In my last blog post, I discussed the basics of ground loop, the most common and yet dreaded phrase field service engineers hear when electrochemistry-based sensors misbehave.
Here is where we left off last time:
The equivalent circuit of a pH sensor in a sample solution, where Eg is the voltage developed at the pH glass electrode of the pH sensor, Er, the voltage developed at the reference electrode of the pH sensor, Rg, the resistance of the pH glass membrane, Rr, the resistance of the reference junction, and Rs, the resistance of the solution.
The signal of the pH sensor is Eg – Er, and we can obtain the signal by measuring it with a voltmeter as connected below:
Now, we have the equivalent circuit of the pH sensor connected to a pH meter (a voltmeter).
The two open-ended nodes are now connected to the voltmeter, and form a closed circuit loop. In other words, the sensor in the sample solution is not an open circuit anymore. Once it’s a closed circuit loop, there will be current flowing through the loop, because there are batteries in the loop.
In this case, the voltage measured is not exactly Eg – Er anymore, but:
Eg – Er – iRg – iRs – iRr (1)
However, a modern pH meter has EXTREMELY HIGH INPUT IMPEDANCE, which means that it will not allow almost any current passing through it, i.e. the current ‘i’ through the closed loop is close to zero. Zero times any number is still zero, so the above equation becomes:
Eg – Er (2)
This means a pH meter can accurately measure the signal from the pH sensor. This is also why you can not use a regular voltmeter to measure the signal of a pH sensor.
Is this circuit loop a ground loop?
Unfortunately no. Very close, but not a ground loop.
However, the practice above is a very good warm up for me to describe the ground loop in my next blog post. And with the practice above, you have learnt the critical requirement of a pH meter, the EXTREMELY HIGH INPUT IMPEDANCE.
In the real world, the EXTREMELY HIGH INPUT IMPEDANCE of the pH meter can be compromised, either by the low quality or deterioration of the cable connecting from the pH meter to the pH sensor, or by the contamination of the terminals either at the pH meter or at the sensor.
Once it’s compromised, you will have an i in equation (1), and when that happens, your pH meter will not measure the pure pH sensor signal Eg – Er anymore, it will have an erroneous reading, and equation (1) tells you exactly why.