ScanWatch 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. That is why it may be difficult to obtain an accurate measurement if ScanWatch is not worn optimally.

To obtain the most accurate results please follow these guidelines:

• ScanWatch should be worn tightly enough that the sensor on the back of the watch face touches your skin.
• Wear ScanWatch 1 to 2 finger width away from your wrist bone.
• Use the wristband Withings provides with your ScanWatch.
• 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. 

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 may not be able to provide an accurate heart rate measurement.
• Skin and blood properties
  The technology used by ScanWatch 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.
  Note that you may have more difficulties to obtain an accurate heart rate measurement if you have circulatory issues.
• Environment
  Environmental factors may affect the ability of ScanWatch to take an accurate measurement. For example, water may obstruct or modify the light dispersion, and cold weather may impact skin perfusion.
• Motion
  ScanWatch 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 to provide an accurate heart rate measurement.