A small tube is placed through the nose into the food pipe with two sensors (located at the tip) that measure acid levels (pH levels).

• Lower Sensor: A sensor is placed in the esophagus above the lower esophageal sphincter (LES) (#3 in diagram) to check for backflow of stomach acid into the food pipe. (Note: This position is also where a probe would be placed to determine whether "classic" gastroesophageal reflux is present.)
• Upper Sensor: Another sensor is placed near the upper esophageal sphincter (UES) (#2 in diagram). Since stomach acid backflow must reach this level to cause reflux laryngitis, measurement of acid level at this site is important. (Note: some physicians may place a sensor in the lower back of the pharynx or hypopharynx (#1).)

This diagram can be found in Reflux Laryngitis: Diagnosis.