Hi Guys

Thanks for all of the input so far, I was going to use two Bronze Probes (1.6mm TIG Silica Bronze Welding Rods) as their Corrosive Resistance is very good against salt water. My water tank will hold 200 Liters of salt water. The water level will be monitored and will remain pretty constant and the temperature will me monitored and kept at 25 Degrees Celsius.

I want to measure the PPM which will be between 27 and 33 which has a Specific Gravity of 1.019 - 1.025. (Resistive measurement taken on probes)
After having a look at your replies and then doing a bit of searching on the net I found a very interesting article on this very matter, with an AC circuit etc have a look at: http://www.octiva.net/projects/ppm/

I will build his circuit and give it a bash with bronze probes, I will report back on my finding and thanks again for all of your help!

Craig