I'm not sure how an NFT system can run out of oxigen. Can you explain?

I'm not really sure how either, but I do know I've read that. I think it was more like 30' than 6'-8' though.
I would reason this wouldn't apply as much to an NFT system open at the top. I've seen pictures of commercial NFT systems where they basically grow the plants in long soft plastic tubes , if you will (almost like a skinny plastic trash bag). in that case I can see it happening as the film would only run between the layers of plastic and the roots would not have anywhere to get additional oxygen from.
We tend to think of NFT as a open gutter with just a trickle of water running along the bottom of it. I believe the way I described it with the 'tubes' is more how it was originally designed. I can't speak to whether one works better than the other or whether either works at all. Until I had several seasons use with a single pump, I wasn't comfortable with an NFT type system. I personally like aero and DWC better, but that's just where my interest lies.

Well If I have a rock stone in my resevoir then I shouldn't have the least bit worries i guess. Oh yeah Grizzman what are your thoughts on the balance of all of the micro nutrients for the plants. I mean after all plants can't live on N. P. K. alone right? Like how are u suppose to account for that when You have homemade h.p systems. Stuff like boron and mg. and sodium and iron and stuff. Last week I spoke to Michael Christian the president of a.m hydroponics. He said that you can buy those ready made "n.f.t" systems. But I get the impression somehow that they use dissolvable tablets that are already formulated u just drop them in the system. Can you help me on the matter grizz or anyone for that matter? Mainly about the micro nutrients though.

Don't confuse a homemade system with homemade nutrient solution. If you're making your own nute solution, you'll need to incorporate the micro nutrients into your formula.
It's very well possible there is a 'tablet' you can just drop in a system. conversely, you could buy a bottle of GH flora micro and incorporate the proper amount of that into your nute solution to ensure it has ample micros.
Oxygen is not a nutrient, but it does need to be provided to the roots. This is typically done by mechanically incorporating it into the water (aka aeration). what I like to do is use a sprayer on a timer. That way the roots get air directly between waterings.

In a properly constructed NFT system, the water should have an area of air above the root system. Using the proper size root area is a must to achieve this goal. I use a 4 inch PVC pipe system and space my plants to allow adequate root area. This allows the pipe to stay open and not packed with roots. The nutrient flow should be of adequate volume to allow a free flow to the return. This will keep it from becoming oxygen depleted.

I also use a "Live-Well" bait type system for my reservoir. This has jets of water that constantly spray into the water and cause massive oxygenation. You can set this up with only a PVC flow-in-frame with small holes drilled in the frame as far up as possible and aimed at the water surface in your reservoir.

Good luck to you!

What they're talking about is that the flow of water past the roots - in the water itself - oxygen gets depleted. See, the first plants in the pipe absorb the oxygen little by little so there's less of it available at the end.

This isn't a major problem for the plants right off, since they've got roots in air that can absorb oxygen that way (though the air in the pipe can get depleted if there isn't some kind of breeze to refresh it). But, oxygen-poor water is more inviting to things like root rot and other problems.

That's why the pipe length in NFT is a concern.

The water flow in an NFT system should be of suficient volume to make oxygen depletion impossible. If you can't get enough water through the system to do that, then your system isn't large enough. In a smaller system, a fast drip from an aereated reservoir that delivers to each grow point will compensate for a smaller volume of water flow from the NFT system. Hybrid drip/NFT are common now.

The rate of flow has no relevance to whether or not it's possible to deplete oxygen. Oxygen doesn't stay more dissolved in faster moving water. It has to be highly agitated to become aerated and then the dissolved gases will outgas at specific speed for the given temperature, humidity, etc.

The greater the surface area of the water the more easily it can outgas the dissolved gasses. NFT systems are ideally designed to maximize the surface area of the water, thus allowing it to discharge the oxygen it's picked up.

And, just in case you need a bigger authority than me, pick up the book "Hydroponic Food Production" by Dr. Howard M. Resh. Turn to page 165 and read the second to last paragraph.

"Generally, with a slope of 1 in 50, the length of the channels should not exceed 20-25 meters (65-80 feet) or nutrient gradients my become evident, and solution oxygenation reduced. While it is possible to introduce the solution into the channels at several points along their length with longer channels, risk of oxygen deficit and temperature buildup in the solution exists."

This guy isn't a doctor the way Dr. Pepper is a doctor. He's a PhD in Plant Science.

Generally, hooked, I agree with you. However, rate of flow does play into that equation as the fluids surface area is a function of how the the nutrient flows through the system.

If you increase the rate of flow without radically increasing the slope of the channel you basically leave the realm of NFT. It's supposed to be a film of nutrient, not a babbling brook. So to keep it a film at a higher rate of flow you've got to have a much steeper channel to permit the nutrient solution to flow away as rapidly as it's poured in without it building up into a fast-moving body of water.

So yes, a higher rate of flow in a system that maintains the requirements for NFT won't see a significant gradient in ppm or oxygen content - not because of the high rate of flow, but rather because you're not going to see a 20 meter long NFT system set at a 30 degree slope unless its on the side of a hill. It's just not practical to build something that steep over that long a distance.

You can always add small amounts of H2O2 to nutrient mixes to increase oxygen content, plus there is an add-on of combating root problems...

True, that's not a bad idea, but if the system is long enough to run into this problem you're still going to have a small differential between one end and the other, meaning the plants at one end grow better than the other.

Something that long is pretty much going to be a commercial application, which means they don't want a lot of variation within a crop.

There is a system called NGS for new growing systems from spain which have hydroponic gullies made up of several layers of plastic film from 3 layer to 5 layer, all the models have a cascade system exept for the three layer which has side slots in the second layer which allows to introduce nutrient using a dripper tape, this allows excess nutrient solution to run to the collector layer which is the bottom one. Also this allows for the roots to hang in the air. They recommend trickle irrigation 2 minutes on 8 minutes off, this will allow the roots to come in contact with air and get oxygen.