# pH Circuit Design

Discussion in 'General Electronics Chat' started by rocketman, Sep 15, 2005.

1. ### rocketman Thread Starter New Member

Sep 15, 2005
1
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I was handed a project to correct a pH circuit design. I have the circuit breadboarded and running.

The problem with the circuit is it is extremely slow to resond to changes in pH, on the order of 10 minutes to go from a pH reading of 7 to 10 (2v to 4.5 v on the op amp output)

The problem with me is that I am not an electronics engineer and do not understand why the circuit has such a slow response.

Can someone please explain to me why the responce rate is so slow and more importantly how do I correct it?

thanks

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2. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Hi rocketman,

It would be helpful to know the setting of the potentiometer that is apparently there to allow you to adjust the gain of the opamp.

hgmjr

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3. ### Yeti Active Member

Jul 26, 2005
35
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I am not an electrical engineer yet, but im working towards it. But for a preliminary guess i would say that the capacitors are slow to charge/discharge on the op amp and are "slowing" the voltage change from 2 to 4.5.
Maybe someone with a little more EE background could give a more or correct answer for you.
And maybe a detailed explanation of how the circuit works

Yeti

4. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
6
How fast is the transducer supposed to respond? The ciruit can't respond any faster than its input.

5. ### hgmjr Moderator

Jan 28, 2005
9,030
214
After thinking about your amplifier response time problem, I believe I have come up with a couple of explanations for your unexpectedly slow settling time.

First, I don't know what the specifications for your pH probe are but it is possible that the output impedance of the probe may be very high. If that were true then the time constant of the integrator network at the input to your opamp would be t = (Rprobe + 10K)*C2. For example, if the pH probe output resistance is 1 megaohm then the time constant t would be 1 sec and since it takes five time constants to settle that would give you 5 secs of your delay right there. If the pH probe resistance is 2 megaohms then that would account for all 10 seconds of your response time.

Another factor that would increase the response time of circuit would be the very small signal output by the pH probe. The opamp integrator response time is dV/dt = I/C. The value of dV is (4.5V-2.0V) or 2.5V. The value of dt is 600 seconds. That gives a dV/dt value of 0.00417 V/sec. Multiplying this result by the value of C1 gives a value for I of 0.00000000417 amps (4.17 nanoamps). This value of I is the estimated current that is output from your pH probe.

Most likely the slow response time is a combination of these two factors. With more details on the specifications of the pH probe is would be possible to verify my figures.

hgmjr

6. ### tonykuphaldt Member

Jan 10, 2004
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Hgmjr is right on target with his diagnosis: the problem is probe impedance. Specifically, the "measurement" electrode in a pH electrode pair is made of glass, and typically exhibits many HUNDREDS of MEGAOHMS of electrical resistance. This, combined with the 1 uF capacitor on the noninverting side of the opamp, creates an enormously slow RC time constant circuit. Since there is nothing you can do about the probe's resistance -- it's the price you pay for the privilege of electrically sensing pH in a solution -- your only option is to use a much smaller capacitor on the opamp input.

This problem is so acute that even parasitic capacitance in the cabling between probe and amplifier can cause slow measurement response in an otherwise well-designed system. Industrial pH measurement applications where cable length exceeds about 20 feet or so usually employ preamplifier circuits located at the probe end of the cable to electrically "buffer" the signal prior to its trip down the cable to the final measuring instrument. And speaking of amplifiers, you'll need something with an input impedance in the order of TERA-OHMS (10^12 ohms) so it does not significantly "load" the voltage signal generated by the glass electrode. I don't have time to check the datasheet on your opamp, but it is worth your while to do so to ensure it is up to the task.

Hope this helps.

- Tony

7. ### chrismicro New Member

Nov 24, 2007
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Hi Tony, I don't know if I have it wrong, but isn't the glass only protection for the metal probes and is it not the resistance of the water between the metal probes that is measured? can anybody tell me, is the probe producing any output signals? and if so what form?

8. ### Papabravo Expert

Feb 24, 2006
10,145
1,791
I feel your pain. Your boss is obviously a dimmer bulb than the fellow with the hair horns that manages our friend Dilbert. Check with Ratbert, the evil Director of Human Resources, for an Elbonian Outsourcing Solution, of the proper pH ...of course! Ahhhh...cha..cha..cha

9. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
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Chris, do you realize you replied to a 2-year old thread?
Anyhow, the answer to your first question is no. For the answer to your second question, see Wikipedia pH meter and Wikipedia glass electrode.