slow response time of Ph circuit

From: aman (aman.bindra_at_gmail.com)
Date: 03/10/05 

Date: 9 Mar 2005 20:04:27 -0800

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 ?

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