If you are going to mix nutrient solution, a TDS meter is a good thing to have.
I have a cheap one like this, which has worked for a few years.
https://www.amazon.com/Dr-Meter-TDS-3C-Quality-Resolution-Accuracy/dp/B00KSTZGNE/ref=sr_1_8?ie=UTF8&qid=1465506180&sr=8-8&keywords=tds+ppm+meter