Problem

[Cuongnd]

Dear Carlos Neves.
I have rebuilt your project with the same schematic, and i use PSoC
Kit to read output voltage (VPH) (i do not have Arduino/Freeduino
board)
But i have a problem with VPH:
- VPH is not stable (it swing largely) and I can not determine output
voltage (my PSoC Kit work well with many different applications)
I checked my circuit many time and it's exactly the same as yours
(http://picasaweb.google.com/lh/photo/GH973jZej_jhEzqNdsHK_A?
feat=embedwebsite)
Can you tell me what's problem?

[Carlos Neves]

Hi,

What is you sensor? Are you using a dummy electrode made using a trimpot with a symmetric power or a real glass electrode? Remember that pH glass electrode needs special cares.

To be sure, make a short circuit between the pins of the input electrode (zero volts between the reference (ground) and the input sense (pH glass electrode)). Then, adjust the R3 trimpot (slope adjust) to generates a potential near from zero at the VSENSE (pin 1 of the operational amplifier). You should have a potential near from 2V at the VPH or pin 7 of the operational amplifier.Measure the noise.

Carlos A. Neves
SMILES: CN1C=NC2=C1C(=O)N(C(=O)N2C)C

2011/3/26 Cuongnd <nguyencu...@gmail.com>

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[nguyen cuong]

thank for your rely!

I'm using real glass electrode

it swing between 2 to 3 volt, but i can't determine the number behind float point 

i can't explain why is that.

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[Carlos Neves]

Hi Cuong,

pH glass electrodes are not like a temperature sensor. They are fragile and need special cares. Try to find how to use a pH glass sensor.

To be sure that your circut is working make a test using a dummy sensor made with a 10k ohm trimpot and powered with a symmetric power from the pHduino shield.

Check the noise at the symmetric power at the operational amplifier.

Make a short- circuit at the input sensor with the ground and test the VSENSE output.

Chech if the trimpots of the shield adjust. Avoid the ends of adjust.

Carlos

Em 28/03/2011 06:00, "nguyen cuong" <nguyencu...@gmail.com>escreveu:

thank for your rely!

I'm using real glass electrode

it swing between 2 to 3 volt, but i can't determine the number behind float point 

i can't explain why is that.

On Sat, Mar 26, 2011 at 9:55 AM, Carlos Neves <can...@gmail.com> wrote:

>
> Hi,
>
> What is you sensor? Are you using a dummy electrode made using a trimpot with a symmetri...

>
>
>
> 2011/3/26 Cuongnd <nguyencu...@gmail.com>
>>
>> Dear Carlos Neves.

>> I have rebuilt ...

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> You received this message because you are subscribed to the Google
> Groups "pHduino" group.

> To ...

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[nguyen cuong]

Thanks Carlos!

My problem is in C3, it not work well, I replaced it and now it work pretty good

But It  respond slowly (approximately 4-5 minutes)

What is the solution for a faster response?

Now i trying to make a glass electrode by myself

I'm making a Ag/AgCl electrode by the way: I immersed a Silver wire in bleach for 30 minutes, but it not work well

Can you help me an other way to make a Ag/AgCl electrode?

Thanks...

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[Carlos Neves]

2011/3/28 nguyen cuong <nguyencu...@gmail.com>

Thanks Carlos!

My problem is in C3, it not work well, I replaced it and now it work pretty good

But It  respond slowly (approximately 4-5 minutes)

What is the solution for a faster response?

Are you sure about the resistors and capacitors values of the filtering stage? To increase the response time you can reduce the time constant of the filter, but you will increase the noise too.
 

Now i trying to make a glass electrode by myself

I'm making a Ag/AgCl electrode by the way: I immersed a Silver wire in bleach for 30 minutes, but it not work well

Can you help me an other way to make a Ag/AgCl electrode?

There is a simple reference electrode that you can make using a Silver wire called pseudo-reference. To make it, remove the oxide layer of a Silver wire using a sand-paper or whatever and leave a in a solution saturated of chloride ions (NaCl solution) during one or two days. You will have an very thin AgCl layer. It is not robust but works. Another way is apply something about +600mV against a real reference electrode in a medium with a high concentration of chloride ions during two or three minutes. You need make some tests. Remember that a pseudo-reference electrode is to be used in a medium with high concentration ions to reduce the drop voltage. It will not work in a aquarium, for example.
 
I suggest you make a real reference electrode. It is not difficult and you will have a precise reference potential. Take a look at this paper.

Miniaturized reference electrodes with microporous polymer junctions
Jairo J. Pedrotti, Lúcio Angnes, and Ivano G. R. Gutz.
Electroanalysys, 1996, 8, 673-675.
http://onlinelibrary.wiley.com/doi/10.1002/elan.1140080713/abstract

Remember: if you don´t have a good reference, you will have a wrong measurement. If you decide to use your reference electrode, you need use a pH glass electrode WITHOUT a reference electrode.

In fact, I suggest you buy a pH glass electrode sensor with a reference electrode integrated. They are ship and works fine. But, we can have fun make one by yourself. Adventure!

Thanks...

Carlos A. Neves
SMILES: CN1C=NC2=C1C(=O)N(C(=O)N2C)C
 

[nguyen cuong]

Thank a lot Carlos