Cheap but good NUTES reader?

I am starting to put nutes in my tub. Don't really know the PPM or when it's getting eaten. I kind of just guess.

Do I need one?

If so, good cheap one to buy?

I would get 1 yes, makes things much easier. Have a look on fleabay for a EC pen or PPM meter, I brought a Fasttech EC pen for £27 and its excellent - Never had a problem with it.

Horticare or Bluelabs aare supposed to be the business but you'll pay quite a bit for these.