Understanding TDS numbers

[TonyV]

So my tap water reads 265ppm which is a no no, and the good water coming out of my RO unit is 7-8 ppm. Is this sufficient or should I be getting 0ppm? I purchased the unit used and was told that the filters and membranes were only a few months old. The sediment filter looked completely clean. I just purchased the TDS meter to check water quality. Will adding on a DI unit bring my numbers down lower possibly to 0 ppm? I currently have 100gpd membranes. I hear that there is better rejection rates with 75 gpd units. Any truth to this? Is 8 ppm. okay to use? I previously used store bought mixed water and had no way to check what the TDS was.

[fayzane]

Quick question - is your TDS meter one that automatically compensates for temperature? If not, you may be getting skewed readings. Failing that I suppose you could assume the membrane is older/more worn than the previous owner stated.

 

That number sounds 'ok' depending on what you got in the tank, certainly TONS better than your tap water... but it was my understanding people liked to get as close to zero as the unit would allow. So the next question is, have you checked online to see what the unit is rated for? I think there's some other things with the setup and flow rate that can affect it's ability to function at maximum efficiency, but I can't remember...hopefully someone else will.

[AZDesertRat]

TDS is a measure of the electrical conductivity of the minerals or contaminants in the water expressed in parts per million or ppm. It has some drawbacks in that it only measures those contaminants that are electrically conductive and it can be skewed by temperatures.

All in all though its a pretty good, inexpensive method of checking water quality. If you are getting a TDS of 7-8 down from 265 with RO only you are doing pretty good, thats about 97% rejection which is respectable for RO only. An add on DI will certainly get you to 0 TDS and DI resin should last a good while at those incoming TDS numbers.

I am a fan of 0 TDS for a couple reasons. One as I mentioned above is TDS only measures electrically conductive substances so there will be others that it misses, the lower the TDS the better the water. Some contaminants are very weakly ionozed or conductive and thses include nitrates, or all forms ofammonia, silicates and phosphates all of which can be harmful in a reef system. A good DI will help get all these.

 

There is a very good sale going on here that includes add on DI kits at smoking prices. I use the MaxCap/SilicaBuster combination myself and DI seems to last forever even here in Phoenix with a tap TDS over 800.

http://www.spectrapure.com/email/customer-...eciation.html#1

[TonyV]

Thanks for the help. I will add a DI unit shortly.