I now use two DI vessels to make my resin last longer and I have just cut and pasted this to explain it well in plain English to help others -
When the pure water coming out of your DI bottle isn't as pure as it was or should be and it's time to replace your mixed bed resin, don't throw away the resin that's in your bottle.
If your water was originally going into the DI bottle at say 84 ppm (parts per million) and was coming out at 0 - 2 ppm but is now coming out at over 11 ppm then it's unlikely that your pure water cleaning system is leaving the windows you've cleaned, spot free.
By obtaining and using a new DI bottle and new mixed bed resin, you can continue to use your first DI bottle with the used mixed bed resin still in it, as a pre-filter for your second DI bottle.
This means that with two DI bottles instead of one, the water that is still coming out of your first DI bottle with a reading of 11 ppm, is what is now going into your second DI bottle with the new and unused mixed bed resin.
Your second bottle is now only having to clean incoming water with impurities of 11 ppm instead of with the original 84 ppm.
This will result in your mixed bed resin in the second bottle lasting a whole lot longer than your original bottle lasted.
The savings in having to replace your mixed bed resin less often, will more than cover the cost of buying your second DI bottle.
When the water you're using starts to come out of the second DI bottle at around 11 ppm again, simply use that as your first bottle and replace the well spent mixed bed resin in your original bottle with new resin and use this DI bottle as your second filter.