How does ScanWatch 2 measure your heart rate?

ScanWatch 2 collects data on heart rate using photoplethysmography (PPG) and calculates a measurement based on dedicated algorithms. PPG uses light reflected by the blood and skin to measure the variation in blood flow to the veins, which can be used to calculate the heart rate. 

Photoplethysmography relies on a fundamental principle: blood appears red because it reflects red light and absorbs green light. To perform this measurement, the optical sensor of the ScanWatch 2 combines green lights (LEDs) and light-sensitive detectors (photodiodes) in contact with your wrist. When your heart beats, the blood flow and green light absorption increase, and between beats, it decreases.

Through rapid LED flashes occurring thousands of times per minute, the ScanWatch 2 estimates your heart rate by computing the light absorption rate of change, supporting a heart rate range of 30–210 beats per minute. Additionally, the optical heart sensor is designed to adapt to low signal levels by increasing LED brightness. It's worth noting that the quality of PPG measurements can vary from one wrist to another depending on each individual and on measurement conditions.

When does ScanWatch 2 measure your heart rate?

When you use the Workout Mode, ScanWatch 2 measures your heart rate continuously during the workout. In addition, ScanWatch 2 measures your heart rate throughout the day when you’re motionless, and periodically when you’re walking. Since ScanWatch 2 takes these background readings based on your activity, the time between these measurements will vary. 

ScanWatch 2 also measures your heart rate when manually recording an electrocardiogram (ECG). It has built-in electrodes in the bezel (the upper ring) and the back, which can measure the electrical signals across your heart when used with the ECG recording. When you place your finger on the bezel, it creates a closed circuit between your heart and both arms, capturing the electrical impulses across your chest. 

To use the electrical heart sensor to measure your heart rate, select the ECG mode on the watch and place your finger on the bezel.

How can I get the most accurate heart rate measurements? 

Since PPG uses light reflected by the blood and skin, it may be difficult to obtain an accurate measurement if ScanWatch 2 is not worn optimally.

PPG quality can differ from one wrist to another. We have demonstrated statistically that non-dominant wrists provide better results. However we have observed also on some users that it can be the dominant. Therefore we advise to test both wrists to have the most accurate HR data.

To obtain the most accurate results please follow these guidelines:

• ScanWatch 2 should be worn tightly enough that the sensor on the back of the watch face touches your skin.
• Wear ScanWatch 2 one to two finger widths away from your wrist bone.
• Use the wristband Withings provides with your ScanWatch 2.
• Make sure that your watch stays in place during exercise. If it moves around at all, tighten your band one notch.
• Make sure your band is secure, but not too tight. It should be comfortable and allow proper circulation.
• Make sure the watch is clean, so the green LEDs are not obstructed by dust or dirt.
• Make sure your watch and your wrist are clean and moisturized but not sweaty or overly wet. 
  Note: Excessively dry skin may result in failure to monitor your temperature. If necessary, wash and lightly dry off your hand before wearing your watch. You could also apply moisturizer to your hand to prevent dryness.  

Additional factors that can cause the heart rate to be measured inaccurately:

• Skin perfusion
  Skin perfusion, or how much blood is flowing through the skin, varies among individuals and according to the environment. In certain circumstances, ScanWatch 2 may not be able to provide an accurate heart rate measurement.
• Skin and blood properties
  The technology used by ScanWatch 2 is based on the light reflected by your skin and blood. Any permanent or temporary changes to your skin, such as tattoos, sweat, or even hair on the wrist may affect the measurements.
• Environment
  Environmental factors may affect the ability of ScanWatch 2 to take an accurate measurement. For example, water may obstruct or modify the light dispersion, and cold weather may impact skin perfusion.
• Motion
  ScanWatch 2 needs to stay securely in place to provide a heart rate measurement. Activities that cause force to the wrist, such as boxing or volleyball, may not allow ScanWatch 2 to provide an accurate heart rate measurement.

Important:

• You may have more difficulties to obtain an accurate heart rate measurement if you have circulatory issues.
• ScanWatch 2 cannot detect heart attacks. If you ever experience chest pain, pressure, tightness, or what you think is a heart attack, call emergency services immediately.  
• If you’re not feeling well, you should talk to your doctor even if you don’t get a notification. Symptoms such as a rapid, pounding, or fluttering heartbeat, dizziness, or fainting, can indicate a serious condition.  
• Do not change your medication without talking to your doctor.
• ScanWatch 2 uses green LED lights to take measurements. It is possible for you to notice the green light it if your wristband is worn loose.
• Each time a heart rate measurement is launched (by you, automatically or during a workout session) LED lights turn on for  a period of 30 seconds.
• If the sensor remains constantly on, even when no measurement is launched nor scheduled, resetting the watch by pressing and holding the button for 20 seconds should resolve the problem.