Before using the OPB350C125Z sensor, the phototransistor load resistance needs to be set to 2.5 Kohm by moving the shorting bar to pins 1 and 2 on the board header. A short length of 1/8” clear tubing is included in the OCB100-KIT. Snap the tubing into the optical path of the OPB350C125Z, calibrate the sensor, and test with water. The blue LED on the board will light when water is present in the optical path. If a dark fluid is inserted into the tube, the green LED will light. The sensor will also indicated bubbles in the fluid by turning both LEDs off.