Hello again karen,
Looks like an easy enough setup. Typical Raft system. If you connect them all together, it makes draining the system somewhat easier, but it creates more points for potential leaks. you have to weigh the pros and cons there. I thought lettuces and such were short length crops and people generally didn't change nutrient solutions during the crop. I'm sure someone with more experience will chime in later.
If you go with piping them together, you need to make sure to include a drain outlet. Also (or conversely) if you pipe them together and only add nutrient to one end, it's not likely the fresh nutrient will find its way to the other end any time soon. Otherwise it seems like a fine enough system.
I'm not sure all the crops your interested in are suited for a raft system. From what I've read, to grow root crops without them looking like mutant freaks your best bet is to grow in a sandy medium with a drip system.
Also taller plants tend to make the rafts tippy and will force your lights higher off the smaller ones thus reducing their available light. Generally, the lux tapers off pretty quickly away from the light source in flourescents.(though admittedly my knowledge of lighting is somewhat dated and new advances may have been made) A light meter in your arsenal might help with that.
A slight overdose in micronutrients isn't generally a problem and, in fact, occurs frequently. The issue is generally that they are expensive so people don't want to use more than they need.
a good source of silicon is "water glass" typically found at small local pharmacies. the box stores rarely carry it, though may be able to get it for you.
Other advice that pops in my mind: when you first mix your nutrient solution, remember to record the concentration (ppm) and the EC reading so you have a baseline for what the EC reading actually means. Also be sure to check you plain waters EC too so you know if it already contains any minerals.
Oh! if you're on a municipal water system, its best to aerate your plain water for 24 hours to evaporate the chlorine.
And finally do a search for the hydroponic calculator on this forum. I believe it will prove extremely valuable in helping you create you nutrient solutions.
that's all at the moment. I'll post back later as more comes to mind.
Cheers, good luck, and post some pictures.

Great tips -- thanks!
* Piping them together: This will be done without any punctures in the pools. Think siphon. Rubber tubing with metal nuts at the ends (as weights to keep them at the bottom), full of liquid, reaching up and over the edges.

* Mutant carrots: Huh, I hadn't thought about that. But as we always love the funny-shaped carrots that come up in the garden, that shouldn't be an issue. :) Ultimately, if we have to leave a few crops out, that's not a big problem; the most important one is lettuce for our lunch salads, followed by some herbs for teas.

* Light height: I'm planning to arrange the plants from shortest on the left to tallest on the right, and then tilt the light as needed. If the dwarf tomatoes prove too much a problem (variety: New Big Dwarf), even with pinching, I'll simply remove them. Fresh garden tomatoes are nice in a salad, but not essential, and I could possibly just put them in a pot outside. Good point about raft stability; I'll need to be careful with balance and perhaps get some weights.

* Micronutrient cost: Apart from the molybdenum source, I was surprised how cheap they were (per unit mass). The big cost was that I had to buy so many different things, and you get ripped off if you buy in small quantities ;) For example, by my calculations, I've got enough copper sulfate to last the rest of my life. ;)

* Silicon: You recommend sodium silicate (water glass) over soluble clays, then? Can do. :) Even if I can't find it locally, it's readily available online, thanks to Cash for Clunkers.

Thanks for all of the other tips (aerating water, pre-and-post nutrient EC measurements, the calculator, etc). :) Any suggestions on how to adjust nut levels for different kinds of plants? I know toms prefer an overall richer solution than lettuce and peppers, for example.

For what it's worth, here's my chemical stock:
* Ammonium sulfate (small amount of ammonium to buffer against nitrate loss -- I'm thinking 10% or so of the nitrogen content)
* Tetrasodium EDTA (chelation)
* Zinc sulfate monohydrate
* Manganese sulfate
* Calcium nitrate
* Copper sulfate
* Magnesium sulfate
* Sodium nitrate
* Superphosphate
* Potassium sulfate
* Citric acid (seems to be one of the main ingredients in dry "pH Down" formulations, the other being ammonium bisulfate -- but since I couldn't find that, I'm using ammonium sulfate, which is less acidic, and I can add locally-bought sulfuric acid if I need a still lower pH)
* Ferrous sulfate monohydrate
* Boric acid
* Molybdenum silicate (this one may be tough to dissolve, but I figure a combination of strong acids and/or chelating factors may do the trick; it was the cheapest source of molybdenum I found at the time)

just remember with siphoning you have to get water into the tube for it to work and if either side ever runs dry, you'll need to make to the tube has water in it again to make it work again. also, placing nutrient in all the baths will be more important as the siphon balance will allow almost no transfer of nutrients between the individual ponds. In fact, in light of that last sentence I'd think connecting them is not warranted as you'll need to add nutrients to each individual pond and possibly drain from each also, thus defeating its purpose. Also, by not connecting them, it will allow you to separate your plants by pond and thus provided each with its own nutrient solution.
If you bought large quantities of micro nutrients and want to possibly recoup some of your cost, shoot me an email with some prices and quantities. you'll find my email address on my profile page.

Whoa.

I got my EC meter recently. My water at home tested about 240 μS. I found a page that said that distilled water should be under 20 μS and that good tap water is under 200 μS.

The water in my office, where the plants will be? It tested at about 1500 μS (!).

Well, at the very least, I already knew it had a major iron surplus, as the water turns yellow and precipitates iron. Now I suspect there's other problems as well. I'll be requesting a full water quality report for this building, and I imagine I won't be able to use tap water here.

