I don't know how accurate those things are- I bought a polar F11 heart rate monitor and it's great- I bought mine used on ebay and LOVE IT
You know if you are that hungry you should make sure you are eating enough protein and fiber to keep you full and going longer, so instead of something like a turkey sandwich eat a huge turkey salad
Or a plate of pasta with chicken sub half pasta for half veggies like zucchini or squash
It'll help keep you fuller and give your body more nutrients you need while working out so much