pH meters

You also may ran into capacitative issues. Something one may cope with, but should be aware of in advance.

AgX said:

No good idea, as the internal resistance of a VOM (=multimeter) likely will be too low for the requirements of such electrode circuit.

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The output of the probe must be buffered with a suitable op amp it seems. Then you can take a reading with a voltmeter from the op amps's output.

Regarding high impedance instruments, instruments such as a Keithly high impedance electrometer periodically appear on ebay. Also, one could build a FET input amplifier. A JFET input stage might be suitable. Gate current of the FET gate might be a factor to consider, though it is generally low for JFETs. MOSFET input amplifiers generally have lower leakage and bias currents, so this is a possibility, though MOSFETs have other issues, especially sensitivity to static electricity.

Vacuum tube voltmeters are another possibility, though this is pretty antiquated technology.

Chopper stabilized amplifiers might be another possibility, though that might be getting pretty fancy/expensive, and with modern FETs this might not be needed. Anyway, if I recall chopper stabilization would typically be more useful for correcting for voltage noise issues than for input impedance issues.

Anyway some of these points might get the creative juices flowing.

We should not overlook that a single classic probe is in the same price range at least as a decend pocketable meter. And there are quite reasonable priced control units that would take most probes. Building a metering circuit would be possible, but may be too much hassle for a side issue.


The main question should be:

-) at what process or emulsion-making stage electronic pH-metering makes sense at which tolerances ?

-) what kind of probe is necessary ?

From then on one can look through catalogues or consider even tinkering.

Sorry guys but it works perfectly. We used them at Kodak. I've used them at home with good results. See my book.

PE

AgX said:

It does not fulfill the OP's reqirements as its accuracy is 10-times worse. (I refer to a requirement of a 2-digit readout and the resp.accuracy.)


Of course, when the OP states to be sufficient with a 1-digit readout the whole discussion is fruitless.
He then even might consider high-sensitive ph-strips.

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Don't confuse read-out with accuracy.

The OP wanted a meter with 2 decimal places. With respective accuracy I meant that with a meter that has a 2 decimal places readout one likely expects a deviation only at the last decimal. But in general some meters are far worse.

In the quoted post above with digit I meant decimal place. Sorry, if my error confused you.