I am getting ready for the big, newly rebuilt LSM 85 S unveiling (to friends and my wife). In preparation, I am trying to get my water situation worked out, but I'm getting some results that confuse me, so, I'm hoping for some help.

First, I have read (and re-read, and re-read ...) Jim's Insanely Long Water Post, so, I think I have the basics. However, that having been said, this "curiosity" doesn't seem to be covered.

I bought an API GH/KH test kit (after reading multiple recommendations from LOTS of wise users on H-B). I started by testing my tap water, which I assumed would be a bit too hard. What I got was:

GH - 5 drops - 89.5
KH - 1 drop - 17.9

That's not terrible, I thought, but, I wanted something a bit softer, esp. since the LSM recommendation is to have <5 fH (my current hardness translates into 8.95 fH, I believe).

Anyway, so I had already assumed that I would use grocery store purchased 5 gallon bottles of water. The first one I bought said it was filtered, run through membranes, etc. AND went through reverse osmosis. Fair enough. So, I tested it. What I got was:

GH - 1 drop - 17.9
KH - 22 drops before I stopped using the test because the water NEVER changed colors ...

What?!? I was ... confused ...

So, I thought ... well, I had no idea what to think. I went back and forth with my scientist wife but we couldn't figure this out. So, I went to a different store with a different bottled water source, bought another bottle of water and tested it. What I got was:

GH - 1 drop - 17.9
KH - 22 drops before I stopped using the test ...

ACK! At this point, I noticed something weird. When I rinsed out the test tubes with regular tap water AFTER trying to run the KH test on the RO water, I saw that the water IMMEDIATELY changed color (like it should have done ...). So, I tried something different. Using the test tube provided by the API test kit, I filled it with 4ml of the bottled water and 1ml of tap water and I tested it. What I got was:

GH - 2 drops - 35.8
KH - 1 drop - 17.9

So, oh wise H-Bers, what is going on here? I have read enough posts that have suggested that one can avoid damage to one's boiler when using RO water by mixing it with a bit of tap water, but ... why would the KH test not register ANYTHING (if that's the right way to say it) in the RO water?

Thanks!

1 drop is not exactly 17.9 but anywhere from 0 to 17.9, etc, etc.

Distilled water is distilled. RO water from a vending machine can get up to about 50 TDS, depending on how well maintained the membrane is, the rejection setting on the RO unit, and the hardness of the water going in.

The KH never changing color is a glitch in your procedure; these tests are quite tedious and easily can go wrong.

For routine water testing, a TDS meter is simpler and cheaper. The TDS reading is roughly the sum of KH and GH, and neutral water is around 125 to 150 ppm TDS.