The Solinst 101D DrawDown Water Level Meter shares the same P7 probe and laser-marked flat tape as the standard 101 Water Level Meter, but adds a toggle switch that reverses the circuit logic. In standard mode, the buzzer and light activate when the probe touches water, just like a conventional water level meter. In drawdown mode, the logic flips so the buzzer and light activate when the probe is in air — signaling the moment water drops below the probe — which lets an operator continuously monitor water level decline during pumping without having to raise and lower the probe to find the current level.