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

Hello, I am working on pH circuit and found this thread interesting. As I am new to this fiedl and trying my best to get the knowledge. I could not understand the circuit shown above. Would you please help me answering my doubts in the photo which I mentioned. Thanks a lot.

 

I am working on pH circuit and found this thread interesting.
As I am new to this fiedl and trying my best to get the knowledge.
I could not understand the circuit shown above.
Would you please help me answering my doubts in the photo which I mentioned.

Triangles are common (ground) of Arduino.
You do not need to add additional components to voltage reference IC.

 

Triangles are common (ground) of Arduino.
You do not need to add additional components to voltage reference IC.

Thank you very much for your prompt reply Mr. Danko....

 

Thank you very much for your prompt reply Mr. Danko....

Dear Danko, I tried my best to build the circuit with real ICs and Arduino board. But I could not get the correct readings.
I thought to share what I did through circuit diagram which I followed. Kindly have a look at it and let me know If I am doing something wrong. Right now I am not considering the temperature.

Will you please answer my doubts below:
1.What will be the out put voltage if I connect/ short A and B (pH electrode) in the diagram? I guess this is going to be pH 7 reading as per my knowledge.
2.How should I get the gain of the LMP7721?
3.What will be the out put voltage if I try to measure pH 4 buffer solution?
4.Can I use ADS1115 to get good precision values at A0 of Arduino?

If anything better than this circuit, please do suggest.

Thank you in advance

 

Hi mram,
Created your own Thread on this Topic.
Moderation.
Link to old Thread.
https://forum.allaboutcircuits.com/...ectly-with-a-multimeter-2.191893/post-1800590

 

1.What will be the out put voltage if I connect/ short A and B (pH electrode) in the diagram? I guess this is going to be pH 7 reading as per my knowledge.

It should be 512 mV for pH 7.
pH of pure water:

2.How should I get the gain of the LMP7721?

In Arduino use 1.1 V internal reference voltage,
then you do not need voltage gain of LMP7721.
https://www.arduino.cc/reference/en/language/functions/analog-io/analogreference/

3.What will be the out put voltage if I try to measure pH 4 buffer solution?

512 mV + (59 mV * (7 - 4)) = 689 mV for pH 4.
https://sensorex.com/ph-sensor-calibration/#How_to_Calibrate_Your_pH_Sensor
https://sensorex.com/ph-sensor-calibration/

4.Can I use ADS1115 to get good precision values at A0 of Arduino?

I think that precision of internal ADC is quite enough (see point 2).
https://www.broadleyjames.com/faq/what-is-the-accuracy-of-ph-electrodes/
==========
Also read:
https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/275690/lmp7721-not-working

Download pH and ORP Learning Handbook:
https://web-material3.yokogawa.com/TI12B00A20-01E_3.pdf

 

It should be 512 mV for pH 7.
pH of pure water:
View attachment 313514

In Arduino use 1.1 V internal reference voltage,
then you do not need voltage gain of LMP7721.
https://www.arduino.cc/reference/en/language/functions/analog-io/analogreference/

512 mV + (59 mV * (7 - 4)) = 689 mV for pH 4.
https://sensorex.com/ph-sensor-calibration/#How_to_Calibrate_Your_pH_Sensor
https://sensorex.com/ph-sensor-calibration/

I think that precision of internal ADC is quite enough (see point 2).
https://www.broadleyjames.com/faq/what-is-the-accuracy-of-ph-electrodes/
==========
Also read:
https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/275690/lmp7721-not-working

Download pH and ORP Learning Handbook:
https://web-material3.yokogawa.com/TI12B00A20-01E_3.pdf

Thank you very much dear Danko for your replies.
But still I am waiting your answer on my circuit diagram which I shared in last post. Is it Ok or anything wrong I am doing?

 

But still I am waiting your answer on my circuit diagram which I shared in last post. Is it Ok or anything wrong I am doing?

I did not notice any errors in your wiring.
Below is your circuit, simulated in LTspice, where
LM4140-1 replaced by voltage source V1, only for simulation.
What you need to do:
___1. Reconnect power wire from 5 V to 3.3 V on Arduino,
because nominal IN voltage for LM4140A-1 is 3 V.
___2. Disconnect pH sensor from breadboard,
connect point A to GND, measure voltages in
points D, C, B relative to GND.
Should be: D=1.024 V, C=512 mV, B=512 mV.
___3. Disconnect point A from ground, connect A to B.
On point OUT should be 512 mV.
Disconnect A from B, connect A to D.
On point OUT should be 1.024 V.
Disconnect A from D.
===
If so, then all is good with your circuit.
BTW, what is model of your pH sensor?

 

Dear Danko,
Thank you very much for your reply.

I did all as you suggested. and I found following results.

I am using Adafruint DF robot pH sensor

 

Seems LM4140A is not working.
How this IC is marked? It should be something like "4140A" and "CM1.0".
Why you turn it 180 degree? After it power 3.3 V becomes shorted.
It is why power voltage reading was 1.21 V instead 3.3 V.

