Likewise, Hydrosense consists of a single device attached to a cutoff valve or water bib that monitors the entire plumbing infrastructure. "When you open a valve, the pressure on the entire system goes down," says Patel. "And whenever you change the water flow from static to kinetic, you get a shock wave that propagates throughout the pipes." He explains that the shock wave, while relatively mild, has a characteristic shape that can be used to identify different fixtures--even the distinction between the toilets in different bathrooms.

Using data collected in nine homes of varying style and age and with a diversity of plumbing systems located in three different cities, Patel and his colleagues have shown that by monitoring these shock waves, it is possible to identify individual fixtures with 95.6 percent accuracy.

"The idea of being able to plug one device into a home and build a picture of what's going on and off is really fascinating," says Adrien Tuck, CEO of Tendril Networks, a company that makes smart meters and plugs for homes. But he suspects that there will be some kinks to iron out before the technology is deployable at a large scale. "If it were easy, it would have been done already," he says, "and that probably means that there are some things that need to be teased out."

In addition to monitoring utility usage, Patel says that the sensors can track human activity within a home, which could be useful for elder care and reducing energy waste. He has also developed a fourth sensor that can be integrated into a home's heating and cooling systems. By monitoring pressure changes that occur when people open and close doors and when they enter and exit a room, a sensor within an air-conditioning unit can infer with relative accuracy where people are within a home or apartment, Patel says.