Hi Trem, Sounds like you have a good unit on the market. Whether you like it or not, it is measuring current. Voltage and current are inversely proportional through a load (the resistance of your CS solution). With a constant current source it is measuring the voltage drop across the solution to trip the comparator. Semiconductors, like comparators trip at a voltage offset. Usually there is a resistor voltage divider to convert the current to voltage. A zener diode or some other voltage reference sets up the trip point. Current goes up, voltage goes down and vise versa. It's just a matter of perspective. Since a 14 pin DIP package contains 4 comparators you could probably make one that compensates for the DW variations without much modifications. Just a brain fart...
Andy (^_^) From: Trem Hi Andy, Since you asked.....we use a voltage comparator to shut our units off. We calibrate the SG6 so the production light does not come on if the water is under about 2 uS which gives the user a general idea of water quality. If the light doesn't come on then the length of time it does take to come on will indicate water conductivity in a roundabout way. Longer time means better water. We do NOT measure current. It is limited and is a constant once the unit is up to speed. That function is indicated by the production light being fully lit. From then on the voltage across the electrodes is continually reducing in order to maintain the constant current. So, it makes little difference what the water volume is......the unit runs until the voltage comparator senses the preset voltage on the front dial is the same as the voltage across the electrode. The unit then shuts down. At that point one can use a PWT meter and get a rough idea of the amount of silver in the water by subtracting the initial water reading and adding a correction factor. If one does not have a meter it makes little difference since the unit checked the water at startup and would have been less than 2 uS. I'd say "it's plenty good enough". If someone waits a while and turns the unit off and back on again it will add more silver to the water. We calibrate them to work properly from startup to shutdown. I hope this helps you. Regards, Trem Williams http://www.silvergen.com ----- Original Message ----- From: ascottsil...@aol.com To: silver-list@eskimo.com Sent: Friday, August 08, 2003 9:03 PM Subject: [silver_list] Re: CS>PWT & its Limits Hi Bob and Dan, When I try to measure CS with a DC ohm meter it acts like either a capacitor or a battery which makes sense. I've got two electrodes in an electrolyte so it is either charging or discharging through the meter (which is supplying current to make the measurement). Just the process of making CS is charging up the "battery" so current restart Bob mentioned makes sense. Measuring the AC impedance rather than the DC resistance (as Ole Bob suggests) would also seem to make more sense. I would assume that the commercial units with automatic shutoff are measuring current by using a voltage divider and a comparator. When a set value is reached they turn off. I doubt that they measure the initial starting current and adjust for variations of DW conductivity. Of course, if the generator runs on a timer, it's probably garbage. Good questions to ask the manufacturer if they provide an "auto shutoff feature." Ole Bob - Just out of curiosity, what kind of variations are you seeing between batches when you try to keep everything exactly the same? Best Regards, Andy From: Robert Berger Hi Dan, With our posteriors exposed we find that when using a DC ohm meter it is necessary to swish the electrodes in short rapid strokes to keep the reading from climbing. Generally when measuring a resistance of a liquid or soil it is done with an AC ohmmeter and preferably one running at 180 Hertz or faster. The commercial units do measure conductivity to determine when the proces is finished. The generators will not start if the conductivity of the DW is above a certain limit. Are you enough of a computer "geek" so that if I sent you off line a program Wplot32.exe that you would install it on the desk top, and then create a file folder "My wplots" under "My documents" to store some files that I would send to you? If you could I would like to send you some data plots of runs that I have made so that you can see some of the strange things that go on with this process. First offf the conductance changes with time i.e. at the end of a run it might be 75 uS/cm and24 hours later if might be 40 uS/cm. If you stop a process for several minutes to wipe the electodes then the current on restart will be 20 to 30% lower. In fact just stopping for a minute and restarting without doing anything and the current readings will drop. "Ole Bob"
