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. 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 Ivan Anderson wrote: > > Tony > > Would it not be much easier to calibrate with a precision resistor? > 100K Ohm = 10uS 200K ohm = 5uS and so on. > > Ivan. > > ----- Original Message ----- > From: "Tony Moody" <[email protected]> > To: <[email protected]> > Sent: Wednesday, 20 September 2000 20:02 > Subject: Re: CS>Measurement of Conductivity > > > List, > > > > Make your own conductivity calibration solution. > > > > To calibrate the meter including probes accurately, a calibrating > liquid > > is required. A saturated calcium sulphate (CaSO4) solution is > eminently > > suitable for this purpose: at 20 deg C, this has a conductance of 1976 > > uS cm-1 (micro-Siemens per centimetre). The solution is prepared by > > mixing calcium sulphate in distilled water until the liquid accepts no > > more of the CaSO4. Note, however that CaSO4 is only slowly soluble in > > water. > > > > By adding an identical volume of water to the solution, the > conductance > > will be halved. Doubling the volume again by adding distilled water > will > > halved the conductance once more. While calibrating the meter, make > sure > > that the solution remains at 20 C, because the conductance varies with > > temperature. > > > > Ref. Elektor 304 circuits. #201 Conductance Meter. 910039 - 12 > > > > > > Tony > > -- > 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]>
