The Mopeka Tank Check Standard Sensor works on 20, 30, and 40 lb (5, 7.5, and 10 gal) DOT cylinders, and on horizontal RV tanks. Allows you to monitor the level of propane using the free Mopeka Check APP on your smartphone or tablet. Also works with Mopeka Monitor. This package includes 2 standard sensor and 8 spacers that clip to the foot ring of cylinders to improve wireless range.