Empty half the bottle and top up with your unused resin, the old resin will still work along with the new
making it last longer.
I would never mix old with new. Old resin when spent leaches impurities back into the water.
I have seen my water going in the DI vessel at 90 and coming out at 140 when the resin is spent.
Once it is exhausted you have to monitor it, it can rise dramatically.
That makes no sense what so ever, who keeps resin until its spent ? most will replace at around 5ppm but even if you
replace it at 20ppm its far from spent.
By your logic running a double DI would be pointless as bottle one would leach impurities into bottle two and save nothing,
seriously a bit of common sense wouldn't go amiss on here at times.
Nonsense it is spent when it reaches the TDS level of the water you are putting through it. Otherwise according to your reckoning the resin would last forever.
If you don't believe me try it.
I put tap water through at 90 TDS once the TDS level coming out reaches 90 it is spent and pointless to use it further.
It's illogical and the chemistry is not understood by me but I can assure you once output reaches the TDS level of the water you put in it doesn't stay at that level but increases rapidly. Very rapidly!
Exactly the resin is spent when its no longer removing anything from the water nobody is saying any different.
But again your missing the point which is most will change the resin long before its removing nothing, so lets say it reaches 10
and like the op you only have enough resin to fill half the vessel, so what's the problem in keeping the half still bringing it down
to 10 which is not spent and adding another half taking it to zero ?