Hullo Ivan, Its good to know that using a precision resistor is practically as good as the calibration solutions. > > Is there a link to this circuit? > I'm not familiar with a link. I will undertake to scan the pages and post it , but not right now, I'm moving house and need to concentrate on what goes where and when. Ref. Elektor 304 circuits. #201 Conductance Meter. 910039 - 12 Regards.
Ivan Anderson wrote: > > ----- Original Message ----- > From: "Tony Moody" <[email protected]> > > Hi Tony, > > Thanks for your reply. > > > Ivan, > > I guess the complication of using a calibration solution is to cater > for > > any oxidation on the probes faces or to account for the probes not > being > > the exact size and distance apart. There must be some reason. > > This circuit uses a 10kHz rectangular wave oscillator which is high > > enough frequency to prevent electrolysis. DC voltage or current would > > distort the measurement. > > You're right... the use of a calibration solution is to compensate for > the discrepencies in the sensing mechanism, and it makes sense to > calibrate in a solution similar to the one you are measuring, but I have > calibration solutions in silver nitrate, potasium chloride and sodium > chloride, and have found that the precision resistor to be spot on. That > may well change over time I suppose, but for the purpose of reading the > concentration of CS, which is different from the theoretical measurement > of Ag+ ions by some margin (although repeatable from batch to batch) > some mis-calibration would not matter much. > > > A 555 chip powers a resistance bridge (including the unknown > > conductance) which feeds a TLC272 opamp with feedback to give X10 > > amplification, signal is then high quality rectified with the other > > TLC272, buffered with another one and pumped through a 50uA meter for > > indication. > > > > BTW They talk of using two pins from a print header, preferably gold > > plated, for the probes. Also talks about a fixed distance apart but > not > > an enclosed volume. > > > > Tony > > Is there a link to this circuit? > > Regards - Ivan > > -- > 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] > Silver-list archive: http://escribe.com/health/thesilverlist/index.html > List maintainer: Mike Devour <[email protected]>
