HRM accuracy -HELP ME understand please

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Can someone please clarify how accurate or not accurate a HRM is in calculating calories weight lifting? My heart rate goes higher(way) and stays in the mid 140's when i'm in between set's and reps and then when I am actually doing them 170+. I lift heavy and i do double and tri sets. Also my heart rate never goes anywhere near as high when I do cardio, it stays right around 90 up to 115. Help please:)