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slow response time of Ph circuit

I have a simple PH circuit following is the description

A pH probe involves measurement of the DC Voltage created between one Silver/Silver Chloride reference electrode - that has connection to the conductive water via a low impedence salt bridge (a porous plug filled with highly conductivie potassium chloride) - that creates a known and very constant voltage.

It has an instrumentation amplifier as it is not possible to measure the voltage directly as it is a high impedance circuit.

It was working fine but lately it takes like 15 minutes to settle down. So it has a long long response time of 15 minutes now.

What can this be due to ? Any ideas ?

Reply to
aman
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What event separated the working and non-working epochs? Is this commercial or one-off?

This symptom frequently follows when a commercial probe has been left in the air overnight or has otherwise been abused. The glass in the bulb has a lot of nuance in it. Perhaps someone else can post their rehabilitation process. If the (ab)use involves strong alkalai (50% KOH) or profoundly strong first group elements (oh, to guess, 15M LiNO3) I would go read the literature.

If you can get a sequence of voltage measurements each second or so, I'd love to try a digital filter on the data to see how soon a reliable reading could be estimated, but that is just for my own curiosity.

Reply to
Aubrey McIntosh, Ph.D.

It sounds like your Silver/Silver Chloride reference is running down. Sensors like that _do_ wear out, and this is what happens when they do.

So, you'll probably need to replace the expensive part of the sensor.

Good Luck! Rich

Reply to
Rich Grise

Bad Reference Electrode !, or bad Combination Electrode

Plugged Reference Electrode Junction

Ultra pure water, Very high Resistance ~10/15 Megohm ! What are you measuring ! Standard Buffers are ~0.05/0.10 Mol. If it's slow response to Standard Buffers then likely a plugged Junction Fast Test, Hold beaker in one hand while measuring pH and shuffle foot across floor, with Bad Junction, readings will swing wildly due to static electric build-up.

Yukio Yano

Reply to
Yukio YANO

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