I have never had the need (and probably never will) for a device that could measure the amount of salts in a solution, but I found the circuit to be very interesting. This EC TDS PPM Meter project provides all the information you need to make your very own.

"TDS is the measuring of the amount of salts in a solution. For alot of applications the amount of salt is indicitive of the levels of other stuff in a solution. TDS/PPM meters sold for gardening and aquariums figure the amount of salt in Parts Per Million by measuring the Electrical Conductivity of the solution under test. So a PPM/TDS meter is a EC meter that converts the EC value into PPM values.

There were alot of design considerations such as needing to pass a minimum amount of current and voltage thru the solution while keeping a good s/n ratio. But most most important it had to use stuff i had laying around the house and be pretty accurate.

The circuit outputs a DC voltage equal to the EC value of the solution being tested. So like 4v on the meter means an EC of 4, 2v means EC of 2, .5V = .5 EC blah blah. To convert EC value to PPM just multiply by 500, so like 4 EC = 2000PPM, .5 EC = 250PPM. Also the output can be connected to an A/D converter without alot much grrrrr."

Thanks Momo