RO water systems

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Denver, the low-pressure membranes I use in the counter-top systems are rated at 93% rejection of 100 ppm NaCl at 20 psi, 77°F and solution pH of 6.5-7.0, in a 15% recovery rate test (the CTRO is double that).

Carol - be careful trusting the readings of any TDS meter - fine for relative comparative readings, but poor for precise ones.

They are inexpensive electrical conductivity (EC) meters with an arbitrary conversion factor to TDS, usually based upon NaCl.

The different ions in fertilizer solutions have different conductivity factors, so the meters will give you different TDS values for the same actual concentration.

And if you are targeting a ppm N, you'd better know your conversion - For example, at 100 ppm N, the MSU Well Water formula has an EC of 0.68 mS/cm, K-lite is 0.71, and MSU RO is 0.80
 
It isn't. I was just pointing out that the absolute reading you get is not particularly reliable. If your pure water reads zero or 20 ppm, it's probably fine. If, in two years, it still does, great. If, on the other hand, it reads 50...
 
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