Just installed a new RO system

[Ray]

Originally Posted by james mickelso

Ray, are you still selling RO units?

Yep, I sure do.

I recently had an odd situation in which systems I had built for folks started not working right, even though the exact, same configuration was fine in the past. No matter what I tried, I couldn't fix them, until.... I discovered the manufacturer of the membrane housings stopped installing check-valves in the pure water outlet port, allowing the tank back-pressure to push the pure water back into the flush water line!

[ALToronto]

The unit I bought (and Ray, I did consider your units, but decided to buy local after all - same price as yours, but no shipping and customs charges) is supposed to have check valves installed - at least the diagrams say so.

In any event, the TDS levels are completely in line with what I would expect. The instructions did say to flush the storage tank after it filled up the first time, and I did that. The water from the first fill-up of the storage tank had a TDS reading of over 200.

Then we filled the tank the second time, and the TDS level was in the teens. Still a little too high, I think, but much more reasonable. And I'm sure it will get better as the manufacturing crud is rinsed out.

This is why the pH is so puzzling. With so little dissolved salts, why would it vary so much? Or is the reading simply unstable at such a high degree of purity?

[james mickelso]

It should become even more stable with purity. Something is not right with the system. But I don't know what it could be without being there.

[DavidCampen]

Originally Posted by ALToronto

And the pH is 8.5. Unfiltered water pH Is 7.3, and when I was using a jug water filter (ion exchange), filtered water was clocking in at 6.7.

I checked my pH meter with store bought distilled water - 7.0 on the dot. What gives with the RO? The 8.5 reading was after the initial tank flush, and the TDS reading was 12 ppm (down from 136 for cold tap water)

The TDS value indicates that the RO system is working correctly. The pH of RO water anywhere between about 5 to 9 has very little meaning precisely because the water is pure. To change the pH of pure water from 7 to 8.5 would require the addition of 3 micromoles of strong base per liter of water; for sodium hydroxide that would be 180 micrograms per liter which would be 0.18 ppm. So as I believe other people have mentioned you just have minute amount of base getting into the water from the membrane or apparatus or even your pH measuring device.

To bring the pH of your RO water back to 7 you could add about 3 milligrams of vinegar to each liter of RO water. This assumes that the pH above 7 comes from a strong base, it could be that there is a larger amount of some weak base that is acting as a buffer at pH 8.5 in which case you would need to add a larger amount of acid but by the TDS you have at most 12 ppm of buffer so at most you would need to add 200 milligrams of vinegar (assuming that the buffer has the same equivalent weight as acetic acid).

Edit:
Thinking about it; sodium bicarbonate solution has a pH of about 8.5 so your 12 ppm TDS could conceivably be sodium bicarbonate. So then you would need to add about 200 milligrams of vinegar per liter of RO water to bring the pH down to 7.

[ALToronto]

Thank you David, it's a relief to know everything is fine. In fact, with less minerals buffering the water, now my fertilizer has an even greater effect on the pH. I'm using liquid Dyna-Grow, and whereas before 1/2 tsp per gallon would drop the pH from 6.7 to 6.1, now the same 1/2 tsp drops it from 7.3 (yes, it is decreasing as I'm using more water!) to 5.6.

So all is good - thank you James, Ray and David.

[professor plant]

I use distilled water for all of my orchids; mostly cool growing miniatures and paphs. I also use dyna gro and plan on switching to K-lite. Should I be adjusting my PH after the fertilizer is applied to the distilled water? And if so, to what? I had previously been unconcerned with it because of the distilled water.

[DavidCampen]

Originally Posted by professor plant

I use distilled water for all of my orchids; mostly cool growing miniatures and paphs. I also use dyna gro and plan on switching to K-lite. Should I be adjusting my PH after the fertilizer is applied to the distilled water? And if so, to what? I had previously been unconcerned with it because of the distilled water.

The nutrient solution I use on my mostly cattleya and laelia collection is about pH 5.5 and they do well.

[ALToronto]

I agree about 5.5 being a good pH level for most orchids. I have some lithophytic paphs, however, so for them I save the runoff water from my aeroponic setup (pH around 8). I haven't had them long enough to see how they like it.

I plan to create a vertical "rock" wall for them (actually, my low-pH concrete - it's around 9, and similar to limestone) and give them regular acidic fertilizer solution - kind of like acid rain on limestone. I'll post it once I make it.