I don't work or think in TDS units (I prefer EC); the levels tolerated by some orchids are fairly well known:
see here for those I've been able to find
Most/all TDS meters arbitrarily (using a particular conversion constant, not always known) convert EC to TDS anyway - you're better off just using EC. (i.e. a TDS meter is an EC meter with funny units).
Adding more fertiliser nutrients isn't always the way forward - eventually you'll hit toxicity from one or more components; depending on the species you grow, this can be (for people used to terrestrial plants) surprisingly low. Most orchids grow in "oligotrophic" environments and do quite well under similar conditions in culture. There is currently a train of thought that we're giving them far too much potassium (K) in culture but it's a topic of new and active investigation.
Some thoughts:
- I suspect that since orchid roots (at least the epiphytic ones) have a velamen layer, the concerns of aeroponic droplet size "too small to support large plants" should be a moot point (the velamen is inherently a wicking layer that will "suck up" plenty of droplets); if you're trying to max out oxygen supply to a root, why are you going to come along and add a medium that is going to interfere with this? The whole point of aeroponics is to do away with the medium and maximise the contact of the roots with air, whilst ensuring sufficient water and nutrient supply.
- If by "support" you mean physical rather than physiological support, no media-less system will support a plant without some kind of structure on the stem; a simple "clamp" or "collar" around the base of the plant isn't a long term solution to stability - but some wires might be.
- Why would you go to the trouble to aerosolise your fertigation water when it's only going to end up running through your media as a film (normal size mist would work the same)? (The aerosol droplets will mostly hit the LECA and adhere, forming a film; not sure just how far a fine mist will penetrate your LECA; you might as well be using normal misters).
Hope this helps
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