heart rate monitor/watch question

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I just got my ft4 heart rate monitor watch and have a question that I cannot find the answer to.

Do you have to have the watch on in order to register calories burned, or is the chest strap all you need?

Weird question I know, but I do tae kwon do and they do not allow jewelry or watches BUT I really want to know my calorie burn!

Maybe if it is in my purse nearby it will transmit the data?

Thanks!

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