Why is Xiaomi’s fitness tracker detecting a heartbeat from a roll of toilet paper?

Weibo users are confused, but the answer isn’t as wild as it seems

Wearables

Xinmei Shen

Published: 9:00pm, 8 Oct 2018

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This article originally appeared on ABACUS

Does a roll of toilet paper have a heart? Obviously not. So why does Xiaomi’s fitness band display a heart rate when it’s wrapped around a roll of toilet paper?

Weibo users have been discussing the phenomenon, with plenty of pictures from mystified users who say the Xiaomi Mi Band 3 fitness tracker is “detecting” a heart rate on toilet paper.

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So we decided to get a Mi Band 3 -- and of course, a roll of toilet paper -- to check it out.

Bizarrely, it’s true.

It didn’t work all the time -- only around a quarter of attempts gave us a heartbeat. The numbers were pretty random (ranging from 59bpm to 88bpm), but they were real.

So what about other objects? We tried wrapping the Mi Band 3 around a mug, because we had a mug, and a banana, because the internet likes bananas. Both gave us a heart rate quickly and far more consistently than the toilet paper did.

But the Xiaomi band isn’t alone. We also tried the banana and mug with an Apple Watch Series 4 and a Ticwatch, an Android Wear smartwatch. Both also displayed a heartbeat for the two heartless objects, ranging from 33bpm on the banana (Apple Watch) to 130bpm for the mug (Ticwatch).

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So how does this happen?

All three models -- Xiaomi Mi Band 3, Apple Watch and Ticwatch -- beam green light on to your skin when detecting your heart rate. Apple says that’s called photoplethysmography (PPG).

The name is long, but the idea is simple: Your blood is red, which means it reflected red light and absorbs green light. When your heart beats your blood flow is stronger, and so it absorbs more green light. By analyzing the reflected light, they can figure out how fast your heart is beating.

When we asked Xiaomi, they didn’t give us an official explanation. Instead, a spokesperson pointed us to a user’s post on China’s Q&A site Zhihu, where the user says that it’s normal for objects to reflect light signals, confusing the sensor. This was confirmed to us by a biomedical engineer in China who published a paper on PPG.

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And just because it’s detecting heart rates on other objects doesn’t necessarily mean it’s inaccurate at detecting your own heart rate. The biomedical engineer pointed out to us that the software is designed for one purpose: To detect a heart rate from a human wrist. They were likely never designed to detect whether they were strapped to a human wrist or a roll of toilet paper, because, uh, who’d do that? (Other than me. And a bunch of people on Weibo.)

The engineer explains that the wearables are likely seeing these strange reflections and trying to interpret them as the only thing it’s designed to look for -- a heart rate.

So feel free to wipe without fear. Your toilet paper doesn’t have a heart.

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