Many factors can affect the performance of your Everion sensors. Skin perfusion (or how much blood flows through your skin) is one of the many factors. Skin perfusion varies significantly from person to person and can also be impacted by the environment. If you’re exercising in the cold, for example, the skin perfusion in your upper arm might be too low for the sensor to get a reading. Permanent or temporary changes to your skin, such as tattoos can also impact Everion sensor performance, making it difficult to get reliable readings. Motion is another factor that can affect the sensor, HRV, SpO2, and respiration rate do not work reliable under motion, for e.g. running, cycling, tennis or boxing.