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I have read quite a few diy Nutrient Film Technique (NFT) guides online. Most are sparse on specific implementation details.

In an NFT setup, does the water need to pool in the trough or must all the water received at the outlet immediately drain away?

Referring to the cross-section below, should height h2 be close to zero (to immediately drain away water), or should it be a positive value (some water will pool in the in the trough).

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To clarify the diagram: Water comes in from the inlet pipe, flows down the sloped trough and drains out the outlet pipe. Raising the outlet pipe above the bottom of the trough will mean water does not drain away immediately.

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You seem to be missing the point that you are trying to create a constant (slow) movement of a thin film of nutrient solution, not a pond. The plant roots should be touching the base of the trough, not sitting in a pool of water.

Typical recommendations are a slope of about 1 in 40 (i.e. one inch drop in 40 inches length) and flow rates of about 1 liter per minute.

If you try out that sort of setup using any type of channel, you will find there is no significant "depth" of water anywhere. You want a thin film of water to make sure it is well aerated.

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There are a number of factors to consider: first will the nutrient solution be exposed to light - if so there will be algae to deal with if the solution sits in the trough, second plant roots will need air and in this case the air is in the gap between the bottom of your pots and the water level in the trough. The trough solution level should be high enough that the roots can feel the humidity and continue to extend themselves, and then as they get longer the water level should go down to expose more root to air. So one solution is to make it adjustable, lowering h2 as the plants get bigger.

Keep in mind too that if there is an exaggerated slope on the trough then there will be a greater distance between the bottom of the first pots and the solution whereever it might be than the last. Also the more solution in the trough and the longer the trough is the more it can be bent out of shape by gravity which might interfere with your plans to make solution evenly available to each plant in the trough.

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