The FD-20 will be replacing the FV-20 soon. Same great performance and price with more durability. Attachable transmitter for use in cast iron or nonmetallic lines and tanks. Operates at 512 Hz and works with any 512 Hz receiver. Available in a continuous tone. Maximum range 20 feet in cast iron, 30 feet in nonmetallic lines. Uses one AAA Alkaline battery.