I bought a Garmin Forerunner 405 about a month ago and using it for my rides and runs. I previously used a polar HR monitor which obviously didn't have the GPS features of the garmin but did give a read out of calories burned. I've noticed a discrepancy between the total calories burned for the same ride between my garmin and polar. For example, for a 90 min ride and a AHR of 150 my garmin calculated that I burned about 150 calories more than my polar.
I was wondering which (if either) is more accurate and why there is such a difference?
Heart rate really can not be converted into calories burned. You must know that different people have different max heart rates. So if you have 2 riders of equal weight riding next to each other and they both are at 60% of max HR but one is at a HR of 120 and the other is at a HR of 100 does that mean they are burning different amounts of calories? This is an easy example.
Take a really fit professional rider. He is riding up a 7% grade at 10 mph at a 140 HR. A recreational rider of the same weight is going up the same grade at 5 mph with a 150 HR. Does that mean the recreational rider is burning more calories?
The notion that HR can be easily converted into calories burned is absurd. For any company to put a "calorie burned" reading on its equipment borders on the fradulent. I will temper this a bit in saying that the consumers want to be lied to because they want the feature, goofy or not.
As others have said, they can't be compared and they are both bogus. Having said that, using one device consistently will give you a relative flavor of calories burned, one day to the next. My polar 720i has a fitness level test that you do, and they then base the calories burned on that.
If you are trying to count calories, use the lowest one (garmin or polar) or use one of the many online calorie burned estimators that are out there.
The notion that HR can be easily converted into calories burned is absurd. For any company to put a "calorie burned" reading on its equipment borders on the fradulent.
It's not absurd. Lab tests have found good correlation between caloric expenditure and HR. The conversion factor used by your HRM may not be perfect but it is reasonably consistent and, in many cases, is the best way to estimate the amount of calories burned for a particular exercise.
My HRM is consistently within about 15% of calories estimated by power output while cycling. Not perfect, but it is consistent and close enough for my needs. It's impossible to precisely know how many calories you use each day but an HRM will help you determine a reasonable estimate that can be adjusted based on your actual weight gain/loss and knowledge of your caloric intake.