the design:

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Old 10-08-2007, 02:10 AM   #2
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So here is how it works:

1) An air pump (~$10), controlled by an electronic timer ($8), powers the dosing. The airflow is split into two via a T.

2) A little airvalve adjusts the airflow through the dosing line. This is not extremely critical, but I find that if the flow is too high, droplets splash through the tubing and can collect after the pump has turned off, preventing an easy refill.

3) This check valve allows nutrient solution to flow into the tubing, and prevents it from exiting when the air pump is running.

4) This check valve prevents solution going up the airline, and therefore skewing the dosed amount depending on the level of the nutrient solution.

5) This tubing contains the volume that will be dosed, which depends on the length and diameter. In my prototype, I am using a 3in piece which results in just a little under a milliliter dosing volume. Extremely easy to adjust, of course.

6) I use mini tubing for this part, to minimize the amount of liquid that fills it to the level of the solution. Also, mini tubing should help to push the liquid out completely.

7) As described in the first post, this serves as a solenoid that is closes while the pump is working, therefore creating back pressure so no more liquid can enter through check valve 3. This syringe shows clearly how many ml are being dosed. First, the liquid is pushed into the syringe. As the "solenoid" closes, pressure builds up and finally pushes the liquid through a tiny needle hole out of the syringe into the dosing line 9 and into the tank.

8) This air valve determines how much air goes into the "solenoid", which is a very small amount, unless you want the little pop.

That's pretty much it, dosing a consistent amount each time the timer kicks of the cycle. I am sure this can be beautified and maybe simplified, but the basic concept seems to work.
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Old 10-08-2007, 01:14 PM   #3
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Hmm, I was thinking a rubber test tube stopper with a hole thru it for the airline tubing might be used in place of your latex glove as a cork for the end of the syringe.

The potential weakness I can see for this design would be the use of cheap aquarium check valves which tend to eventually fail and get stuck especially when exposed to liquids with stuff dissolved in them.

I'd be curious to know how long it runs for before it is in need of repair.
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Old 10-08-2007, 02:21 PM   #4
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This is exactly what I have been looking for to dose my kno3 ect while im away from the house!
pictures please wasserpest I really want to build myself one!
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Old 10-08-2007, 04:06 PM   #5
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Quote:
Originally Posted by vidiots View Post
Hmm, I was thinking a rubber test tube stopper with a hole thru it for the airline tubing might be used in place of your latex glove as a cork for the end of the syringe.
It might work. The difference in my design is that the latex glove collapses, which opens up the syringe part completely. If you use a rubber stopper, there is still the back pressure from the air pump membrane which might prevent the refill.

Quote:
Originally Posted by vidiots View Post
The potential weakness I can see for this design would be the use of cheap aquarium check valves which tend to eventually fail and get stuck especially when exposed to liquids with stuff dissolved in them.
You are right... that might or might not be an issue over time. Here they sell check valves with a different design, might be worth to check out.

Quote:
Originally Posted by vidiots View Post
I'd be curious to know how long it runs for before it is in need of repair.
Me too. It's still in the trial phase. Once I have it running for a few weeks or months I will let you know.
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Old 10-08-2007, 04:08 PM   #6
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Quote:
Originally Posted by garuf View Post
This is exactly what I have been looking for to dose my kno3 ect while im away from the house!
pictures please wasserpest I really want to build myself one!
I'll post a picture once I have it finalized. For macro dosing, check out the "Cheap Dosing" link in my signature. Works great.
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Old 10-08-2007, 10:00 PM   #7
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Quote:
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You are right... that might or might not be an issue over time. Here they sell check valves with a different design, might be worth to check out.
Those check valves sound like they work the same way, but they are not see thru so you can see how they work. I would imagine though that an residue buildup on the diaphram or walls will also cause that design to fail.

I got sick of using check valves on brine shrimp hatchers cause the salt water would cause them to fail after just a month or two and went to just puting the pump up higher than the hatcher water level to prevent a back syphon.
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Old 10-08-2007, 10:10 PM   #8
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I updated the original graphic, did some changes that simplified the design a little bit.

Here is a photo of my "prototype".

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Old 10-08-2007, 10:17 PM   #9
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Quote:
Originally Posted by vidiots View Post
Those check valves sound like they work the same way, but they are not see thru so you can see how they work. I would imagine though that an residue buildup on the diaphram or walls will also cause that design to fail.

