The OV-Watch Sensor Pack contains the specialized biosensor to be used only with the OV-Watch Fertility Prediction Device. This sensor is a non-allergenic device used by the watch to gather readings from your skin under sustained contact. Each Sensor is good for one cycle and should be replaced at the beginning of each cycle. SPEE01-01 contains a 1-month supply of sensors and English language instructions. The expiration date for an unopened sensor is published on the foil package and Sensor Box label.