We tested the latest optical technologies to provide you with the most reliable solution. Some factors can, however, reduce the reliability of the heart-rate measure: YOUR WATCH IS IN THE WRONG POSITION: Your must wear your watch in a comfortable position above your wrist bone. If the strap is too loose, the watch will be unable to correctly measure the signal. See the section: "How to position the watch on my wrist.” THE SENSOR IS DIRTY: If the back of your watch is dirty (soil, sun cream etc.), this may affect the sensor. Your HR500 watch is waterproof. You can clean it by rinsing it in clean water. YOU PRACTISE A NON-REPETITIVE SPORT: Measuring your heart rate with a monitor on the wrist (optical reader) is optimal for sports with repetitive movements such as running, or using a cross trainer or exercise bike. It is not recommended to use a heart-rate belt for fitness class, body-building, etc. THE WEATHER IS VERY COLD:Blood vessels reduce in size when the weather is cold, making reading the heart rate more difficult.Blood circulation improves as you work out, improving your heart-rate measure. INTERVAL TRAINING:The watch is not very close to the heart so the blood will take some time to flow to your wrist. For interval training, there will be a delay in the time it takes to measure the signal. For very short minute-long intervals, it will be difficult for the watch to measure the maximum heart-rate. TATTOOS: If you have tattoos on the area where the watch is located, you may notice errors in your heart rate measurement.  USING IN WATER:The heart-rate monitor does not function correctly in water. OTHER REASONS: The blood vessels in your right and left arm may be different. If the results on one arm are not optimal, try the other arm.