Our sensors are digital and have a built in serial number to identify themselves. The first time you use a new sensor you simply pair it with your App. The process of pairing is determined by the particular App so please refer to the App for specific instructions. When pairing a newly purchased sensor, be sure to stay 30 feet away from other operating sensors to avoid pairing with the wrong sensor. With certain older sensors, the serial number will change when the battery is replaced and will have to be re-paired when a new battery is installed.