The custom heart rate monitor on the back of the Apple Watch helps the device get readings for your daily activity and specific workouts. Apple has launched a new support page detailing how the monitor on the watch works. The technology at work is called photoplethysmography, which uses light to measure the blood flow in your wrist.
From Apple (opens in new tab):
Apple also highlights some tips to get a good reading from the monitor. The fit of your Apple Watch is important. If your watch sits too loosely on your wrist, the heart rate monitor won't be able to get an accurate reading, so Apple recommends a snug fit.
The company also notes that things like your environment might impact your reading, based on how much blood flows through your skin. Rhythmic motions like those from running or cycling will give more consistent results than motions from activities such as tennis, where motions are irregular.
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Joseph Keller is the former Editor in Chief of iMore. An Apple user for almost 20 years, he spends his time learning the ins and outs of iOS and macOS, always finding ways of getting the most out of his iPhone, iPad, Apple Watch, and Mac.
I'm curious to see just how accurate the heart rate monitor will be for the average user.
Uses the same tech that Fitbit uses and that is suppose to be pretty accurate.
though fitbit recommends a very lose fit instead of a snug one.
Joanna Stern was testing Apple Watch HRM accuracy and she claims it is on pair with best chest straps heart rate monitors.
I just found out that HRM monitor in Apple Watch can't be used in third party iPhone fitness apps. It is a terrible news for me, I love running, cycling with Runtastic and they claim that to monitor burning calories within their app I will have to use another heart rate monitor than the one in Apple Watch. Having Apple Watch and buying the other device just to use with fitness apps that I've been using on iPhone for a very long time is ridiculous.
That may change in the future. I use the Digifit app.
From Apple's page
And throughout the day, Apple Watch measures your heart rate every 10 minutes and stores it in the Health app.
</quote> This should me mentioned in the article above, as it implies you can monitor your heart rate without having an app running
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