Timex Heart Monitor

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Okay I purchased a Timex Heart Monitor WR30m model and it is giving me crazy readings. Said i in 40 minutes I burned 513 calories on the treadmill going from 4.0 to 5.1 in intervals for 30 minutes. The treadmill claims it is 257.
I don't know what to believe and I can't get this thing to enter my weight. Don't even see that I can on the watch but the silly manuel that came with it claims it does!

I bought this so i could get a better idea of calories I am burning because I am stuck at my current weight for going on three weeks now.

This model has the strap and I make sure it is in contact with my skin and that I have wet the sensors so that it has contact.

Anyone else have issues with heart monitors?