Low TDS, high alk.

Earl Karl · Feb 27, 2018

So my RO/DI water is very weird. The TDS comes out at 2-3 ppm (due to terrible water), but also comes out with high carbonate hardness, which is 8-9 DkH. Right now my water is 3 ppm and 8 DkH. This test has been done by many test kits, including the LFS testing it. However, according to @Randy Holmes-Farley, this is probably not possible. Can someone explain this?

Randy Holmes-Farley · Feb 27, 2018

You earlier said 0 ppm, which is impossible.

As a rough test, 3 ppm tds is roughly 3 mg/l of some salt. Assuming it is nothing but sodium bicarbonate (which itself is impossible, but assume it is) then 3 mg/l is 0.04 mM = 0.04 meq/ L = 0.1 dKH.

So it is still not possible and must be alk test error or tds measurement error.

Brew12 · Feb 27, 2018

Earl Karl said:
So my RO/DI water is very weird. The TDS comes out at 2-3 ppm (due to terrible water), but also comes out with high carbonate hardness, which is 8-9 DkH. Right now my water is 3 ppm and 8 DkH. This test has been done by many test kits, including the LFS testing it. However, according to @Randy Holmes-Farley, this is probably not possible. Can someone explain this?

Has to be testing error. Alkalinity is a dissolved solid and will read on a TDS meter.

I don't know enough about the way Alk test kits work to know if it is possible for a small amount of contaminant to show up as high alkalinity. Regardless, one or the other isn't accurate.

Earl Karl · Feb 27, 2018

Just checked with my LFS. Alk test says 8 DkH but tds is 260... whoops. Guess I need a new tds meter lol. My tap comes in at 500. So, my ro/di system is terrible, well it is old so it served its purpose. Time to get a new one I guess. Dam.
I don't know if I can trust any digital meter anymore.

Randy Holmes-Farley · Feb 28, 2018

Earl Karl said:
Just checked with my LFS. Alk test says 8 DkH but tds is 260... whoops. Guess I need a new tds meter lol. My tap comes in at 500. So, my ro/di system is terrible, well it is old so it served its purpose. Time to get a new one I guess. Dam.
I don't know if I can trust any digital meter anymore.

Yes, it sounds like there's an issue with it. Does it read 0 ppm TDS in air?