Tom Handley wrote:
>   Gavin, while I'm no expert on pH probes, you are probably looking at an
>impedance of 10^9 to 10^13 ohms. Analog Devices has an example pH probe

Gavin,

those numbers are probably 20 years old, todays ordinary pH probes fall into
the 10^8 ohms class (the one I use is specified for 200 Mohms maximum
internal impedance). OA with input bias current of several pA (like
TL061...) will make at most few mV voltage drop. And since your pH probe is
always calibrated with buffers, the same error is encountered during both
the calibration and the measurement, so it actually cancels out. Althouhg OA
featuring units of fA Ib current will also work, it will bring very little,
maybe insignificant improvement.

On the other hand, there could be special high impedance pH probes, for
special purposes like high pressure, complicated shape, microprobes, for
strongly alkaline solutions and I don't know what else. But you probably do
not have any of these, do you ?

If you could describe your circuit in better detail, I may be more helpful.
Is it powered from a battery or from a wall socket ? What is the whole
instrument connected to ?

Josef

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