What do you think!

Hi All,
Having some issues trying to get readings on distilled water. I have a Hanna PH pen that usually works well-but since i changed over to distilled water i'm having trouble getting accurate reading. Tap water just was too hard for girls. I'll put pen in water and get reading of 7.6-swirl water around get 7.4,swirl water around get 6.9 and so on til maybe i go into 5.8? Some water readings go to 5-5.5? Pen does calibrate correctly though! I really think my meter is the prob but want second opins really don't have the money for another meter but i'll get one to save this grow, also talked to store worker and he said that as long as the meter is calibrating the factor will say that it is ok! If it is ok then why the hell am i getting f@#ked up readings?
All replies appreciated

I have that issue if the battery is weak...

I was thinking of taking it to the store with a gallon of water and see what happens with the results of my meter vs a brand new Bluelab pen that they also sell but with a replacement battery in my Hanna. I read somewhere someone was having the same issue but never solved it or never replied back with results of how they fixed it