Measuring Nutrient Strength

You won't be doing hydroponics too long before you discover that you can't do it very well if you don't know how strong your nutrient solution is. The plants need to be fed consistently at an optimum strength and the usual way to control this is with a conductivity meter. (See the picture at right of the Dipstick which is about $85 and a slightly more recent version of the first instrument I got for doing hydroponics).
When nutrients (non-organic salts) are dissolved in water they disassociate into ions and make the water slightly conductive. Using a meter you can tell how much dissolved nutrient is in solution. Unfortunately you can't tell the composition, i.e. how much of different nutrients make up the solution. So ever few weeks it is a good idea to dump the nutrient and remix to reestablish a known nutrient composition. This can be done when the tank has gone down a good bit so that relatively little nutrient is lost.
If you want to see a selection of meters you can CLICK HERE and it will take you to a Portland Oregon firm, American Agriculture. This is just an example of what is out there.