All bedwetting alarms work on a simple principal. If an electrical insulator such as air or dry briefs separates two electrical conductors having a small voltage across them, this results in an electrical open circuit. So no current will flow between the conductors to any electric circuit to which they are attached. When the insulator (briefs) gets wet, electricity flows more easily between the two conductors, and this triggers the sensor to create the alert.
Urine contains electrolytes (salts) which makes it a better conductor of electricity than plain water. For any sensor, there will be a minimum threshold of wetness and salinity, below which the sensor will not send its signal to the alarm, as the low voltage will not let enough current flow through the medium. This minimum threshold is desirable to reduce false signals that may be caused by water or light perspiration. Better alarms will distinguish between water and urine (or salt water). Better alarms can also distinguish between a “rush” or quick increase of urine as opposed to a slow increase in wetness as may be caused by perspiration.
Always remember that if the urine does not reach the activating part of the sensor, the alarm cannot “know” that it should alert.