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09-09-2011, 06:56 PM
Originally Posted by XSiVE
this seems odd, I just got a dual inline TDS meter and of course Im playing with it... I have my RODI set up into my ATO bucket with a float valve... when the float valve closes, the back pressure causes the auto-shutoff valve to turn the RO off.. typical configuration.
In normal running situations Im getting 5TDS out of my RO, but here's where I think it's a little strange. When the autoshutoff shuts the system down, the TDS starts creeping upward to around 100. once the system starts flowing again that number goes back down to 5 eventually.
Im really not sure what this could be from, unless the DI resin is leeching its leftovers back ward as the water sits there not flowing.
What you are seeing is TDS creep. When the ASOV starts to close it places backpressure on the treated water side of the membrane causing the water to osmose or equalize on both sides of the membrane. Since RO water has fewer ions and is considered agressive it attracts the untreated water right through the membrane fabric, or osmosis.
Once the system starts again it takes a few minutes for the TDS creep to flush itself out and the treated TDS to drop back to the normal 96-98% rejection rate if its in good shape.
One big reason I do not recommend float valves is in normal operation they start and stop frequently since they have such a narrow operating range compared to two float switches set a disatnce apart. With float switches you don't necessarily eliminat TDS creep but you reduce its frequency dramatically so the effects are minimized and DI resin lasts much longer.
One thing you can do to reduce the effects, if you have sufficient ATO storage capacity, is shut the RO/DI unit off manually after the ATO reservoir is full and allow it to drop some distance before turning the water supply or ball valve back on again so it cycles less often.
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