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How Does My Apple Watch Know My Heartrate?
In this day and age, it’s easy to become blasé about technology. Understanding why a common device works has become less and less important as our gadgets become integrated into our lives and are more reliable than ever but occasionally my curiosity just gets the best of me.
I remember from when I was younger going to visit people in the hospital and seeing the little white plastic clothespin-like thing clamped to a finger with a glowing red light emanating from it that fed heart rate data to the machine that went blip-blip-blip with each beat of the heart.
Fast-forward a few years and now I wear that same technology on my wrist under my Apple Watch. Some sort of sensor sits against my skin and does the same thing. But how does it actually work?
Short answer, long word: they use photoplethysmography, a method of detection which measures differences in light absorption.
The bottom of those watches have a mixture of infrared and green LEDs and light-sensitive diodes that pulse back and forth hundreds of times per second and measure how much light is absorbed or reflected.
Most of the time in order to sense your heartbeat the IR LEDs, which are just on the edge of our visual spectrum, are activated. If they aren’t doing the trick because of sweat or other interference the green LEDs are activated. Why green? On the color spectrum, green is the exact opposite of red and therefore is a more efficient color to use to detect the presence of blood in your veins.
When blood is in your wrist after a heartbeat, it absorbs the light and signals to the watch that a heartbeat is occurring. In between beats, there is less light absorbed due to the absence of blood.
That’s why it’s important that your watch fits snugly to your wrist and is unobstructed.
How accurate is this method? A 2017 study showed that the displayed result was within 5 BPM of an ECG reading 95% of the time so reliable enough to track your activity and workout effectiveness for sure.
Speed Up Those Apple Watch Updates
I run Apple beta software on my Apple Watch so I probably update the software a bit more often than most Apple Watch users. But I know you Apple Watch users will agree with me that the Apple Watch updates are painfully slow. As long as I am complaining, I don’t know why Apple requires a charged Apple Watch to be on the charger in order to complete the update. It probably is because that 12-hour charge on your Apple Watch might not be enough to cover the slow update.
I can’t do much about the charger part but I can help show you how to speed up the updates. Your Apple Watch communicates with your iPhone via both BlueTooth and Wi-Fi. The Apple Watch prioritizes BlueTooth over Wi-Fi to preserve power. BlueTooth uses less power but it also is a lot slower transferring data than Wi-Fi in almost every case.If you disable BlueTooth at just the right moment you can force the Apple Watch to use Wi-Fi to do your upgrade. Believe me, it is a LOT faster. Unlike most Apple devices your Apple Watch does not download its own upgrades. You have to think about your Apple Watch as an extension of your iPhone, even if you have the cellular version.
Let’s say you get a notice on your Apple Watch of an update, and you also see that notice on the iPhone. When you go to the Apple Watch app on your iPhone you can download the Watch update to your iPhone and it will then update your paired Apple Watch. This is not a tiny file and sending it over BlueTooth to your Apple Watch just takes a really long time. I am sure you have watched that update circle move like maple sap in the dead of winter.
You can speed up this process by utilizing Wi-Fi instead of BlueTooth, but to do this, you will have to turn off BlueTooth at the right moment in the process. Here’s the steps to faster updates for your Apple Watch:
- Make sure that your Apple Watch is on its charger and charged to at least 50%. Your iPhone should be close to the Apple Watch. We are assuming from the start that both BlueTooth and Wi-Fi are active on your iPhone and your Apple Watch
- Open the Watch app on your iPhone and click on the My Watch tab
Tap on General and then Software Update. If there is an update available you will see the version number, some notes about the new release and a Download and Install button - Pushing on that button may trigger a request for your passcode, go ahead and enter that
- As the Watch app prepares to send the update to your Apple Watch you will see a message with “Estimated time remaining” just under the version number
- When the rough estimate of time remaining shows (i.e. “about 2 hours remaining”) and not before, it is time to switch to Wi-Fi by temporarily disabling BlueTooth. The timing is important here – don’t disable BlueTooth before you see the estimated time remaining alert.
- Open Settings*on your iPhone and choose *BlueTooth from the list
- Slide the toggle switch to Off. This shuts off BlueTooth on your iPhone. That means all your BlueTooth devices aren’t going to work while you do this upgrade but it isn’t that long.
- Go back to the Software Update screen on the Watch App on your iPhone and you will see a warning to turn BlueTooth back on. Click Cancel to continue. Without BlueTooth your iPhone and Watch will now communicate with the faster Wi-Fi protocol.
Now watch that circle close up! You should see the estimated time decline dramatically. Once, the download has completed – meaning the iPhone has sent it to the Watch you will see the “Estimated Time…” change to “Preparing…”- You will then see it say “Verifying…” and then you will have an active “Install” button. Tap the install button and enter your passcode if requested.
- Once the progress wheel appears on your Apple Watch your iPhone is no longer needed. Go back to Settings on your iPhone and slide the BlueTooth toggle back to on.
This work-around is actually pretty easy, the main thing to remember is to turn off BlueTooth on your iPhone at the right time (just after the estimated time appears) and then to remember to turn it back on.