De-ionized R/O systems will put you at 0 PPM
These are not the same thing.
Distillation: the separation of water from dissolved solutes (salts, etc.) by vaporizing the water, and condensing the vapor.
Deionization: the use of
ion exchange columns to remove dissolved solutes. I've used these before to make ultra-pure water (better than 3 megaohm resistance; distilled is anywhere between 100,000 and 250,000 ohms, while "perfect" ultra-pure water is 18.3 megaohm resistance); the only problem I've had with them is a trace of fluoride making it through somehow. Never did figure out how, but it was in the low part per trillion level, so it wasn't important to us- just a nuisance.
Reverse osmosis: the use of
pressure across a selective membrane to purify water.
Of the three, RO produces the lowest quality of water; a well-tuned system will produce very high quality water, but it still pales in comparison to distillation. However, RO water (unless the membrane is very old or punctured) is still more than good enough to produce exceedingly high quality of water for horticulture.
So, while your meter may read 0 ppm, I can assure you that solutes still make it through an RO membrane, and that as that membrane ages, the concentration of solutes will continue to increase- membranes are not immortal, and must be changed.
For example:
Chloride: rejection rate of 94-95%
Calcium: 96-98%
Magnesium: 96-98%
Sodium: 92-98%
All of these ions are of interest because they are common to water in the desert southwest; feed water at the consumer level (that which runs through an RO system) can have total dissolved solids levels at several hundred PPM.
If your meter is reading 0 ppm, then it is probably reading water from a relatively new membrane and/or the meter doesn't read very low levels of solutes; these meters simply aren't designed for good resolution at the low end of conductivity. I'd be happy to run any samples of RO water you might have on an ion chromatograph to show you this.
