Garmin HRM Swim - Anyone seeing really low HR values and calorie burns?

daj150

I recently got the HRM swim for my 920xt since I thought it would be cool to get even more swim data. I have the belt on tight and in the correct position. However, I am seeing my manual pulse check HR anywhere from 10-30 BPM different than the HR strap captures (wish I could compare right away, but HRM stores data until you sync after workout). Anyway, it then says, for an interval swim where I feel like my heart is going to burst out of my chest and I am exhausted, that a 45 minutes session was around 150 calories burned. Anyone else seeing this? Anyone know why?