ph problems

hello, i have 2 ph pen testers and i have calibrated them both in the 7.0 and then the 4.0 solution, but when i go to test them both i get a different reading of .3. One will say 5.8 5.9 and other will say 6.1 in my solution, im curious on what im doing wrong and/or the .3,.2 difference will make a huge difference considering ph levels fluctuates daily?
thanks in advanced.

ph will move up or down. after you add your fertilizer you need to check it then. if your in soil. your ph needs to be 6.0 to 6.7 ish any where in between. you can go as high as 7 I don't like to. best number is 6.5 dead in the middle. if its close to this it will be fine. plus or minus a few points. just check it every watering. I use http://www.amazon.com/General-Hydroponics-GH1514-Control-Kit/dp/B000BNKWZY/ref=sr_1_9/175-2143798-3243211?ie=UTF8&qid=1379636942&sr=8-9&keywords=ph+tester+for+soil it has a gage on the bottle what color you want. its very easy to use,

You get what you pay for in ph pens.