Hi all, I am trying to determine whether my pH probe is giving me accurate data or not. I was hoping to measure the voltage with a multimeter. I have the probe in a pH buffer of 4 and according to pH to voltage scale. 1 pH is about 59 mV/pH, Starting at 7 pH = 0. IF thats the case i should be seeing around 150mV but i only get about 10mV

not sure if this is a good place to ask but everyone here is very helpful.

Thanks!

Hello,
Have you got the solution? If yes, can you please tell me as I am facing the same problem
Thank you

Mod: created new thread, previous poster not replied for 2 years.

Old thread link.
https://forum.allaboutcircuits.com/...irectly-with-a-multimeter.172977/post-1555235

 

Hello,
Have you got the solution? If yes, can you please tell me as I am facing the same problem
Thank you

"The source impedance of a pH electrode is very high because the thin glass bulb has a large resistance that is typically in the range of 10 MΩ to 1000 MΩ.
This means that the electrode can only be monitored by a high-impedance measuring device."

Read PDF file:

 

Accurately measuring pH is highly problematic for maintaining accuracy. Keeping and maintaining probes is in itself labor intensive. Even with the best quality probes and proper maintenance scheduling you still need specially designed metering electronics. Working as a Process Control Engineer for a chemical refinery for many years, pH was what gave us the most trouble as we were measuring pH continuously and controlling acid feed to maintain a pH setpoint. In addition pH was measured by the area lab techs by taking field grab samples for comparison as we had gotten into trouble with the city's wastewater treatment plant from our effluent killing all the bugs in their digester due to excessive acidity. Our solution was to install a large agitated neutralizing sump with NaOH injection to control pH close to neutral before sending the effluent out to them. On the process side we were using Foxboro instrumentation and their pH guy was acknowledged as being one if not the best guy in the field. I can't remember his name but I had the privilege of meeting him just before he retired long ago when he was in his 80s. I think they kept him around for bragging rights long after his use by date. Foxboro may still sell his pH book which used to be the gold standard reference for pH measurement and control, I've been retired quite a while so not current. You did not mention if this is a field or lab problem and just what problems you are having...

Seems they have been bought by Schneider Electric and looking at the Foxboro Site I don't see any literature other than their manuals. Try contacting them @ Foxboro's history | Schneider Electric Global (se.com)

 

i have a pH electrode and i want to measure it's input/output impedance, can anyone please tell me how to do it?

 

i have a pH electrode and i want to measure it's input/output impedance, can anyone please tell me how to do it?

1. You do not need to know impedance of your pH electrode at all.
2. If you want to obtain accurate voltage reading using multimeter,
then you should make electronic circuit like that:

------------------------------------------------------------------------
3. But cheapest and easiest way is to buy this pH meter for EUR 5.52
(Approximately US $5.92), free shipping:
Portable PH Meter LCD Digital Tester Pen

 

i have a pH electrode and i want to measure it's input/output impedance, can anyone please tell me how to do it?

Has it been well-cared for? What technology is the electrode?

 

1. You do not need to know impedance of your pH electrode at all.
2. If you want to obtain accurate voltage reading using multimeter,
then you should make electronic circuit like that:
View attachment 287202
------------------------------------------------------------------------
3. But cheapest and easiest way is to buy this pH meter for EUR 5.52
(Approximately US $5.92), free shipping:
Portable PH Meter LCD Digital Tester Pen
View attachment 287228

I am using this circuit but it is not working, instead of LMP7721, I am using LTC2050HVIS8#PBF

 

instead of LMP7721, I am using LTC2050HVIS8#PBF

Therefore it is not working.
Input bias current of LTC2050HVIS8#PBF about 3000 times bigger, than input bias current of LMP7721,
so input of LTC2050HVIS8#PBF simple shorts signal from pH electrode.

LTC2050HVIS8#PBF ___________ versus _________ LMP7721

________

 

I too did a lot of process work with pH and other specific ion probes.
An interesting one was the ammonia probe which was just a pH electrode separated from the test liquid usually waste water, by a PTFE membrane.
When the sample was treated with a strong alkali, ammonia gas would go through the PTFE and change the pH.
The high impedance problems and ground issues were easily solved by putting an op amp close to the electrode, i.e. on top of it, and run it all off a lithium cell.
The amp, a Maxim MAX406 OR 407 did the gain and offset and you could run the signal up long distances with bog standard two core cable.
The amp drew 2.4 uA and the cell lasted for a long time.
Yes, proper calibration with buffer solutions, two point minimum three point sometimes.
And of course, proper temperature compensation.

 

Therefore it is not working.
Input bias current of LTC2050HVIS8#PBF about 3000 times bigger, than input bias current of LMP7721,
so input of LTC2050HVIS8#PBF simple shorts signal from pH electrode.

LTC2050HVIS8#PBF ___________ versus _________ LMP7721

View attachment 287246________View attachment 287280

I tried it with lmp7721, but it's still not giving the desired output, I give it a input of 504mV and it's giving me output of approx 2 V at 4 ph, and the expected output was 670 mV approx

 

I tried it with lmp7721, but it's still not giving the desired output, I give it a input of 504mV and it's giving me output of approx 2 V at 4 ph, and the expected output was 670 mV approx

Change polarity of electrode (switch blue and orange wires).
What voltage you see in that case?

 

Change polarity of electrode (switch blue and orange wires).
What voltage you see in that case?

It's 716 mV approx, still not correct

 

It's 716 mV approx, still not correct

You need to get the basics right.
You could buy a pH amplifier module but learn not a lot.
Any of the operational amplifiers mentioned could easily be set up to do the job.
All you would have to do is make sure the high impedance side follows the rules for this type of output.
Look up electrometer and electrometer amplifiers.
That could include shielded short coax TNC or BNC connectors input guarding etc.
A pH probe puts out about 60-mV per pH unit temperature dependent.
So that's plus or minus 420mV for 0 to 14 pH.
A simple unity gain amp would give you that measurable on your meter.
The beauty of using a dual amplifier is that you could do the buffering with some gain using half the amp and do the offset to give you 0 to 840mV linearly proportional to pH or 0 to 5-volts, whatever you like.
There are loads of application notes available from amp manufacturers complete with circuits and explanations of all the precautions.
Your figure of 716-mV is in the right ball park.
You can always use a millivolt source to an amp to simulate a probe and take it from there.
If you get it working it's very rewarding.

 

It's 716 mV approx, still not correct

See how pcb should be designed:
Read here

 

Exactly. Very nice.

Maim did a nice application note for the MA406/7 and pH.
Analog Devices and Burr Brown were sticklers for guarding techniques.
I think all I did with the MAX406/7 was bend the input pin of the DIP8 package out at right angles.
pH is such a fragile sensor signal wise that you have to treat it gently until safely into the op amp.
At that point, it's straightfoward.

 

@va28, read this:
https://sensorex.com/2016/05/09/ph-sensor-calibration/
After calibration, tune your bias voltage up to 512 mV on output at pH=7.

 

@va28, read this:
https://sensorex.com/2016/05/09/ph-sensor-calibration/
After calibration, tune your bias voltage up to 512 mV on output at pH=7.

The circuit is working properly, thank you for your support