The Optical Bubble Sensor Assemblies function as a drain sensor and monitor fluid lines at five locations. Four sensors are used to confirm drain of the vessels. Those used for vessels 1, 2, 3, and 4 are marked 31, 32, 33,34, respectively. One marked 35 is used to monitor fluid exiting the Process Pump. Specify Vessel 1, 2, 3, 4, or Pump at check out.