Been keeping track of gallons throughput of my RODI since setting it up in April '09 on an Excel spreadsheet. Other than the poly pre filters I prefer to use gallons throughput rather than months as criteria for when to change my carbon filter. The documentation for my 0.5 micron carbon block says it's "Highly effective for chlorine.... for up to 20,000 gallons at a rate of 1 GPM".
According to my calculations, I've put an estimated 12,600 gallons through my system in just under 3 years. I say estimated since I'm using not only gallons of pure water produced, but also adding 4 times the amount of pure to the total since there's an estimated 4 gallons of waste for every gallon of pure water produced. This all goes through the carbon block. The 1 to 4 ratio is an approximation & I can only guess that since I don't measure this every time I produce water. I'm a cheapskate & don't fancy the idea of changing my carbon filter after 4 months if I haven't come anywhere near the rated gallon amount the filter is supposed to be good for. When feasible, I also like to verify manufacturer's claims so that I don't knacker my RO membrane before its rated life. So a few months ago I decided to check for chlorine at the tap source as well as after my carbon block. Results follow, I feel warm & fuzzy that my carbon block is still good & can go for a few thousand gallons more.
Sorry getting a little off topic from chloramine, but there has been some info provided in posts here regarding when to change certain filters.
Two photos since it may be difficult to tell the colour difference, one made with flash & one without. Should be able to tell on the strip in left of photo chlorine is 0 whereas the one on right shows a touch of purple indicating the presence of chlorine. This of course is the one dipped into the source water & the left one is after the carbon block.
