pH module modification

[martisek101]

Hi,
I decided to add a pH sensor for my arduino controlled aquarium.
I bought the PH4502C pH sesor module.
The accuracy isnt very good, so I started thinking about upgrading the module.
Its my first meeting with opamps and amplifiers so I have just basic knowledge and Im unsure about my thoughts.
I hope to get more precise readings when I modify the module for measuring pH 4-10 (I dont need 0-14) to get more resolution.
I studied the circuit. Int the next picture is written by black color what I know about the board and by red color is what I would change.

If I understand how opamp works that would change the pH- voltage to 0,05-0,5V regulated by pot instead of 0,45-2,6V. And the amplification of P0 would be 11x instead of 3x (I counted with 10x then but whatever ) So the voltage for pH 4-10 would be 0,8-4,2V. Or maybe i can chose different resistors and go even further.
Would that work like that? Or is there something more which is above my level? I didnt studied the datasheets of the TLC4502 opamp because I really dont understand it.
One bad thing is that full range of the measurement (pH 0-14) would go to -1,5V to 6,5V but that in normal operation cant happen...or should I add some diodes for protection?

Big thanks to everyone who will try to help me.
I hope that its not totaly dumb idea.

PS: I just realised i totaly overlooked the TL431 D3. Some kind of voltage reference which I also dont know anything about. I will just post it now and look at it later. Maybe it isnt even problem.

[gilshultz]

Keep going, you are on an exciting adventure and learning a lot.

[MarkT]

Firstly accuracy is not resolution - increasing the resolution wont change the accuracy.

Secondly opamp circuitry like this is pretty repeatable in performance, limited by the resistor tolerances in fact - improving it won't help if the source of the inaccuracy is the sensor.

pH sensors are very very very high impedance and you have pay proper attention to this (ordinary wires will not work well, teflon insulation is required for instance). The TLC4502 opamp shown in the circuit has an input bias current rating of 1pA, ie 0.000000001mA, and this is needed for this type of sensor.

If the PCB isn't teflon you'll be limited by leakage over the surface of the PCB, which is very dependent on the atmospheric humidity and how carefully the PCB was cleaned.

You may also have neglected to properly galvanically isolate the supply for this circuit - if you haven't you'll need to isolate the solution being sensed from ground properly.