RO/DI Water quality - what do you use as your guide to replace your filter media?

So I purchased a three stage RO/DI filter from another GIRS member.  It was used and I didn't know how long the filters had been used.  I borrowed a TDS meter and it's reading 3-4 for the water that it produces.  I also tested the Wal-Mart water that I buy and it was reading the exact same thing.  Then I tested my home water (just to make sure that it wasn't giving everything the same reading) and I got 165-170.  My question is, what is considered a safe number to use...and at what point do you place the order for replacement filters?
--AJ

 

Good info Rick...but I'm testing the water to determine how much to sell this unit for... So a little background on the situation. I bought this unit just to get me by for a little while and to get around the inconvenience of lugging water from the store. Since then, I have ordered a new 5 stage unit that arrives next week so I no longer need the three stage unit. I don't want to sell something without knowing if the filters need to be replaced or not as that would impact what I asked for it. So that goes back to the root of my question...at what point do you replace filters, and is 3-4 TDS OK? I can't imagine that you can expect much better out of a three stage unit. I thought that it was interesting that the Wal-Mart water and the water out of this 3-stage unit produced the same results.

--AJ

 

I didn't test off of the RO...just off of the DI. I'll test that. It looks like the DI is color changing resin and it still looks like it has some life in it so it must be filters or membranes...I'll check that tonight.

--AJ

 

Good idea Jeremy. I ordered a pressure gauge to put between the refrigerator and the 3.5 gallon pressure tank (in case the water or ice maker started acting weird, I could make sure they were getting adequate pressure) but it sounds like maybe I should order another to put between the filters and the RO membrane.

--AJ

 

That's a good idea, but I would have to but the modified inserts for the canisters which means additional expense.  I have a friend that still buys his RO/DI water so this would be better than the nothing that he has right now and I could get close to enough for a dual chamber reactor from BRS that comes with the inserts and the ball valve. 
--AJ

 

So I measured off of RO and the TDS is 7...seems in line with what you are seeing h2so4hurts...are you running dual DI or should my DI be taking it down to 0? I've ruled out that the TDS meter needs to be recalibrated because it tested the same on two separate meters.

--AJ

 

I'll give that a try. It's color changing resin and looks like it has quite a bit of life left..but I'll change it out and see if that helps.

--AJ

 

I asked the guy I bought it from and he really didn't know. It has color changing resin in it and according to the colors, it should still have about 1/2 life in it..but it sounds like that's not really the case. I'll replace and take new readings.

--AJ