I've always wondered about the accuracy of TDS meters, specially when we're to be getting all freaky over a couple of PPM either way

i have used my well water in my reef in the past (for 1.5 years) with no problems, but since upgrading (merged tanks with the bf) and investing many k's into our livestock we have switched to store bought RO water (the 5gal drinking jugs) to be safe :mrgreen: and so we know exactly what is going on with our water chemistry, and it has paid off.

however we plan to use well water again once we get our new FOWLR up and running, it will just have easy softies (shrooms, anthelia etc) and of course fish.

oh and we use well water in our bio-cube, but it hasnt anything other than shrimp in it so thats why.

In cases like this, it's wise to put a second RO cartridge. Test the water coming out of the second RO cartridge on a regular basis, and when it gets up to 15-20 ppm, throw away the first RO in line, put the second RO into the first spot, and put a fresh one into the second spot. Be sure to change out the sediment and carbon filters every 6 months (no matter how much you use it). Change out the DI resin as soon as you get 1 ppm tds. You should be able to achieve 0 ppm tds this way, and if you swap the two ROs like I suggested you actually won't burn through many more RO cartridges than if you were only running one. :)

You mean run them in series? Do I would have the clean water from one membrane go into the other? 2 RO waste drains? Or parralel them?

Don't wonder, unless you spent some serious $ on your meter the standard deviation is ~ 10ppm.

For the OP, keep in mind that carbon reduces any volatile organic not just chlorine. Pesticides and organic phosphates are always a possibility in farm country.

Very good question! I actually haven't noticed how these units have run, although I do know that generally having dual TFC membranes is considered "commerical". I did a quick Google, and couldn't find any pictures that were detailed enough to figure out how they are hooked up. I'm very interested to find out as well, as I think I will need to add a second TFC membrane when I move to Saskatoon.

I have never heard this before. AquaFX advertises their digital hand held meter as having accuracy +2% from 0-999 ppm.

For instruments I'm use to seeing accuracy stated in units, eg +/- 0.1Volts.

For the AquaFX a percentage of what? Could potentially see this as +/-2% of full range so +/- 20ppm.

personally i would keep the carbon and just put it down to 1 micron. RO/DI units with out 1 micron fiolters for marine aquariums is just taking away from your membranes life. Also flush kits help lengthen the life of a membrane. Carbon doesn't just take chlorine out either.

Housed in a pocket sized ABS case, this electronic Total Dissolved Solids tester is very durable.

- Wide Measurement Range: 1 to 999 ppm
- Excellent Accuracy: + 2% Full Scale
- Long Battery Life: more than 1000 measurements
- One-Point Calibration

That's how I interpret it too Mark.

The instructions for my Milwaukee CD97 states both:
+/- 10ppm @68F
1% EMC deviation of full range (999ppm)