Sportline Heart Rate Monitor

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I'm hoping someone else with experience with a Sportline heart rate monitor can help me. I was given a Sportline women's heart rate monitor with a chest strap for Christmas (it's blue). I can not find the model number, otherwise I would list that too. Anyway, I have started using the RunKeeper Ap on my Droid phone while running. I usually average a 10 minute mile, and run about 30 minutes.

Today, after running I plugged in my calories into my exercise and MFP calculated I had burned about 350 calories. The RunKeeper Ap calculated 328 calories. My Sportline monitor calculated 566 calories burned. I have programmed the monitor with my correct age, weight, etc. I'm really unsure which calculation is most accurate...

Can anyone give some advice if they have ever seen something like this happen. I just want to be as accurate as I can be with my calories, as I've hit a small plateau in my weight loss and I was really hoping the heart rate monitor would help. Thank you!