What wouldn't show on a TDS meter?

[Greg Hiller]

Randy,

I own an El'Cheapo TDS meter, which I assume is probably just a very simple conductivity probe. Are most TDS probes just conductivity probes? The meter I have will read down into the single ppm range, but with no decimal places. I typically change my DI cartidge when I get a reading of 1 ppm (as opposed to 0 ppm) from water just as it exits the RO/DI system.

Anyhow, if one were using a typical TDS meter to monitor the output from an RO/DI system, and the meter read 0 ppm. Is there anything that could be coming through the system that would not register on the TDS meter? Some type of non-charged compound that would have very low conductivity?

And, if the answer is yes, is it likely that that compound would come through the DI cartidge (assuming the cartridge is just about spent) before something else that would give a signal on the TDS meter?

 

[Bojan]

Re: What wouldn't show on a TDS meter?

<a href=showthread.php?s=&postid=6651090#post6651090 target=_blank>Originally posted</a> by Greg Hiller
Are most TDS probes just conductivity probes?

YES.

 

[Randy Holmes-Farley]

Double yes.

I typically change my DI cartidge when I get a reading of 1 ppm (as opposed to 0 ppm) from water just as it exits the RO/DI system.

A perfect plan, IMO.


Is there anything that could be coming through the system that would not register on the TDS meter? Some type of non-charged compound that would have very low conductivity?

Small neutral organics that got through the RO membrane. Chloroform, etc.

If the carbon is bad or inadequate, chloramine might get all the way through. Gases like O2 will pass through.

H+ or OH- might get through if one half of the resin becomes depleted first.
FWIW, my RO/DI water ALWAYS has high pH. I'm not sure why.

I discuss all these issues in:


Reverse Osmosis/Deionization Systems to Purify Tap Water for Reef Aquaria
http://reefkeeping.com/issues/2005-05/rhf/index.htm


and

What is TDS?
http://www.reefkeeping.com/issues/2004-04/rhf/feature/index.htm

 

[boxfishpooalot]

<a href=showthread.php?s=&postid=6652570#post6652570 target=_blank>Originally posted</a> by Randy Holmes-Farley

If the carbon is bad or inadequate, chloramine might get all the way through. Gases like O2 will pass through.

Carbon removes oxygen? Even in tank water!

 

[Randy Holmes-Farley]

No, that's a new sentence. I meant that is passes through an RO/DI.