TFA HiTrax Tip Handbuch Seite 10

7.0 About Heart Rate Measurement
In exercise or sports, your heart naturally speeds up in pumping
blood to the body in accordance with your increased energy level.
This watch can calculate that increase expressed as the number of
beats per minutes (bpm) to determine a safe target heart rate for
each individual.
PRECAUTIONS/TIPS
1. A heart rate sensor is located on the back of the watch. The back
sensor must make firm contact with the user's skin during
measurement.
2. DO NOT take heart rate measurement when diving or under water.
3. Clean the back cover occasionally by applying a few drops of water;
wipe it dry with a paper towel, removing any residual grease.
4. DO NOT use hand cream; it will insulate the signal between the skin
and the sensors.
5. Clean skin and fingers with soap and water for better signal
transmission.
6. DO NOT use the fingertips where the skin is dry and thick. For those
with extremely dry skin, moisten fingers and wrist with tap water or
apply a conductive gel.
7. Hold the watch firmly when taking measurement in motion,
8. During the measurement, avoid any awkward motion. Awkward
motion creates undesired muscle noise; hence an erogenous heart
beat reading may result.