View Full Version : conductivity of RO water

01/23/2007, 06:18 AM
I don't have a TDS meter but have the ability to find the conductivity of my water at work. It reads at 5.8uS/cm, read microSiemen/centimeter. I am unsure if this is the same unit as the TDS meters read, but I think that is ppm. So is 5.8 uS/cm bad and need replacement of the filters or is it within reason for conductivity?

01/23/2007, 06:44 AM
well to answer my own question, i found out TDS in mg/L is about 2/3-3/4 of the EC measured in uS/cm, so ~.7 x 5.8 = 4. I this is too high correct? Also is TDS in the hobby measured in mg/L or mg/ml, this I am confused about.

Randy Holmes-Farley
01/23/2007, 07:34 AM
If that is post DI, the DI needs to be replaced.. If it is just the RO, then see the discussion below.

TDS is ppm TDS (mg/L is close), although the actual units of TDS vary somewhat as detailed in the article below.

The usefulness of TDS with respect to an RO membrane (no DI) is to make sure that the conductivity (TDS) drops at least 90% from the tap water to the effluent. Better units will drop it at least 95%.

These articles have more:

What is TDS?

Reverse Osmosis/Deionization Systems to Purify Tap Water for Reef Aquaria

from the second one:

If you are evaluating an existing RO membrane and can collect water from the tap and after the RO membrane, the conductivity (in mS/cm or ppm TDS) should drop by a factor of more than 10 across it (to as much as 100), relative to the tap’s water. If the drop is less than a factor of 10, it is not working properly, and may have holes in it.

Monitor the DI resins by measuring the effluent’s conductivity, either with an inline meter (set to its most sensitive level), or by measuring the effluent manually. If you are using a TDS or conductivity meter, then the measured value should drop to near zero, or maybe 0-1 ppm TDS or 0-1 mS/cm. Higher values indicate that something is not functioning properly, or that the DI resin is becoming saturated and needs replacement. That does not necessarily mean, however, that 2 ppm TDS water is not OK to use. But beware that the flow of impurities and the conductivity may begin to rise fairly sharply when the resin becomes saturated. Do not agonize over 1 ppm versus zero ppm. While pure water has a TDS well below 1 ppm, uncertainties from carbon dioxide in the air (which gets into the water and ionizes to provide some conductivity; about 0.7 mS/cm for saturation with normal levels of CO2, possibly higher indoors) and the conductivity/TDS meter itself may yield results of 1 or 2 ppm even from totally pure water by not being exactly zeroed properly. Also note that the first impurities to leave the DI resin as it becomes saturated may be things that you are particularly concerned with (such as ammonia if your water supply uses chloramine or silica if there is a lot in the source water).