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Calibrating the Meter
on March 22, 2011
This review is on both the meter and the calibration fluid designed to calibrate the meter.
I bought the TDS-3 from Amazon as a check on the efficacy of my Brita filters. Shortly after I bought it I misplaced it (don't ask, it's a lesson in applied stupidity) so I bought another. You guessed it; I found the first one immediately after the second one arrived.
Having the two meters did provide one benefit though, I could compare the two. I was quite disappointed to note that there was a significant difference between readings for the two meters.
If you have this meter, be sure to go to the web site for the manufacturer (Google TDS-3). They have an excellent discussion on how to use the meter to take measurements, what the meter's limitations are, i.e., what it measures, and how to calibrate it.
Using their instructions, i.e., taking measurements correctly, the two meters were much closer in their measurements. There was from 2 to 3% difference in the numerical readings for tap water, Brita-filtered water and distilled water. Both read zero for distilled water. Not too bad for home use. It's really important to do it right, though.
Since there was a difference between the two meters, however small, I decided to calibrate them using TDS's own calibration fluid. This is a mixture of water and NaCl to a known concentration. They offer 1000 ppm and 342 ppm. The TDS-3 is supposed to be calibrated against the 342 ppm fluid.
Amazon offers TDS calibration fluid. The product is pictured with two bottles, one at 342 ppm and behind it in the picture, 1000 ppm. Since the 342 ppm was in front and much larger, I assumed that's what would be sent. Well...we all know about assumptions.
The fluid I received was the 1000 ppm. When I contacted the company the representative said 1000 ppm was all they offered. This disagrees with their web site but I didn't press it; it's not that expensive (shipping was almost as much as the fluid itself) and I made some 333 ppm fluid by diluting what I had 3-to-1 with distilled water. The meters read low with my jury-rigged calibration fluid, 304 rather than 333. Not tremendously scientific but good enough for my purposes. The company did offer to take the fluid back - with me paying for shipping.
So I have rated this meter 3 because of the misleading approach to selling calibration fluid, the claim that they only offer 1000 ppm, and the discrepancy between the two meters when their web site says "no need to calibrate out of the box".
The meter is OK for my purposes. I can measure filter efficiency relatively rather than absolutely. I don't need precision; it's good enough to note that the filter reduces TDS without actually knowing the precise value of the reduction. In short, if I lose both of these (not inconceivable) I'd buy another.