I am using Adafruint DF robot pH sensor

If you are using that kit, then you should connect "-" to Arduino ground,
"+" connect to +5V and connect "A" to A0. That's it.

 

Dear Danko,

Thank you very much for your reply and your time.

1.Yes, I think LM4140A is not working. I thought I might be making some mistake, so turned it 180 degree.

2. I tried DF robots module which I posted the picture above. I tried to use it as per the instructions given. The results were not as accurate as I wanted at first and after two or three days it stopped working and was giving random results.
So, I thought to build my own robust circuit which can give accurate results. Then I started searching on internet about how to build a strong, robust and which can give accurate results. Fortunately I came to know about this forum.

3. If possible can you please send me Digikey exact part number/any other supplier link of LMP7721 and LM4140A...etc. So that I can purchase the right part for the circuit.

4. As I want to build industrial/medical device grade circuit. Which should not fail. Can you please suggest any other best circuit/Circuits diagrams which you might know other than above which I trying? That will help me move faster.

5. One more thing, I tried to use HX711 module of lad cell for pH sensor. But I could not get the method of how to get analog input to A0 of Arduino. So that I can convert those into pH values. Please guide me if you know any method to get the values from HX711. As it uses SDA and SCL pins. I don't know how to get values from these pin to measure pH.

Thanks again.

 

Here is a few links I have been reading to learn about the matter at hand:
https://www.ti.com/video/4793150528001#tab-1

https://www.ti.com/lit/ug/tidua47b/...60312&ref_url=https%3A%2F%2Fwww.google.com%2F

https://www.ti.com/product/LMP91200

The LMP91200MT is specialized in measuring ph, but because it’s a SMT component it’s hard to use without designing a PCB. In the chip data sheet you can find useful info on PCB design and stuff

the ph shild for sensor tag is good because the provide schematic and even Gerber file, so you can open in kiycad and see exactly how the have designed the PCB, also, it uses the LMP91200MT

ps: I allso tryed to use a LM4140 and it came broken from factory (I think )

 

Here is a few links I have been reading to learn about the matter at hand:
https://www.ti.com/video/4793150528001#tab-1

https://www.ti.com/lit/ug/tidua47b/tidua47b.pdf?ts=1708988560312&ref_url=https%3A%2F%2Fwww.google.com%2F

https://www.ti.com/product/LMP91200

The LMP91200MT is specialized in measuring ph, but because it’s a SMT component it’s hard to use without designing a PCB. In the chip data sheet you can find useful info on PCB design and stuff

the ph shild for sensor tag is good because the provide schematic and even Gerber file, so you can open in kiycad and see exactly how the have designed the PCB, also, it uses the LMP91200MT

ps: I allso tryed to use a LM4140 and it came broken from factory (I think )

Thank you very much roci75..... for your kind reply.
I am trying my best to make it work. I have ordered LMP91200MT and let me try.
Thanks again..

 

Thank you very much roci75..... for your kind reply.
I am trying my best to make it work. I have ordered LMP91200MT and let me try.
Thanks again..

Hi, I have been researching and tinkering, I don’t know if it applies to you, I was using a breadboard and normal flex wire to connect the bnc connector to de op amp. This is wrong, you should use a coaxial cable (like the one from the ph electrode) and solder it directly to the break out board.

After doing this I was able to mesure the mV with the multimeter ant it’s working relatively well.

I will try to send you photo tomorrow

Best regards

 

Hello, I am working on pH circuit and found this thread interesting. As I am new to this fiedl and trying my best to get the knowledge. I could not understand the circuit shown above. Would you please help me answering my doubts in the photo which I mentioned. Thanks a lot.
View attachment 313397

Can anyone please explain the circuit diagram? I am just a beginner.

 

Can anyone please explain the circuit diagram? I am just a beginner.

the LM4140A-1.0 is a voltage reference ic, it outputs a stable 1.0 v +/-, afer the voltage divider it is connected to a opamp that serves as a buffer, the op amp outputs +/- 500 mV. in this case because the circuit is supose to use only positive power supply you need to connect the 500 mV output to the reference lead of the probe.
the secund op amp is just a buffer in other to be able to read the output voltage of the probe, witch is very high impeadance.

You need to use a good instrumentation aop amp with low revesebias voltage and high input impeadance or it wont work, allso you cant use any cable to conect the bnc conector of the probe to the op amp, you need to use coaxial cable or solder the bnc connector directily yo the board (in witch case you need to take in acounting board levle shilding.

I will leave you this links so you can read more about ph data aquesition circuits

circuit using LMP7721
circuit using LMP91200MT

best of luck, keep in ming I am not a pro in circuit design and what i said could be wrong

 

For most IC voltage sources there should be a low leakage bypass capacitor across the supply to the source, located close to the device. In addition, the calibration of Ph measurement systems is usually done with reference liquids, because of the measurement system requirements.
And the reference of "The Precision" of the arduino brings the question of does it mean accuracy or resolution? Precision is such a vague term that it should not be used other than to describe a general property of data.