The patented Squarewave Resilite is a positive, reliable and economical method of monitering water quality via conductivity. Bright Red/Green visual output is easy to read. The monitering 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 100v to 9v to insure safe operation and longer life. 220v Eurpoean is no longer available. When ordering make sure to specify coductance. Available in 2 Meg ohm, 1 Meg ohm, 500k, 200k, 20k, 10k, and 5k.