Comment by ein0p

Yes, but be that as it may, at least it shows whether _any_ filtering is occurring at all. Which in the case of even a somewhat spent carbon filter is not a thing. RO does the job so well, you have to remineralize afterwards or the water doesn't taste like anything. You could literally pee into a jar and get fresh water on the other end. That's sort of what happens on sailing vessels that are equipped with water makers - they simply run sea water through RO.