I just want to give you all a quick prod in a good direction on this... The TDS-1 from hannah reads about 1000 ppm at full scale. There is another meter, the DiST-1-WP, that was recommended once, but is going to be even worse to use, with a full scale of 2000 ppm. Both of these meters are not working at their best trying to measure less than 30 ppm.
I suggest looking at their PWT, which stands for Pure Water Tester. It measures, not ppm, but nanoSiemens, which is a unit of conductivity. If I remember correctly, the full scale range is something like 100 or 200 nanoSiemens (nS) which corresponds to either half or twice that much ppm for the calibration solutions they use. I don't remember exactly which way that goes. The point is the PWT has electronics that are set up to measure a much lower range of conductivity than the others, so will be more accurate for our application. PLUS, it has the advantage of *forcing* you to make up a calibration curve between your readings and ppm measured by some other method. This will make it much harder to fool ourselves into thinking the device is magically measuring ppm when it actually isn't. I believe it costs about $40 (US). The other thing I'd like to mention is there is an alternative to the 800 ppm calibration solution (HI-7032P) they recommend for the TDS-1. The sales people won't recommend it, but the HI-7033L conductivity standard (84 uS/cm) is *MUCH* closer to the range we're working in, and is equivalent to (again, I can't remember exactly) either twice or half that number of "ppm" when measured with a TDS meter. I have messages from a fellow at Hannah that confirmed both of these approaches. I'll try to dig them up and post the info, but I don't have time right now. In the long run we'll be better off measuring conductivity, and calling it that, when using these instruments. We can reserve the term "ppm" for techniques that actually deliver that measurment. Mike D. [Mike Devour, Citizen, Patriot, Libertarian] [[email protected] ] [Speaking only for myself... ] -- The silver-list is a moderated forum for discussion of colloidal silver. To join or quit silver-list or silver-digest send an e-mail message to: [email protected] -or- [email protected] with the word subscribe or unsubscribe in the SUBJECT line. To post, address your message to: [email protected] List maintainer: Mike Devour <[email protected]>
