Whenever I put some distilled water in a cup and use my ph pen..it starts out at 8.75 and then slowly goes to like 6.5..it takes like 3-5 mins for the numbers to stop moving...is this normal..IT seem like the longer I leave the pen in the water ..the number slowly decreases by .01 and .02..IS this normal?? I have had in in there for 5 mins and in the past min it has weent from 6.5 to 6.46..now it looks like it finally stoped