The Solo Collar Tags come in 3 different sizes. All sizes of tags work with ALL door sizes. The different sizes are for different sizes of pets. The Solo Automatic door is triggered by a magnet. There are no batteries or circuitry in the tag that your pet wears. It is a magnetic field that triggers the door to open. This method of triggering the door is what make the Solo so reliable. It doesn't matter if the collar tags get wet. Because they are only magnets, they are very inexpensive to replace if they are lost.