Hi
@irie ivan , that's a fair question. The simple answer is that it gives me peace of mind and makes me feel warm and fuzzy inside.
I will share my thought process as well though: (remember this is just my personal opinion).
The fist thing to consider, is that "0tds" as measured by our TDS meter's is actually a big lie. The meters that we use are in fact EC (electrical conductivity) meters that are measuring the electrical conductivity of the water, taking that value and calculating an approximate TDS based on a conversion factor. This would be precisely accurate if every single molecule was charged either -ve or +ve... but in reality there are a number that are not charged, and thus not measured by EC and not reported in the TDS result.
As these uncharged molecules move through a mixed resin bed (Anion & Cation) the more contact time they have with the negatively and positively charged resin beads the more likely they are to pick up a -ve or +ve ion, and then adhere to the very next cation or anion resin bead that they pass now being removed from the water completely.
So a brand new resin chamber can result in a 0TDS and have very few uncharged molecules that manage to pass through the bed, but a well used resin chamber can also result in a 0TDS reading, but have alot more uncharged molecules that have passed through the bed, and because the bed does not contain nearly as many charged anion and cation beads the molecules pass through into the final product. So both scenarios' are resulting in 0tds water produced, but the first scenario has very few uncharged contaminants and the second scenario has a lot more uncharged contaminants.... when do you change your DI resin? especially if it is not the colour changing kind? Most would change it when they start getting something other than a 0tds result.
By having 2 DI resin chambers filled with colour changing resin you increase the volume of the resin by 100% but you increase the contact time for any uncharged molecules by more than 100% because part of the first chamber will be already saturated and no longer active, those uncharged molecules must move through another entirely active resin bed in order to escape to the final product.
It also means that as soon as the first resin chamber has changed colour completely I can remove it, swap with the fully active resin chamber, refill it and replace it as the 2nd chamber and so on.
One of the main culprits for uncharged molecules is silicate, its is also something that very few people test for. Thankfully, it is also something that is easily removed with GFO, so people who are running GFO to remove phosphate are also removing the silicate that the probably didn't know was in the tank to begin with.