View Full Version : TDS Reading

03/23/2007, 06:10 AM
I hope this is the right forum to post this question. But any way...

What is the lowest acceptable TDS reading I can have before changing the filters on my RO/DI? I know the best is 0, but do you guys really change your filters when the TDS meter starts reading 1 or 5 or 10 of almost pure clean water coming out of your RO/DI?

03/23/2007, 07:50 AM
I use a DI only system and my TDS readings get near 5 right before I change it.

But then again I'm using color change resin to tell me when it needs to be replaced.

03/23/2007, 08:15 AM
Prefilters and carbon filters have almost no effect on TDS. They are there to protect the the membrane by removing suspended or large particles and to remove chlorine and VOCs and nothing else. You change these two every 6 months regardless of the amout of water you have made. This is to help prevent any form of bacteria or virus growth inside the filters and filter housings as well as to ensure sufficient pressure is getting to the membrane. If you wait too long the prefilter can become plugged reducing pressure available to operate the membrane correctly.
DI resin is changed when you consistently see anything other than 0 TDS. The reason I say this is TDS is only a general indicator, you have no idea what that 1, or 5 or whatever TDS is made up of so don't chance it, change it. Color changing resin is notoriously inaccurate. It can turn too late, in streaks, in layers, not at all, too soon etc. Its merely an indicator that something is going on and you need to get your TDS meter out to check it out. I will not use color indicating resin myself just because it contains dyes and can actually contribute to TDS at low levels.

03/23/2007, 08:42 AM
:thumbsup: Hey Rat !

03/23/2007, 11:19 AM
I can only afford to change the membrane on my RO/DI right now. If I replace the membrane and leave the other filters, will I still get a reading from the TDS meter other than 0? I can see that the other filters are still white. They do not look dirty at all.

03/23/2007, 11:53 AM
How old is the membrane ? Depending on incoming water, membranes can last for years. If your TDS is creeping up, it is likely your di cartridge. (the last one before output).

Like the Rat said, it is a good idea to change all 3 filters at the same time but if your 1st two filters don't look discolored, you could possibly get away with just changing the DI.

03/23/2007, 12:34 PM
Right you are Billy. If a RO membrane fails it will usually manifest itself in two ways:

1. Suddenly you need to replace the DI cartridge at a far more frequent interval
2. the output of the RO unit severely drops off

A gradual rise in TDS is almost always due to depletion of the DI resin and unless there is reason to believe the RO has failed then changing the DI is all that is needed.

03/23/2007, 01:01 PM
The only way to answer your question is for you to provide TDS readings from the tap water, RO only without DI and RO/DI. These numbers will tell you what needs attention.
If the prefilter and carbons are more than 6 months old change them regardless of how they "look". Carbon get exhausted adsorbing chlorine and VOCs, especially if it is granular carbon or a higher micron rated carbon block. Better carbon blocks can adsorb about 20,000 gallons worth of 1 ppm chlorine residual while granular carbon can often be exhausted at 200 to 300 gallons and you can't tell it by looking at it.
Prefilters can plug internally and again you can't physically see it. A pressure gauge is a good tool to have on the membrane so you can see pressure drop and a low range chlorine test kit is a good tool to help determine carbon condition. Neither are must haves unless you are trying to stretch filter changes which to me is not a good idea. Filters are cheap, RO membranes and DI resin are not, and fish and corals definitely are not.

03/23/2007, 02:43 PM
OK, I will get those #'s today or by tommorow. Thanks for the help guys.