Ambulatory PH Monitoring measures the acid that refluxes back up from the stomach. A very thin tube is inserted up through the nostril and then down the throat and esophagus until it reaches just above the stomach. The tube has a very small probe at the end that will register any acids that are refluxed from the stomach. An x-ray is taken to make sure that the probe has been positioned correctly. The other end of the thin tube is attached to a small computer (small black box) for 12 or 24 hours. During this period you are given a diary sheet to complete, on which you should record the time of each activity that takes place, basically a running history.