I called Hanna Instruments and spoke to their chemist to find out exactly what the TDS-1 does and how it does it. It was instructive.
Their chemist said the TDS-1 actually measures conductivity between the two probes and uses this to estimateTotal Disolved Solids. According to the chemist, it is usually used for measuring salts and things like that, but it can be used for testing solutions like Colloidal Silver also. However, she was careful to tell me to use the TDS-1 only to give relative measures and not to compare the result to a commercially tested solution. She said the TDS-1 is not specifically made for testing CS and will not give an exact measurement. Furthermore, she said that it tests solids rather than ions. I think that is somewhat different from my previous understanding. The TDS-1 puts out 6 volts from one probe and measures how much current reaches the other probe. The measure is then multiplied by a conversion factor of 0.5 to get the parts per million (ppm). She said a different conversion factor would have to be used for silver, but wasn't sure what the correct factor should be. I suspect that the conversion factor for silver should be lower because metals (like silver) should be more conductive than salts and would therefore give a higher reading relative to salts. This might explain why I got a result of 18 ppm for my home-made brew and got a whopping 98 ppm for a commercial brand that claimed it was 10 ppm. For all I know, though, the commercial brand has other stuff in it or was made from water that wasn't distilled first. It seems to me that I need to find a water lab that can tell me exactly what I have and then I'll have to calculate a conversion factor to measure my solution in the future. If any of you experts out there has better information, or can give me some direction, I'm anxious to hear from you. I think I had assumed that the TDS-1 would give me an accurate measurement of the ppm in my CS. Have a great weekend everybody. Bob Wells
