Warning for RO-only users

[gregzz4]

Nothing to panic over, but if you use a shut-off float valve on your RO holding tank ( as I do ) to shut off your system when the HT is full, this may apply to you ...

A few weeks ago I tested my HT water and found it to be a crazy 6PPM
That's nearly my City Water value

I washed out the drum (HT) and refilled it with a fresh run of 0 TDS from my RO only system. Since then I've topped it off a couple of times, so I've gone through approx 50g ? Doesn't matter
I tested the water in the drum this last weekend and found 2-4 TDS, depending on how the probe was positioned. I think these inline ones are sensitive to us ( interference ). Anyway , WTfudge

After some reading, and a bunch of PMs with 11purewater about Permeate Pumps, the cause and solution came about

Turns out that, because I use a float valve to close my RO water to the drum when it's full, the back-pressure on the Membrane kinda overwhelms it's ability to give me 0 TDS
What's happening is not really important. How it happens is

As the valve starts to close, it puts back-pressure on the Membrane and the TDS starts to climb
However long it takes for the valve to close is beyond me as I'm never around when it happens
Also, I have no idea what the TDS is when it's finally closed
But, I can tell you it must be very high. High enough to turn 45 gallons of 0 TDS RO water into 3-4 TDS crap

To confirm this, I ran a simple test tonight ( Thanks for the idea Andy )
I have an inline shut-off ( manual ) valve to my drum, and a Tee with another valve ( also manual ) that goes down the drain
I ran the system down the drain until I had 0 TDS. Then I opened the inline valve to the drum and closed the valve to the drain
Now the drum is being filled with 0 TDS RO water
Next, to simulate the float valve closing, I started closing the inline valve a bit at a time and, once it started to drip 4-6 drops/second, the TDS coming out of my Membrane started to climb
Who knows what kind of TDS I've been getting before the float valve in the drum is fully closed, but, as I stated above, it has to be HUGE to turn 45 gallons of 0 TDS RO water into junk

Suffice it to say I am going to revamp my system
I'll start by removing the float-shutoff valve and let the excess RO go down the drain until I shut off the water supply manually

In the near future I'll come up with a better way to turn off the system once the drum is full, but at least for now I found out where my TDS is coming from

And before someone jumps in and tells me to use a DI canister ...
I don't need one for my city water, so I'm not adding one just to solve this issue