The AP Products LPG Tank Check is a propane tank gas indicator that is easy to use and even easier to install. The magnetic sensor mounts to the bottom of the LP tank to indicate the level of propane left in the tank. The sensor is monitored by downloading the free Tank Check app on your smart phone or tablet which connects via Bluetooth delivering an accurate tank level. The level is updated every 5 seconds when your device is within 50 feet of the sensor. This is for one (1) XL sensor. NOTE: Requires the free Tank Check App, which can be installed on a smart device.