I got sick of using check valves on brine shrimp hatchers cause the salt water would cause them to fail after just a month or two and went to just puting the pump up higher than the hatcher water level to prevent a back syphon.
Judging from the description, they work completely different from the valves we are commonly using. In the design that we use for airpumps and such, you have two thin rubber lips (diaphragm) that open up one way and are pushed closed the other way. The linked design seems to be based on a little movable plate that covers up the hole one way, and leaves it open the other way.

Residue buildup might or might not be an issue. In diluted solutions you often find some sort of growth (fungus/bacteria) which clogs or even attacks the rubber. Dosing full strength, there might be no such growth possible, and as long as it does not dry out or contain particles, maybe the check valve keeps working.
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Old 10-12-2007, 07:08 PM   #10
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Trying to simplify the design a bit... a smaller nutrient container will be beneficial with small dosages.



Currently searching for a different design check valve. The ones we commonly use have two rubber lips and need a little bit of cracking pressure to get the flow started. There is a different design with a movable flap or cylinder that covers up a hole in one direction, and allows flow in the other direction. That design has almost zero cracking pressure, that is, allows fairly free flow as soon as the pressure is zero. That would be optimal and might even alleviate the need for my super latex glove solenoid.
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Old 10-12-2007, 07:17 PM   #11
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Wasser, do you have any concerns about having the micros settling, or coming out of solution?
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Old 10-12-2007, 07:19 PM   #12
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Not much... I am using Flourish and Flourish Iron, they might settle a bit, but giving the container a quick shake once every week or other should fix that.
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Old 10-13-2007, 03:00 PM   #13
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Quote:
Originally Posted by Wasserpest View Post
... The trick to making this work is to have a restriction ("small hole") while the pump is pushing the fertilizer out (which keeps the tubing free of more solution), and having open flow when the pump turns off, so the tubing refills readily.

After many hours of thinking I came up with a somewhat redneck, but simple and cheap solenoid valve. I used a piece of latex glove, wrapped around the end of some air tubing with a rubber band. That went into a plastic syringe. As the air pump pumps, some air is diverted to blow up the latex bubble, which blocks the flow through the syringe, therefore restricting the flow and keeping the tubing clean.
I've read this a number of times and I still dont understand exactly what you mean by "which keeps the tubing free of more solution" or "therefore restricting the flow and keeping the tubing clean"

I've played with these airlift dosers myself and by "keep the tubing clean" I assume you mean preventing the retention of solution in the uplift tube so that air can escape as the submerged dose tubing refils?? ...if so... I dont understand why restricting the outflow prevents solution being retained in the narrower uplift tubing. Can you perhaps take another crack at explaining exactly how the solenoid improves the dosing and exactly which bit of tube you are trying to keep "clean"?

.. or have I missed something else?

oh and btw I think those disk based checkvalves sound perfect for this application!
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Old 11-06-2007, 10:45 PM   #14
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Yeah, it is a bit hard to explain.

With the original, simpler design you have only one tube going straight up from the container. It fills up with solution on the bottom, and when the pump kicks in, that solution is pushed into the tank. Now as soon as all the solution has left the tubing, the back pressure against the lower checkvalve becomes zero, and slowly, more solution trickles into the tubing and is pushed out to the tank. When that happens, little droplets remain in the tubing and prevent it from refilling.

So if you restrict the outflow a little bit, all solution is pushed out, but then no more solution enters the tubing since the air pump creates some back pressure against the check valve. Once the pump stops, the back pressure is gone, and the tubing fill again.

The issue was that I couldn't get that to work. Either I restricted the outflow too much, which prevented the refill, or it was too open, which lead to droplets making it through the check valve while the pump was running, and then prevented the refill.

The work around with the DIY rubber glove solenoid makes that work. Back pressure while the pump is working, no back pressure while pump is off.
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Old 11-06-2007, 10:52 PM   #15
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A picture is worth a bunch of words, so here is one of my currently working prototype.



It has been dosing my 100gal tank for the last 3 or 4 weeks. Weak points are the check valves. The one on top dries up a little and it takes quite some pump strength to unglue it.

I really want to try one of the other check valves, but I can't get them without incurring a $10 shipping charge, and that goes against my frugal nature. If I knew they would work I would buy a bunch to distribute the shipping cost, but I don't know that.
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