Calorie expenditure in a hot environment

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Hi all,
I have signed up for a hot yoga beginner class (1 hour long), and plan to wear my heart rate monitor to see the number of calories burnt.

But my friend told me that you cannot use a heart rate monitor to estimate calories in any type of "hot yoga" class. She added that even though heart rate is increased due to thermal stress, calorie expenditure does not increase.
Is this right?

So I have a question for all you "Hot Yogis" out there:
How do you calculate how much calories you burnt in a 1 hour session?

Thanks ! Appreciate all the advice.

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