It's easy enough to use a stopwatch to time a shower, or set an alarm to go off after your allotted shower time. But devices that measure time aren't the most exact way to monitor water use, because different showerheads use different amounts of water. Depending on the water efficiency of your chosen spout, your shower might use anywhere from 1 gallon to 7 gallons (4 to 26 liters) of water per minute [source: FYP].
If your goal is to limit your water consumption, and not just your shower time, it's helpful to monitor your actual water consumption. That's what Aqualim does: It monitors volume, not time.
Aqualim is an attachment, not an actual showerhead. You screw it into the pipe your shower water flows from, and then screw your showerhead onto Aqualim. The attachment is preset to limit your shower to a certain number of liters. In the Aqualim prototype, the amount is 40 liters (11 gallons), which gets you about five minutes with a water-conserving showerhead (that amount could change if or when the device comes to market).
The unit counts volume using a hydraulic motor. As water flows through the Aqualim, it spins a motor. The motor runs a liter counter. When the counter nears 40 liters, water pressure decreases. That's the "warning mode." When the counter reaches 40, the device cuts water flow down to a dribble.
That's not the end of the story, though -- if you're still soapy, you can resume your shower by turning off the water faucets and then turning them back on. That resets the counter, and the process starts all over again.
This feature, while certainly handy, begs the question: If you can just turn the shower right back on, how does it reduce water consumption?