Here's why heart rate monitors are getting bpm from bog rolls

You might have noticed photos on social media of smartwatches and fitness trackers wrapped around toilet rolls this week, after it was found that some devices take ghostly heart rate readings from inanimate objects.

It started with the Xiaomi Mi Band 3 when a group on Weibo noticed that the budget fitness tracker's heart rate monitor was somehow able to provide a heart rate reading while strapped to the toilet roll.

Essential reading: Best heart rate monitors to buy

But how many wearables fall foul of this, and why does it happen? We thought we'd power up a bunch of wearables, put them on a toilet roll to see which were heart breakers and which were...heartfakers (just go with it, it's all a bit of fun).

No reading – the heartbreakers

Garmin Forerunner 935

Last up in the test was the Forerunner 935. We're long time fans of Garmin's triathlon-friendly sports watch so it's good to hear that this is one wearable that's not falling for the fake wrist trick. Making our way to the resting heart rate reading screen the Forerunner 935 didn't pick up any measurements in our multiple tests.

Fitbit Versa

The Versa passes our toilet roll HR test with flying colors. By accessing the Today dashboard on Fitbit's latest smartwatch we attempted to take multiple resting heart rate readings. The Versa however did not produce any data whatsoever. Good work Fitbit.

Samsung Galaxy Watch

The Samsung Galaxy Watch like the Fitbit Versa wasn't going to be fooled by toilet roll masquerading as a human wrist. When we strapped it on and scrolled to the Samsung Health watch screen to take an on the spot heart rate reading, it was not playing ball. The Galaxy Watch simply displayed the message that we should clean the sensor and attach the watch securely. Samsung Galaxy Watch, you passed the test.

False reading – the heartfakers

Fossil Explorist HR

The Fossil Group added heart rate monitoring to its fourth generation smartwatches and it's fair to say we didn't have the greatest of experiences with it in our Fossil Explorist HR and Venture HR reviews. So it's perhaps no surprise to find out that this Wear watch managed to produce readings of 60BPM and 89BPM through the Google Fit app. It did also throw out a few requests to tighten the band, but the fact remains that its sensor can be fooled.

Apple Watch Series 4

It's not good news for the new Apple Watch in the great HR toilet roll test. While our proper chest-strap comparison tests gave the Apple Watch's optical HR sensor a great bill of health, we were able to get readings that ranged from 29BPM all the way up to 176BPM.

What's the science – and why is it happening?

So now that our very scientific (okay, not scientific at all) testing is completed, it's clear this is a common issue. So why is the tech responsible for taking heart rate readings able to be deceived by things that are clearly not a human with blood pumping around their body?

Dr. Steven LeBoeuf is cofounder and president of Valencell a company that develops biometric sensors for the likes of Samsung and Suunto. We asked LeBoeuf to shed some light on why your wearable thinks an inanimate object has a heart.

"It's all down to how the sensors that read heart rate work. PPG (photoplethysmography) sensors have different methods about how they process the information and generate that heart rate data. One thing common with all PPG sensors is the technology assigned with finding a signal. A signal that it wants to find and amplify.

"What happens is that if the PPG sensor doesn’t see a signal from blood flow or a motion artefact, it’s going to find the closest thing to a signal it can. That could be the flickering of light above you or vibrations in the room that might happen from the AC unit or shaking of a table. If you put that sensor down on an inanimate object, it's going to find a light signal from somewhere.

And Ilkka Korhonen from Firstbeat continued the explanation:

"Signal-to-noise ratio of a real blood flow is sometimes very low so the algorithms are very sensitive in screening for any non-random components in the light variation, and when such are found they are interpreted as HR. So, the algorithms pretty much always find something which is not completely random and interpret that as a HR."

In other words, to avoid crapping out when it's on your wrist, sensors are constantly looking for signals when blasting your skin with green light. And while we've found them to be accurate at medium intensities at least, all bets are off once they're off your wrist.

And as LeBoeuf explains, ECG, which unlike PPG isn't a light based technology doesn't have this signal problem because it requires conductivity between the electrodes. Otherwise it won't register a reading.

So does this actually matter?

In a word – no. It's all about the accuracy when it's on your wrist, something we take very seriously in our testing here at Wareable, by comparing all wrist-based monitors to a chest strap under a variety of physiological loads. And LeBoeuf agrees.

"I do think though that for PPG, used for fitness and running, it’s not a concern" LeBoeuf said.

"Right now on the consumer side, for folks people using the Apple Watch there’s not a use case that's putting people at risk. In the case of the Apple Watch, the way the its FDA approval works is very narrow. It’s screening to see if people have the atrial fibrillation.

"If the wearable is going to be used as a medical device, let’s say a hearing aid and the use case is to relay data about your vitals, it’s really important to know that the information is coming from a human and not an inanimate object like a banana."