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aquaman05
11/18/2009, 07:55 PM
I have a RO/DI question.

I'm setting up my ro/di and was wondering when everyone test their water are you testing tds or ohms? I know when you are running a di cartridge and if its a good one tds is out and ohms are in. Im guessing your looking for around .05 on tds meter?
I want to install a guage of some kind on the out side of my di cartridge letting me know when i need to change the filter. I can get a inline tds but it only goes to .05. If i want to go any lower than this i need to go with a red light green light ohms meter light.

Any thoughts?

Thanks

Aqua

natedogwv1
11/18/2009, 08:22 PM
i use a dual tds meter that monitors input and output. it will read down to .00 it is made by HM Digital model DM1.

aquaman05
11/18/2009, 08:29 PM
So what is everone looking for when running a rodi.... .00 ?

sjames
11/18/2009, 08:32 PM
pretty much, anything more indicates your di is exausted

Buckeye Hydro
11/19/2009, 06:39 AM
Your tap water may range from less than 50 to over a 1000 ppm tds. Your RO water should show a 90%+ reduction of the tap water tds, and after the DI the TDS should be 0 ppm.

Frogmanx82
11/19/2009, 08:12 AM
Most industrial users use resistance and measure ohms. TDS meters are ok for our purposes, but 0 on an TDS meter does not mean there are no dissolved solids, they are just below the detection limit. To really know how pure the water is, you should test ohms. 1 megohm resistance is pretty pure. 18.3 megohms is the theoretical limit for ultrapure water. I'd be curious to know what sort of resistance we get out of these home units.

Buckeye Hydro
11/19/2009, 08:38 AM
TDS meters are ok for our purposes
Yes - agreed.

Wilberheim
11/19/2009, 08:47 AM
How would one go about testing the resistance of water?

Frogmanx82
11/19/2009, 08:53 AM
Conductivity meter.

Randy Holmes-Farley
11/19/2009, 11:16 AM
How would one go about testing the resistance of water?

These have details on how to do it, what it means, where to do it for RO/DI, etc:

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

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

Frogmanx82
11/20/2009, 12:00 AM
Interesting article. So TDS really is not what is being measured. All the talk of TDS should really be replaced by a direct measurement of the conductivity. Then you at least don't have to worry about what conversion they used or delude yourself into thinking there are no dissolved solids when several neutral solids aren't measured.

What conductivity would typically give a 0 TDS on a meter?

Randy Holmes-Farley
11/20/2009, 05:43 AM
I think they are perfectly adequate for RO/DI monitoring.

If the meter can only read whole numbers of ppm TDS units (some can read to 0.1), then any true value of about 1 uS/cm or less will read 0 ppm TDS.

jayinh
11/20/2009, 05:57 AM
i have a duel TDS meter on my RODI it says 90 something input and 0 outout, sometimes the output goes to 3 or so then i flush it out and it comes down.

dygaar
11/20/2009, 07:20 AM
I use a duel TDS meter on my RODI it's usually 7 after RO and 0 after DI.

Frogmanx82
11/20/2009, 08:09 AM
I think they are perfectly adequate for RO/DI monitoring.

If the meter can only read whole numbers of ppm TDS units (some can read to 0.1), then any true value of about 1 uS/cm or less will read 0 ppm TDS.

So 1 uS/cm is 1 megohm resistance and is typical of the performance of a home rodi system. That's about what I figured. I think it is deceptive using TDS values of 0 since it implies there is nothing in the water when in fact ultrapure water which has 18 megohms of resistance is very hard to get.

TeePee
11/20/2009, 08:22 AM
I use TDS as well. The book with my RO/DI unit (Spectapure) says to replace the filters/DI cartridge after a certain volume has passed through, discoloration, or when you start seeing a reading on the TDS meter.

menthol
11/20/2009, 08:32 AM
I use TDS as well, working out pretty good. My water gets delivered to me by a local drinking water station, TDS reads out 0, sometimes 2.

mikeBurchett
11/20/2009, 04:52 PM
tds of zero is ideal. Never tried testing for ohms.

Frogmanx82
11/21/2009, 08:42 AM
Yes, TDS of zero is ideal, you're not getting that. You're just getting a 0 on the meter.

As already stated, for our purposes that's good enough.