Nut PPM meter Nut Lock problem improvise ppm meter

HI..
I want to know how to make a ppm meter at home improvised.

http://www.amazon.com/HM-Digital-TDS-EZ-Quality-Tester/dp/B000F5M6VG

Totally not worth it, just buy the one I linked
And why did you put the word nut in your thread title
Did you mean nute as in nutrient I'm guessing?
This is a nut:

who wants to see my nuts?

my nuts have grown pretty nice to but yea nuts= nute= nutrients

also I quess Whats the cheapest ppm meter recommended

The one that I linked in my first post
I have it and it works good

any one know the suggested reservoir size for 3 plants all between 2-3ft high... I use 55liters I use the advanced Nut calculator so my ppm should be good. How do you adjust ppm in mid weekly cycle in a hydro system without changing out entire reservoir. I dont know how big thew will get its week 4 bloom... one is super bushy the other two are bonsi-ish. almost scrawney