In Depth: Apple Watch heart rate sensor: everything you need to know

How it works and user compatibility

If you peel back the layers of the Apple Watch, tossing its premium look and features like the Force Touch screen and the Digital Crown to the side, you'll eventually be left with one piece of tech that's par for the course in most of today's smartwatches: a heart rate sensor.

Not to be outdone by the Microsoft Band, Basis Peak and other competition in the wearables category, Apple Watch can track your heart rate with the sensor located on the flipside of its watch face. But the main attraction isn't how Apple Watch interprets the heartbeat data, but (read this in your best Jony Ive voice) how Apple is trying to power a wrist-sized evolution in the way we communicate through technology.

Before we get too ahead of ourselves, let's do an overview of how the Apple Watch's heart rate sensor works.

How does the Apple Watch's heart rate sensor work?

Fitness isn't the sole focus of the Apple Watch, but don't count it out. Its sensor, which rests in the circular back of the watch, is a tag-team effort comprised of infrared light blasters and green LEDs. They work together to employ a tried and true (but difficult to pronounce) technique called photoplethysmography to give users an accurate heart rate reading.

Apple's support documents explain the complicated process for tracking your heartbeat:

"Blood is red because it reflects red light and absorbs green light. Apple Watch uses green LED lights paired with light‑sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment. When your heart beats, the blood flow in your wrist - and the green light absorption - is greater. Between beats, it's less. By flashing its LED lights hundreds of times per second, Apple Watch can calculate the number of times the heart beats each minute - your heart rate."

Wouldn't flashing the green LEDs "hundreds of times per second" plague the Apple Watch battery life to die in a jiffy? Yes, it would. But the green LEDs kick on only when they're needed. For the rest of the work, the infrared light comes into play. This low-power tracking mode sends your heart rate to the Health app every 10 minutes, which compiles the data into a graph for you to study, if you choose.

But if this method of heart rate monitoring isn't producing consistent, reliable data, the green LEDs will kick back into gear as a failsafe.

Apple Watch packs guts capable enough to track several types of workouts, but like most trackers, it has limitations.

Will the Apple Watch heart rate sensor work on my wrist?

Apple is upfront about the likelihood that, for some users, the heart rate sensor just might not work very well – or at all – even under favorable conditions. In the same support document, Apple stresses that wearing the smartwatch correctly on your wrist is key, but you probably guessed that much. The heart rate sensor can't detect much of anything if the Apple Watch isn't "snug and comfortable."

Still not getting a good reading? As it turns out, there are also a host of other factors that could be to blame for a lousy reading. Some are environment-related: for example, if it's cold outside, the Apple Watch will have a hard time giving you a reading when circulation to your wrist is reduced. That said, if circulation to your extremities is an ongoing issue for you even in a warm room, an external Bluetooth heart rate monitor will give you better, more consistent results.

Lastly, Apple points out that the Watch might be choosy when it comes to tracking during your preferred activities. "Rhythmic movements, such as running or cycling, give better results compared to irregular movements, like tennis or boxing." Sorry, jousters and parkour athletes.

Taptic Engine and future applications

At launch, the fitness functionality in WatchKit, the OS that runs Apple Watch, is basic. The Watch's heart rate sensor does provide the beat to back simple readings and more in-depth graphs to track patterns over a period of time, but it's similar to what many other smartwatches can output.

Fitness aside, the heart rate sensor brings along a few surprise use cases that could help separate the Apple Watch from the bunch of wrist-wrapping competitors. To do this, the Cupertino-based company is having a go at making communication on your wrist more "genuine." But how?

New-age methods of communication continue to become more rich and complex, and this doesn't bode well for the limited screen real estate of a smartwatch. To challenge this trend, Apple wants you to try dumbing down your communication and talk to others without using words, but rather, your heartbeat.

The Taptic Engine, Apple Watch's vibration motor, translates your measured heart rate into pulses of vibration that can be sent to your contacts for the most basic of status updates.

This feature, of course, has limited use unless you know someone else with an Apple Watch. But for the purpose of demonstration, let's just say you do. Simply navigate to a contact using the Side Button and Digital Crown and select the middle option to send a Digital Touch.

At this point, holding two fingers on the screen sends your measured heartbeat along. The recipient will feel your heartbeat on their wrist. Creepy, awesome, whatever your opinion is, the utilization of the tech inside the Apple Watch is unique. If anything, it tries to cut out the unnecessary manual chit-chat like "hello," "I'm thinking of you" or "I have an Apple Watch" from your texting lexicon.

Putting a humanizing spin on a basic feature

Retina Display, FaceTime, Touch ID. At their core, these are just mainstream features -high pixel-density screen, video calls and a fingerprint scanner- wrapped in Apple-generated phrasing that's designed to tug at our heartstrings.

The heart rate sensor in Apple Watch is just a heart rate sensor, but paired up with the vibrations from the Taptic Engine, it becomes more human. At the very least, it's a simple yet ingenious way to communicate in a fun new format. One use case I could see is helping to bridge the gap between long-distance lovers.

But will this idea spin off into anything meaningful in the future? It's hard to say. The touchscreen and accelerometer in smartphones inspired app developers to think up some awesome uses for the tech. Hopefully, with the heart rate sensor and Taptic Engine, developers are intrigued enough to get to work on some equally cool ideas for the Apple Watch.

Here's a few ideas that I thought up:

• A Metal Gear Solid-inspired stealth action game where your heartbeat can give up your location
• A status indicator in iMessages that, like the "online", "away" and "offline" status indicators, will display your heartbeat in real-time
• A rhythm game where the app procedurally generates music around the tempo of your beating heart
• A Life Alert-like feature where if your heartbeat becomes highly irregular, a service is alerted

Do you have any million dollar ideas? Submit them here and we'll steal them and not give you any credit whatsoever.

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