Heart rate monitor, calories, and workouts

Alright, so this is potentially a very dumb question, but I'm just curious...

I have a polar F11 HRM (which, yes, I love!), and I've been using it for a few months.  Great.  Now, I guess I'm wondering what all, exactly, it takes into account when it calculates how many calories are burned.  I know it uses my height, weight, age and general fitness level (from the test things), but does exercise duration have ANY effect on it?

Some days at the gym, I'll run on the treadmill for half an hour, then do the elliptical for another half an hour; other times, I'll only run or only do the elliptical (for longer).  When I split up the workout into those two, 30-minute sessions, I usually stop the HRM, then re-start an exercise "session"... of course, when I start on the elliptical, my heart rate is already elevated, since I was just running.  However, a few times I've just paused the workout session on my HRM, then continued it for the remaing 30 minutes of elliptical.  In theory, they should both record the same amount of calories burned, right?  Does the HRM know this?  I guess I'm not entirely sure why it would count more/less calories burned after exercising for a while, but I was just curious.

Ok, so I'm not even sure if my question makes much sense anymore, seeing as how even I barely know what I'm asking!  So, to make it easier I suppose, what would YOU do?  Start a new "workout" for the HRM, or just continue with the same one?

I don't have an answer but I was wondering the same thing as well.  Sometimes I will run on the treadmill or the elliptical for 15-20 min and then I will do weights and my heart rate will drop.  I was also wondering should I leave my heart rate monitor on while doing the weights or just while I am doing cardio. 

So sorry i can't answer your question and wasn't any more help but I am also wondering about what you were saying as well.

I would recommend letting the monitor run. Your heart rate normally stays slightly elevated while you are changing machines *and walking is an exercise =)* so you can continue to feel good about your burn.

It's not so much a question about leaving it on as I switch machines-- rather, should I RE-START an exercise "session" on my HRM, or pause the current one, and as I start working out again, continue on?  I'm realllly picky about counting only when I'm doing the cardio (just the way I am...I don't know, I know that the time in between technically is burning more, since my heart rate is elevated, but that's just the way I do it), so I wouldn't be counting the "in between" time, either way.

Thanks for the input so far, though!

i have one, and I would say keep it the whole time you exercise

I have a F11 as well and usually start the recording when I step on the first piece of equipment and don't turn it off until I'm walking out of the room after my workouts.  I keep my HRM running while I am doing weights resting in between sets even just talking to others in the room.

As far as how the HRM measures the cal burn its all a guestimate anyway.  Also after doing a good hard cardio workout you are going to have "afterburn" expecially if you do HIIT - that is you will burn some extra calories over and above your BMR for a time after working out so the little bit of time it takes to go from one machine to the next probably isn't worth the extra wear and tear on the HRM.

Thanks for the input.

dbackerfan, that does make sense.  Good reasoning.  ;)