For reliable heart rate monitoring, the contact between the skin and the sensor needs to be firm. Please make sure that you have positioned the device correctly, at least 1-2 finger widths from the ulna bone towards your elbow. If you have tried this already, you may also try placing the sensor even more towards the elbow, some 5-8cm/1,9-3,1 inch from the ulna bone. In some cases the wrist anatomy does not provide solid skin-sensor contact when the sensor is placed too close to wrist and that causes compromised reliability.
PulseOn sensor measures your blood perfusion close to skin on your wrist. In cold conditions, when the skin is cold, the blood perfusion close to skin may be dramatically reduced or even stopped, and despite its advanced technology, this may affect also PulseOn sensor. For optimal results also in cold conditions, warm up some minutes before your actual training (this is also recommended for best training results!) and wear proper clothing to prevent extremely cold skin.
Make also sure that the strap tightness is optimal – the device should be firmly against your skin even during rapid hand movements. Also, try to avoid (if possible) fast hand movements – for example, in racket sports, use PulseOn in the opposite hand to your racket.
A good tip for verifying the correct tightness of the strap is to shake your hand - in case the device feels like moving against your wrist, you should tighten the strap a bit further (please see the attachment). The strap should still feel very comfortable against the skin thanks to the elasticity of the carefully selected technical material.