ph probes are 4-20ma or 0-10V or ?

[bprewit]

I am curious how ph probes work exactly. Do they use 12V and return a 4-20ma signal or 0-10volts? I have access to some PLC controllers that will accept 4-20 or 0-10v inputs and have output cards that could be used to drive alarms, calcium reactors, etc etc and could also use them for light timing and lots of other not needed but really cool things that would be way overkill for my tank but fun to have regardless! Anyone have an idea?

[bprewit]

ah hate to reply to myself but found that probes produce millivolt signal per ph point but high internal resistance makes it difficult or impossible to read with a meter without using a op amp. Hmmm maybe this isnt such a great plan eh?
E=Ex + 2.3RTK log (ai)
nF
where:
Ex = constant depending upon reference electrode
R= constant
TK = absolute temperature (Kelvin)
n = charge of the ion (including sign)
F = constant
ai = activity of the ion
For pH measurement, we are interested in the hydrogen ion for
H+: 2.3RTk
______ = 59.16 mV
nF
where: n = 1 and T = 25°C. This term is commonly known as the Nernst coefficient. Since pH is defined as the negative logarithm of the hydrogen ion activity, the general equation at any temperature can be expressed as:

E = Ex- 1.98 Tk pH

[NaCl_H2O]

:zzz Wow, I'll bet this thread hits 20 pages quick :roll

[bprewit]

ya think? >_<

[javajaws]

There's some info on RC I and others have been accumulating over the monthes on PLCs and ph transmitters, etc. over on RC: http://www.reefcentral.com/forums/showthread.php?s=&threadid=546258&highlight=PLC

Hope this helps.