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I went over and read the posts Matt. I am pretty sure that the point about TDS meters using conductivity to estimate TDS is probably accurate. I am going to call the folks over at Hanna and get the scoop on that tomorrow. I now how to phrase the questions now... 8)
Conductivity measures exactly that... the ability for the aquarium 'solution' to conduct electricity. What does that is the ions in the water. No ions... no conductivity. That is pretty clear, and it is also clear how to measure it.
TDS, from the definition that will most likely be presented to me by Hanna, will not include carbonates because those substances do not dissolve (break into ions) in water. I think that the big confusion all along has been the difference between water HARDNESS and TDS... they are not the same thing (though... I guess common sense would say they are :? ).
HARDNESS is defined as the level of carbonates in the water, and results in water have a high alkaline buffering capacity. Since carbonates will not show up on an EC meter, then they are likely not included in the TDS reading of the meter.
I know that this is different from what I posted before, because I have always considered carbonate hardness to be a part of total hardness. I think that the answer I am going to get tomorrow is that carbonates do not count in TDS....
So what do we need in a meter? I want a meter that allows me to reconstitute r/o water accurately. I personally an not overly concerned with test tap water... I know that mine is pretty nasty 8O. I will want to reconstitute in such a way that the pH will be low, but not crash. I can use a small amount of Alkalinity Buffer, then adjust the pH using a good meter and Acid Buffer. Since I am starting with r/o, I know what is going in as far as carbonates are concerned. The meter will tell me when adjustments are necessary. I also want the EC to keep track of ions that inevitably build up in water. I can also use it to make sure my r/o-DI equipment is working well. I can also use it to reconsitute up to specific TDS levels using salts.
One thing that I have discovered during this search for meters is that there are A LOT of meter options. However, not all meters are created equal. I have chosen the Hanna meters (combo pH/EC/TDS/temp) because it is comparably prices to single parameter meters of the same quality.
The Hanna pHep4 pH meter uses the replacable electrode technology, is waterproof, has a simple automatic calibration and will automatically adjust for temperature... it retails for about $70.
The Hanna EC meter with the same bells and whistles is also about $70; as is the TDS meter (though we have pretty much figured out that TDS and EC are really measuring the same thing and converting).
The combo meter has the same features as all three single parameter meters for a retail cost of about $130. Even if yo only wanted EC or TDS, not both, the combo meter at $130 is a better buy than $140 for two separate meters.
I will let everyone know what Hanna says about the meters and carbonate hardness tomorrow sometime.... but my money is on the 'carbonates do not count in TDS' answer. Even money...any takers :wink: .