Why Is It So Hard for the Bathroom Faucet Sensor to Detect My Hands?

They’re a perfectly normal size and shape… I think?

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There are certain experiences — like waving back at someone who wasn’t actually waving at you — that leave you feeling like slightly less of a person. Chief among them for me is the sign-language seminar I’m required to conduct in front of an automatic faucet’s sensor when trying to wash my hands in a public bathroom. Why must I do this phalangeal dance for minutes at a time to trigger the briefest splash of water? I know from bitter complaints from family and friends that I’m not alone in my very 21st century bathroom despair.

So what gives?

According to Peter Jahrling, the director of design engineering at Sloan Valve Company, “Every faucet motion sensor is different, but most use the same basic principles: The sensor emits a beam while waiting for the user. Once the user’s hands break the beam, the water will begin to flow.”

And is this supposed to take several minutes? Jahrling says not — ideally, in fact, these motion sensors should recognize you very quickly indeed. “Something in the 100 to 200 milliseconds range is an acceptable delay time,” Jahrling says. “That’s what you experience when you put your hand in a valid detection zone.”

This means, if the motion sensor works the way it’s supposed to, it should detect my hands faster than I can blink. But since, nine out of 10 times, the faucet only starts working when I’m on the verge of screaming and pleading with it to stop patronizing me with its voodoo workings, something else is going on here.

“The biggest problem we have is maintenance — the sensors scale up with lime and calcium,” says Jahrling, referring to minerals that typically come from hard water and often cause problems in daily use. Then again, I may just not be getting in close enough, since, explains Jahrling, most sensors have a range of just three to five inches. “You have to be close enough to set it off,” he says.

As to how to tell whether it’s your proximity, or the fact the sensor’s crusted over with mineral deposits? You’ll just have to keep that ceremonial metacarpal dance going until you find out.