Check this out Leo-when I mix and test my nute solution-I use a gallon milk jug with a air pump tube stuck down inside(down through the handle part) after adding nutes-I stick my PH meter( which happens to fit perfectly in the neck of the container) into the gallon with the air bubblin up the solution, I then wait about an hour to get a solid reading-I think I've been doin this all along without knowing what exactly it was that I was doing.No problem as usual, just happy to see the girls growing.
Ok here's the deal here below is a link to the exact PDF back issue with the article, it is on page 07 of 41
http://maximumyield.com/assets/pdf/back_issues/usa/MY_USA_May_09_d.pdf
Everyone who has ever had to deal with pH read this article it'll give you clarity as to why you have such wide pH flux.
In case a mod removes the link it's the May 09 back issue of Maximum Yield, the 4th PDF out of 4, on page 07 of 41.