ph meter

i need to build a simple ph meter i wonder if someone can help me?!?!?!

i have found this

http://www.cyberreefguru.com/electronics/phmeter1.gif

it is not completely clear to me.

on the left there is the probe rite? why does it look like a resistor? is
actually a resitor or what do i have to use?
second question. how do i mesure the ph? multimeter is that correct? the so
which are my terminal for measuring?

thank you for your kind help!

 

Measurement would be from the output of the opamp to gnd. I guess a multimeter could be used. You would need to use two samples of known ph like an acid and a base, and hope to hell the circuit is linear.

 

This schematic is very misleading. A pH probe is a two terminal
device, much as a volt meter has to have two leads. One lead is
essentially a contact to the liquid, to act as a reference point,
while the second lead is a half cell that generates voltage when it
comes in contact with the liquid. The voltage it produces gets more
positive as the hydrogen ion concentration of the liquid increases.
The pH meter is a high impedance volt meter that amplifies and
displays the magnitude of this voltage. See:

http://www.omega.com/techref/ph.html

So the circle with a resistance through it, labeled pH probe is
actually a pair of probes, one that is the reference probe and one
that is the hydrogen ion half cell. It is shown as a resistance,
probably because the cell formed by these two probes and the liquid
between them has a very high internal resistance, on the order of 100
meg ohms.

I think with a bit of searching, you can find a better design of a ph
meter than this.

 

Just Buy one, Cheaper in the long run. Additionally, be prepared to
buy a new one every few years. These PH cells don't last more than a
few years.

Take care.....Gary

 

The high-impedance meter may be the easy part.
The pH electrode is made from special permeable
glass, and is something you would be best buying
anyway. They tend to be very fragile and need special
care in handling and storage. Last time I checked
I think you could get one for around US$50, but that
was a "few" years ago.




Bob Masta
tech(AT)daqarta(DOT)com

D A Q A R T A
Data AcQuisition And Real-Time Analysis
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