For some people, heartburn becomes a chronic problem. The medical condition associated with chronic heartburn is known as gastroesophageal reflux disease (GERD). In patients with GERD, the band of muscles [called the esophageal sphincter] at the bottom of the esophagus just above the stomach does not close tightly enough, allowing the stomach contents to leak back into the esophagus on a regular basis. A wireless sensor helps to diagnose and evaluate treatment of GERD. Using a tube that goes down through the throat , a physician attaches the sensor to tissues in the patient's esophagus. The sensor reads pH-a measure of acidity we discuss in sections - a transmits the readings to a recorder worn on the patient's body. The patient goes about his or her normal business for the next few days while the recorder monitors esophageal pH. The sensor eventually falls off and is passed in the stool. The record of esophageal pH can be read by a physicians to make a aiagnosis or to evaluate treatment.