The patented Squarewave Resilite is a positive reliable and economical method of monitoring water quality via conductivity. Bright Red/Green visual output is easy to read. The monitoring is simple: the green light indicates the water conductivity is below the threshold value the red light warns that it is above. Uses a wall transformer to convert 110v to 9v to insure safe operation and longer life. 220v European is no longer available. When ordering make sure to specify conductance. Available in 2 Meg ohm 1 Meg ohm 500k 200k 20k 10k and 5k.