How to use an elec. ph meter

Hello again everyone at Cannabis forums :D

I saw what looked like a good 4 WAY SOIL ANALYZER pH Light Moisture NPK Tester but it is only 7" long. I assume that the seven inches includes the whole thing but I'm not sure. How deep into the soil do I want to go?
Do I have to worry about damaging my roots? I have thought about having a few small holes around the pot to check all sides of the soil.
Can I just stick it in my soil a few inches and the meter determines the ph level for the whole pot?
I have posted before that I have never grown before and know very little about what I am doing. I just want to be as educated as I can possibly be before I begin my indoor small closet grow operation next month. :thumbsup: