RO/DI When to replace cartriges

[Osama]

When do you know it is time to replace the various cartriges & when do you know it is time to replace the membranes on a Kent RO/DI 100GPD unit. I am getting hairline algae after 3 years without such problems. My population of hermits & snails is down but I am also thinking it maybe my water quality. I replaced the various cartridges 6 months ago and plan on replacing them in the nest few days, I am assuming that the membranes would need to be replaced when the output flow becomes too slow. Your input is appreciated. If I need to test ther output water quality; thenwhat tests should I run & what brand name of testing eqipment do you suggest.

 

[blownalcohol]

I usually change my prefilters every 6 months or when my pressure drops below 40. I use a hand held hanna TDS (total dissolved solids) tester. I have pretty good water to start with, a reading of 35 before ro/di with the hanna tester.
I don't think you can go by the output for changing the membrane.
As far as changing the membrane or how long they will last depends on a few things. How good is you're well or city water to begin with? How much water do you make? Do you have a flush kit, to flush the membrane?