In Digest #1117, "Results of ppm calculator lab test", Herx wrote: "I recently posted a link for a spreadsheet I wrote to calculate the ppm of homemade CS. "Ole Bob" was kind enough to offer to test samples I made, and compare his lab results to the predictions of the spreadsheet. Well the the results of the Ole Bob test are in, and Im sorry to say they were rather disappointing. As Guenter suggested, Ag depositing on my cathode had a significant impact, one that I failed to take into account. I hope to compensate for this in future versions of the ppm calculator. Ole Bob and I discussed the results and here is our conclusion: The ppm calculator spreadsheet uses Faradays equation to calculate how much Ag is liberated from the anode. It does this accurately, but not all of the Ag liberated winds up suspended in the distilled water. Some gets deposited on the cathode, so the actual ppm is going to be less than the alculator predicts. How much less depends on several factors. Bob and I believe that the most important factor is the surface area of the cathode (size of the anode is not relevant here). Unfortunately, I used rather large cathodes when generating the samples I sent to Bob for analysis. Time is also a factor; the longer current flows, the faster Ag seems to plate to the cathode. So shorter brewing times may result in more accurate ppm prediction than longer brew time.
Bob and I hypothesize that a cathode with very little surface area will not allow significant amounts of Ag to plate, and so the ppm calculator results would be reasonably accurate. We intend to test this hypothesis, hopefully by the end of this year. We will post the results. Bob noted that since high voltage CS is not as prone to plating, the spreadsheet results may be much more accurate for HVCS. Ive asked him to test this if he has the opportunity. But most of us low-tech types (myself included) use low voltage (under 40V) and I hope to find a solution for us." A while back I came to the same conclusion re cathode area and plate-out. After having little improvement with going from large area stainless cathodes to 1/16" diameter stainless wire cathodes, toyed with borrowing an idea from the Microwater people - cocoon the cathode(s) in a membrane such as chamois, that allows current flow but restricts access of silver ions to the cathode surface. However I believe the best solution is to borrow another idea that a CS vendor (can't recall who) uses - reverse the polarity regularly. I think that vendor's machine switched polarity about once a minute. An adjustable timer is preferable, as optimum switching period probably will vary depending on the particular setup used. It neatly solves a number of issues. No need to clean the cathode surfaces of oxide buildup. No need to filter the brew. No need to worry about how much silver is lost via plate-out - should be negligible. The disadvantages are increased complexity and cost, and the need to ensure that all electrodes are silver. To me, benefits outweigh costs. Any electronics enthusiasts on the list willing to come up with a DIY circuit design?
