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                                  ELECTRODES

 

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.

 

                                ELECTRONICS

 

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 this application to justify replacing the very low cost 741. However replacing the 741 with an AD797 high frequency amplifier gives a 3-fold increase in sensitivity and a small increase in S/N ratio. At a cost of £10.90, compared with £0.6 for the 741, this replacement does not represent good value for money!

 

* 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.

 

PCB

 

A PCB is being developed to carry all the circuits shown at this site on one board.

 

A prototype design was not successful owing to signal loss by transfer between tracks on the PCB, with signal getting to earth by coupling because the tracks were too close to each other. I now need to redesign the layout and try again!!

 

TEMPERATURE EQUILIBRATION

 

Using plastic tubing for the walls of the electrochemical cells means that the thermal equilibration with the air in the box will be relatively slow. By using metal tube instead of plastic the time taken to reach equilibration (relaxation time) is much reduced. Thus the time reduces from 166 secs with plastic to 28 secs with stainless steel, a factor of approx. 6. This means that it is necessary to wait only 2-3 minutes for equilibration to reestablish if it disturbed during an experiment.

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