Casio WSD-F21HR User Manual page 121

2. Position the watch so there is at least one finger width
between it and your wrist joint when you bend your hand
back (Figure (2)).
3. After you determine the best wrist position, tighten the
band snugly so the watch does not slide on your wrist.
To improve measurement accuracy, keep your arm still with the touch
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screen facing upwards for about the first 15 seconds immediately after
the start of a heart rate measurement operation (such as when you start
an Activity app measurement).
Important!
A band that is snugly tightened for heart rate monitoring can cause you to
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sweat and make it difficult for air to pass under the band, which can lead to
skin irritation. During normal wear, when you do not need to monitor your
heart rate, make sure the band is loose enough to allow you to insert a finger
between it and your wrist.
Figure (2)
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