The AP Products LP Tank Check is a propane tank gas indicator that is easy to use and even easier to install. The Tank Check monitor uses Bluetooth to connect to one or two sensors and can be used instead of or in addition to the Tank Check App on your smartphone or tablet to deliver 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) monitor only. NOTE: this unit requires at least one (1) standard Tank Check sensor to function (not included).