I got the vernier device information from threads I had searched on the forums here and had posted it up. I was hoping some progress had been made with PPM since that thread. I will try to post some more information.
I am really aiming to use a premade probe for PPM readings. What a PPM probe does though, is generate a current in a solution and the strenght can be measured and displayed in PPM after some conversions. You can make your own probe, but most of the methods I've seen leave me skeptical of their accuracy.
The competition, GroBot uses Sensorex sensors. Right now, I can interface their pH sensor through a wonderful device made by phidgets. pH/ORP Adapter - 1130_0B at Phidgets . It allows me to plug in a sensorex probe or a slightly cheaper one for development purposes and the analog goes into the arduino. My goal is to use this probe from sensorex : http://www.sensorex.com/products/conductivity_sensors/lab/conductivity_lab_sensors.html for tds readings. I am just at a loss of getting the TDS probe to talk to arduino.
I hope that clarifies things and apologies for being short in my earlier posts.
Thanks,
-Ryan
edit:
I called sensorex and got a wonderful guy on tech support that recommended one of these: http://www.sensorex.com/products/conductivity_sensors/accessories/conductivity_transmitters.html and an op amp circuit. He didnt go into any more detail than that.
more data on box: http://www.sensorex.com/support/specifications/download/CT1000Specs.pdf