What little I know about maximum TDS tolerance has been from my reading here and there. What follows is from my impressions and memory only; I cannot give references.
My impression is cooler growing plants are more likely to be intolerant of higher TDS in water. Plants from the beach tolerate a lot more (accustomed to, but not requiring, salt spray.) Some are just surprises.
I think all epiphytic orchids can be grown well with rain or reverse osmosis water. If either be readily available and not too expensive I would use only those - and I do. I use my tap water for non-fertilizer watering of my Vandas (see below.)
Some people have tap or well water with low TDS. This is great for orchids. New York City is an example. A friend in the extreme east of metro Phoenix has well water great for most orchids, under 100 ppm TDS. I have tap water with 600-1200 ppm TDS depending on time of year. A gardening friend four blocks from my house actually measured his at 900 ppm on several occasions.
Some guesses at routine TDS acceptable for orchids. This is for regular watering, not fertilizing.
I have been told 1/8 tsp / 0.625ml, of MSU for pure water dry fertilizer powder, per gallon / 3.78 liter pure water, yields around 50 ppm TDS. I have neither measured nor calculated this number. If somebody demonstrates otherwise I will come back and change this message.
Paphiopedilum 50 ppm and under preferred. (Advice from Fred Clarke at SVO.)
Phragmipedium even less.
Cloud forest plants 50 ppm or less.
Cattleya, Miltoniopsis, Oncidium, Phalaenopsis 200-600+ ppm max but lower always better.
Former Odontoglossum (and hybrids?) need well below other Oncidium.
Tolumnia seem to do fine with Phoenix tap water 600-1200 ppm TDS (I use rain when I fertilize.)
Vanda are grown commercially with Florida well water, among the highest PPM water in the US. They do fine here with Phoenix tap water 600-1200 ppm TDS (I use rain when I fertilize.)
Paphs with Phoenix tap water brown from the tips and die over 3-6 months. Catts and Phals live for years with Phoenix tap water but often have brown tips; they don't thrive.
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