Recently added is a new section detailing the construction of low cost electrodes made from pencil leads. Use of these in place of the original commercial (RS) electrodes saves some £50 in the overall cost of the apparatus. They are easy to construct, work very well and are robust & reliable in use.
Following a re-examination of the use of a 741 OPAMP in place of the long-tailed pair that was originally used as the differential amplifier, the 741 has been found to be better, particularly in the removal of common mode signals that (inevitably) arise from transients on the input square wave signal. A lower balance null voltage* is achieved and the noise is lower. I tried a number of higher spec, more expensive OPAMP chips but none was sufficiently superior in ths application to justify replacing the very low cost 741.
* Never quite zero owing to residual ac components that arise from electrode polarisation (see next section).
The effects of electrode polarisation can be reduced by increasing the frequency of the input square wave. By changing just one resistor the frequency has been raised from 10.3 kHz to 27.8 kHz. A transient on the rising edge of the input waveform is effectively attenuated by the use of the 741 (above).
CALIBRATION, Linearity etc.
Following detailed studies of the above new information is being added to the calibration section. Guidelines have been evolved for allowing very sensitive measurements over a 100,000-fold range of conductivity.
A PCB is being developed to carry all the circuits shown at this site on one board. This is taking longer than I expected but should now be available soon.
News & Recent Additions to Site