View Full Version : TDS Reading

11/15/2007, 07:43 AM
I just purchased a TDS meter from MarineDepot and used it on my RO system. The reading was 017. Does anyone know if this is good???? My filters were all changed recently and I just want to make sure. I have a RO line that goes to a 5 gal bucket behind my tank and is gravity fed to my sump (via a Kalkwasser Reactor).



11/15/2007, 07:46 AM
My reading was 048. I assume that's PPM. I would think that 017 would be excellent...

Would be interested to hear from the experts...


11/15/2007, 07:48 AM
Is that is coming out of the RO...yeah its good. DI output would be closer to zero.

11/15/2007, 07:57 AM
To determine the effectiveness you need to know the tap water TDS also. With a properly functioning RO membrane you should be seeing a 95-98% reduction in TDS from the tap water. NOBODY can say it is good or bad without knowing what the starting point was.

Using my numbers as an example:
Tap water TDS = 806
RO only TDS = 05.2
RO/DI TDS = 0.0

I am getting a 99.35% rejection rate or removal rate presently.

Subtract your RO TDS from the tap water TDS then divide that number by the tap water TDS for a final number

806-5.2=800.8 divided by 806=.9935 or 99.35% rejection.

Another tip is always use a clean container to test in. I suggest a squeaky clean clear drinking water glass with absolutely no water spots or soap residue as they will throw the readings off. First test the tap water then triple rinse the meter and glass with DI if you have it available. If you do not have DI use RO for the rinse or buy a jug of distilled water for that purpose as you want to keep the meters probes clean at all times as well as your glass.
Next test the RO TDS and again triple rinse. If you have RO/DI test the DI TDS then do a final triple rinse and you are done.

Never test out of a storage container or topoff vessel if you are testing the efficiency of the RO/DI unit, the readings will be skewed by air, CO2, contaminates in the plastic or vessel etc. Those are good numbers to have and certainly test from everywhere except salt water(which is way above the measuring range of a TDS meter) but this will not tell you how your unit is working. Always test straight from the unit into your glass.

11/15/2007, 08:23 AM
For RO only, 17 isn't bad. but if you use a DI cartridge as well, it should be 5 or less. I change my DI resin when my TDS reach 5.

hope this helps!

11/15/2007, 09:04 AM
Again, you CANNOT say if it is good or bad if you do not know the tap water TDS. If the tap is only 50 to begin with its not doing much in the way of treatment. If the tap is 1000 is doing OK. You must know the starting point to make a statement like its OK.

In my mind 17 is pretty high considering the national average tap water TDS is around 250. That would be a 93% rejection rate which is not spectacular by any means, 96 to 98+ is where you need to be.

11/15/2007, 09:12 AM
Thanks a bunch! Your replies were helpful!!! My tap water TDS was 274 and my after RO TDS was 017. I called the people at Buckey Field Supply and ordered the recommended BFS-72 DI Kit to add on to my existing RO system (very simple add on by the way), with color changing resin and they told me my after RO/DI TDS would read close to ZERO with what I'm already testing at. That's what I'm talking about! Can't wait for it to arrive!! (By the way, the add on kit was $38.00). Not bad for perfect water for my crew!!! Got some nice corals coming in and want to make sure they get the best. Again, thanks for your quick and great replies!!!


11/15/2007, 09:17 AM
You might want to look at improving your RO membrane soon. At 17 the DI resin will not last much more than 150-200 gallons. By improving the RO TDS even only 2% you could possibly double that DI life to 400 gallons. The cost of 3 or 4 DI replacements is about equal to the cost of a membrane.