As a followup to this old thread: I just thought I'd show the status of my setup today. Sorry for the quality -- cell phone camera.

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The water level isn't as high as it will be on final approach -- we're filling it with rainwater and air conditioner condensate since our water here has too much sodium (thank you, water department for the detailed mineral breakdown!). Naturally, rainwater and AC condensate takes time to collect. We did discover some pinhole leaks in the plastic -- cheap plastic sheeting, you kind of have to expect imperfections. Those were patched with caulk. Additionally, we had to add a bit of extra flotation to some of the rafts because we forgot to account for the fact that the added weight of pots, plants, and water would lower how high they rode in the bin, putting more of the base of the pot underwater. Apart from that, everything seems to be going according to plan just fine.

For now, we're avoiding a bubbler and just relying on the natural porosity of the MicaFlake vermiculite insulation for air (only the very bottom of the pots are underwater). This is in an office, so we want the system to be as quiet and low-maintenance as possible. I.e., if the power went out or the bubbler died, we don't want the plants to drop dead. But if we ultimately have to, we will. The holes in the bottom of the pots are sealed off with the rubbery anti-slip webby-mesh liner that you put in your kitchen cabinets. That lets the water in but keeps the vermiculite out of the solution -- and it should be non-biodegradable.

As for the pics: On the far side is a bin for growing microgreens (a bit overplanted even for that, but oh well ;) ). The next two are herbs. After that is a bin for super-dwarf broccoli, a dwarf pepper, and two herbs. And furthest to the right is the bin for the dwarf tomato. The PVC pipes are for carrots. There are only two light fixtures overhead (440W), but there's room for one more if we decide we need it. The hood is made of aluminized bubble insulation. We put a quilt over the top for aesthetics.

We've made our own nutrient mixes -- a lettuce mix, an herb mix, and a tomato mix. We also have a low-NO3 mix and a low-PO4 mix to add should the nitrate or phosphate tests, respectively, show high levels relative to EC (should they show low levels, we'll just add some nitrate or phosphate salts). We'll be adding water whenever the level gets too low, and adding nutrients whenever the EC gets too low. Combined with the occasional NO3 and PO4 test, we hope to be able to get the nutrients to last a whole growing season. We'll see. I'll check in in the future with updates as to how things are growing.

P.S. -- This micaflake vermiculite insulation seems awesome so far for this application. Dirt-cheap, plus amazing wicking, water-retaining, *and* aeration capability, as well as being totally inert. :) I'll have to see how it holds up over time, of course.

I'm speculating here, but I believe your plants will get leggy with the lights that far off them.

PVC pipes for carrots. Good idea. I was using coir for carrots in a bin with a drip. I really like the pipe idea, though. Less medium, so less weight and waste from evaporation.

Grizzman: The hood is adjustable up and down. I had to have it up for the pics. I also leave the light on 24/7.

Well duh! **smacks forehead**
that makes all the sense in the world.

Update:

There's an utterly gorgeous crop of lettuce growing in the system right now. Most other plants aren't as far along; the lettuce got an early start. Some lettuce roots stick out the bottom of the pot, but rather than those exploratory roots dying off like I expected, they still look white and healthy. I guess they're getting O2 somehow. Well, I'm not going to complain. :)

Germination was poor in several pots, although almost all of my seeds are old.

My spouse dropped our (non-waterproof) pH meter in a bin. It shorted out. She took the batteries out and let it dry out, then put them back in. It behaved very strangely -- not only was it off on the pH, but it still ran when the meter was turned off -- just with a dim screen! Thankfully, this latter symptom has gone away, so I think all we need to do is recalibrate it.

Tip #1: When suspending the light fixture with a taut hitch, tie an additional half-hitch at the loose end of the rope. This will prevent the taut hitch from accidentally pulling undone over time. We lost a taut hitch and our light fixture fell down (thankfully, it didn't break any bulbs, since what it ran into was the floating trays, which absorbed the shock).

Tip #2: Silicon gel should not be used to patch leaks. It seems to work at first, but it doesn't last; the leaks will just open back up in time. Instead, use "3M High-Strength 90". It's one of the only adhesives in the world that your average consumer can readily acquire in small quantities which can get a good polyethylene bond (I should have gone straight to it the first time, as I have dealt with polyethylene film before for different purposes and came to the exact same conclusion). And it's really convenient to use, since you can just spray it out in big swaths across the plastic, and it'll foam up a little and fill in the gaps.

Tip #3: When making homemade nutrient mixes, a spare coffee grinder will work *wonders* for dealing with the pelleted nutrients.

Tip #4: When dealing with calcium nitrate, never leave it exposed to air for more than a few minutes. It'll absorb moisture from the air until it liquefies itself!

Tip #5: To get dilute mixes like lettuce to have a proper (read: very low) EC, you'll want to keep all of your cations down to the bottom end of their normal range except potassium, which you should have below the normal range (lettuce doesn't like it much). Try to hit the minimum sulfate (~60ppm), then make the rest of the anions nitrate, since lettuce loves nitrate. High-EC plants like tomatoes are easy, but tomatoes also don't like much nitrate (~80ppm); for the most part, use sulfate salts in a tomato formula composition. Remember that (from what we've read) seedlings like to start off with extra-diluted macros, but micros should always be at full strength. Also, to keep EC reasonable, we found we could only manage 30 ppm silicon in the dilute and semi-dilute mixes, but we were able to manage 100ppm silicon for the tomatoes.

Apart from all of that, it's all pretty noneventful. :) Plants just sit over there growing.