Hi,
Here is the manual for your heart rate monitor:
http://www.omronhealthcare.com/img/shop/hr-100c---2003-hr100cinst-rev-a.pdfThe size of your wrist shouldn't matter, because you can actually mount the monitor to a bike handle, so you don't have to be wearing it at all. It's just acting as a receiver, displaying the info. Are you wearing the transmitter belt, is it positioned correctly, and tight enough? Are you putting water or gel on the backside of the transmitter so that you get good conduction and contact? Is the transmitter belt within 20 inches of the monitor receiver display?
The transmitter battery might be low. If so, the symptom would be that your readings can be erratic. I know you said that you just bought it, but batteries can be low in new products. ..How many times have you bought a new digital watch only to find that it had a low battery...You can replace the battery in the transmitter with a lithium 3v battery #CR2032. Do you see a low battery indicator? If so, the display (receiver) battery also needs replacing and it uses the same battery as the transmitter.
Hope it helps, please let me know.
Thanks
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