PLC PH measurement

Hello,

I'm searching some info about connecting a ph probe to a plc.
The PLC i'm going to use is the logo from s....ns
I does i've to do this.
i've already heard that it is not that easy.
I'm going to use this plc to control and autom. a marine aqua

Thnx for all the input
gr

bart

Connecting the probe to the PLC is easy. Most of them, such as Endress & Hauser, have 4-20 mA or 0-10 VDC outputs, linearized with respect to the pH. These signals can go right to a standard PLC analog input card. You can use standard functions to scale the signal to engineering units of pH.

The problems you have been warned about have to do with controlling pH. This is a difficult problem unless you are working in a very narrow range. That is partly because pH is itself a logarithmic measurement. Also, things like buffering capacity, concentration of reagent in the solution, etc. make the respones un-predictable. I'm not an expert, but from what I've heard the best bet is to not use a P-I-D loop, but instead work with tabulated values or an empirical equation.

what you need is analog input 0-20mA (4-20mA or 0..10V or similar) and proble that will convert PH into current or voltage ... like this
http://www.globalw.com/products/waterquality.html

pH probes need solution temperature compensation, have a very high output impedance, and have a low mV output that limits the length of the connecting cable (and makes that cable noise susceptible)

Hence, pH probes are always connected with a shielded cable to an electronic box, usually called an analyzer or transmitter, that takes care of conditioning the signal from the probe itself into a linear signal that has a typical industrial output, like 4-20mA or 0-10Vdc.

You get the analyzer/transmitter from the same manufacturer as the probe itself.

Dan

hello, all
i dont mean to steal the thread, but i am looking for a pH probe and TDS meter that outputs 4-20mA to an analog PLC module. Does anyone have more links to some inexpensive probes? i've only been able to find $500 probes like in the link above. i was hoping for 100-200 or less.
is this possible?