I have had this test done and its okay.
The nurse had a great technique of getting you to drink through a straw while she popped the tube down. I actually had to do this twice, once for the big tube which measured the PH while liquid was dripped down and then a second time for the small tube which sits there for 24 hrs. The tube was attached to a walkman type device which I carried around for 24 hrs.
The tube never moves and the only problems I had was that some food, like bread, might stick to the tube and when I went to bed I had to find somewhere to place the recorder.
Don't worry its okay, but you might not want to plan a lot of activities with other people as you have this tube sticking out of your nose