Ph range

[Robert Jacobs]

Hello,

After building the system myself, I am wondering what the ph range of the system is.

I can see on this group page that it can either sense alkaline or acid ranges, but can it sense both at the same time?

I would like to use for an aquarium setup, but I would like to sense both above ph7 and below ph7 in a range of about 1ph. So ph6 to ph8.

While nulling it with ph4 and calibrating with ph7 I cannot measure ph10.

While nulling with ph7 and calibrating with ph10 I cannot measure ph4.

Is it possible to extend the range with changing resistor values?

Thank you,

Robert Jacobs

[Carlos Neves]

2012/4/24 Robert Jacobs <in...@robertjacobs.eu>

Hello,

After building the system myself, I am wondering what the ph range of the system is.

I can see on this group page that it can either sense alkaline or acid ranges, but can it sense both at the same time?

Yes, you can calibrate it to works in a range from acid to basic.
 

I would like to use for an aquarium setup, but I would like to sense both above ph7 and below ph7 in a range of about 1ph. So ph6 to ph8.

Perfect. But, It is very important to know how precise need be your measurement. For example, you can have from 0.2 pH units (at the acid and middle range) to 1 unit of precision (basic range has a low hydrogen ionic concentration).

While nulling it with ph4 and calibrating with ph7 I cannot measure ph10.

Right. Up to 9.
 

While nulling with ph7 and calibrating with ph10 I cannot measure ph4.

Right.

Is it possible to extend the range with changing resistor values?

There are two points: (1) input impedance and (2) calibration circuit.

(1) You can improve the input impedance mounting a voltage follower using a electrometer (for example, OPA 129). It is a operational amplifier with a very high input impedance. Connect the pH glass electrode to the follow voltage input (with high impdeance) and the output is connected to the input of the pHduino. The problem is you need +12V/-12V to power the OPA129 up. The OPA129 datasheet says the minimum power is symmetric 10V.

http://www.softwareforeducation.com/wikileki/index.php?title=Voltage_Follower
http://focus.ti.com/lit/ds/symlink/opa129.pdf

(2) You have a small imprecision in the calibration because the adjust of the offset and the gain is in the same operational amplifier. I found that I could put all together with a small disadvantage. For a short range, this is not a problem. I wanted a circuit very small, cheap, and easy to mount. The next version will be an analytical device.
 

Thank you,

Robert Jacobs

Carlos A. Neves

 

--
You received this message because you are subscribed to the Google
Groups "pHduino" group.
To post to this group, send email to phd...@googlegroups.com
To unsubscribe from this group, send email to
phduino+u...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/phduino?hl=en

[Robert Jacobs]

Hello,

Your information was very helpful, thank you! :)

Basically my "problem" was that I wanted a third reference point (ph10).

But, since I can only measure up to PH9.**, I was searching a way to achieve this.

Basically the range is more then enough for my application.

I have bought a small pocket ph meter as reference.

Thank you for the great help!