I switched over to air atomized after that C99 grow, and thats what Im doing now. When I was doing HPA, I did it all on the cheep. Actually the AAA build was on the cheep as well.
My HPA system used all 1/4" push-fit tubing, valves, etc. The same stuff you use for home RO systems. It would be better to use larger tubing. At the low flow rates most home growers will be using, the small diameter tubing doesnt represent that much of a pressure drop between your accumulator tank and the nozzles. Larger tubing will result in less pressure drop, but its not all that bad if you lay things out so the runs are not all that long. I also used all 1/4" solenoids, pump fittings, etc. Going up to 3/8" tubing wont cost all that much more than 1/4", so might be worth it.
By the way, its especially important to keep the solenoids as close to the nozzles as possible.
One thing that will kill your budget is adaptors. Try to get all your parts and pieces, pumps, filters, pressure reducers, pressure switches, solenoids, one way valves, etc etc, to use the same size pipe and fittings.
The nozzles that worked best for me were from a company called Ecologic
cloudtops.com
I used the stainless 0.02 nozzles, plus a nozzle body, swivel base, stainless spring and anti drip valve parts. There are several other sources for the same basic nozzles, but I liked these folks best. It works out to around $6 per nozzle total IIRC.
You might want to try the next larger size orifice. I think my median droplet size may have been a tad small on average. Either than or I was just running a bit too dry as far as on/off times. Thats partly why the root mass spread out so drastically so early in the grow. On the other hand, it may have just been me. I am addicted to those fuzzy hairs and sideways growing shoots, so I tend to adjust on/off times to get more or those when I should be considering things like over growing the root chamber and swallowing nozzles.
On pumps - I used cheep coffee machine solenoid pumps that got my accumulator tank up to 140 PSI or so. I then used a pressure reducer to drop it down to the 100 PSI range. I dont really recommend using this type of pump. They do work, but they are noisy, and mine only lasted about 6 months of constant use.
You need your pump to be able to get the accumulator tank up to well above your normal nozzle operating pressure. If your pump can only do 80 PSI, then that limits how hi you can run the nozzles. I dont know how well the nozzles I chose will work at say 60 PSI. They may be fine, but I never ran them that low. It was always at 100 PSI or above. That seemed to produce the most uniform droplet sizes with the least amount of wasted drips, dribbles, sputters etc. There are diaphragm pumps available that do higher pressures. You just need to dig into the specs.
So far Im liking the air atomized option better than HPA. It has a lower parts count, but will cost you a little more to get started. A lot will depend on the nozzles you choose. Unfortunately, finding info on good air atomizing nozzles is not at all easy. The few folks - like Atomizer - with lots of experience in AAA refuse to share any specifics on nozzles.
I have been using relatively inexpensive Delavan oil heater nozzles with some success. They are not ideal, but they do work. I used those EcoLogic hydrolic nozzles on the C99 grow and the Delavans on all the auto grows since then. There one big advantage over some of the other nozzles Ive looked at is they work at relatively low air pressures and flow rates - under 10PSI easily. Im now running mine at around 6 PSI.
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The all stainless ones were closer to $100ea. You will either need to buy a matching brass adaptor at around another $20, or make your own. I went cheap again and made my own. The stainless adaptors are more $$.
After about a year of use, and 3 grows so far, these brass/stainless ones I have are starting to show some corrosion effects. The brass shows some external signs which dont bother me, but the internal air passages are starting to build up some corrosion on two of them. Ive cleaned them up enough to get through another grow, but I will probably have to replace them after that.
It would probably have helped a lot if I had a water removing filter in the air lines, but I cheeped out there too and just used a standard 200 micron filter. I may add a water trap in the air lines when I replace the nozzles, but that will depend on how $$ the filters are vrs the cost of the nozzles.
The only other major expense you have with AAA is an air compressor. If you can hide one out in a shed or garage where the noise isnt a problem, then a normal construction type compressor will be fine. If you are like me and have to keep it inside the house, then you want a quiet or 'silent' type compressor. Those run a bit more $$. I am using the smallest model from California Air Tools.
That should get you started. If you want more info, I will suggest you check out OverGrow.com. I spend most of my time over there. I have build threads for the HPA and AAA systems. You can follow along as I stumble through trying to figure this stuff out. That will save me a lot of re-typing stuff - which Im less likely to have the energy for these days.