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  1. #1
    squatchy
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    Calorie burning,,, Garmin -vs- Power meters

    Hi

    I'm wondering if any one has compared the two machines. I just can't believe how many calories my Garmin Edge 500 says I burn on my work outs. Do any of you have a Garmin and a power meter to compare the values of each?

    Ryan

  2. #2
    Senior Member ericm979's Avatar
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    Unless you have a power meter (or lab equipment) the calories burned number is just a guess. Most algorithms guess high, I suspect because it makes people happy.

    Power meter measures work, not calories, but physiological efficiency does not differ that much between individuals. And by making an assumption that you're on the high end of the normal range makes Kcal ~= KJ.

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    You'll get different answers depending on a person's fitness and particular HR behaviour. For some the Garmin 500 will be reasonably accurate. For others it is not as good. To give it a fighting chance you need to change the resting and Max HR settings from their defaults.

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    My Edge 500 reads +/-10% of my PowerTap. KCals for the Edge are consistent over a regular course I ride, and usually the Edge reports high compared to the PT, but it'll occasionally report less and I've never figured out what the difference was.

    Garmin 705 and 305 both report 2-3x higher than the PowerTap with all the same athlete info set.

    i think I saw on DCRainmaker's blog an explanation of the different algos used over the years, including the newer devices that allows a company to do some analysis of the rider then generate a profile that is loaded on the device to provide a more accurate calorie calculation.

  5. #5
    squatchy
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    So apparently I am uninformed about certain aspects of a power meter and how they work. Is not a calorie a quantified amount of energy value. I thought it was determined how much fu le needed to burn to raise the temperature a certain level. And then once applied a value could be attached to a volume of food particles based on the food particles ability to burn hot enough to raise said temperature a certain amount.

    So, if the power meter knows how much power was /is generated during a certain period of time it can also determine how many calories were burnt to generate that same amount of power. I don't see how it's objective with a power meter!!

    Any help here??

  6. #6
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    Quote Originally Posted by squatchy View Post
    So apparently I am uninformed about certain aspects of a power meter and how they work. Is not a calorie a quantified amount of energy value. I thought it was determined how much fu le needed to burn to raise the temperature a certain level. And then once applied a value could be attached to a volume of food particles based on the food particles ability to burn hot enough to raise said temperature a certain amount.

    So, if the power meter knows how much power was /is generated during a certain period of time it can also determine how many calories were burnt to generate that same amount of power. I don't see how it's objective with a power meter!!

    Any help here??
    The powermeter measures the power output, however, your body is not anywhere close to 100% efficient. Studies have found human efficiencies in the 18-24% range. If for example you are 20% efficient and you put out 100W as measured by a powermeter, your body will be burning 500W in order to generate 100W. Most powermeter users assume an efficiency of approx 24% which results in 1 Cal burned for every 1 kJ of measured work.

  7. #7
    Senior Member mr_pedro's Avatar
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    There might also be some loss in the drive train of the bike that a power tap can not measure.
    The efficiency of the human body is not known exactly, was already mentioned.
    And also, if you are trying to measure how much extra food you can/should eat for a given run, it is not exactly known what net amount of calories your body is able to extract from the food; it does not need to be the full 100% of the calories on the food label as some will leave your body again and some will be used to process the food. But this uncertainty applies to any way you measure calories.

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    As has already been pointed out, the power meter is measuring the work. Your machine is far from efficient. Probably somewhere around 25% or less. The basic Kcal=KJ is in the ball park. To get accurate you actually need to start gathering data on your personal physiology.
    Birth Certificate, Passport, Marriage License Driver's License and Residency Permit all say I'm a Fred. I guess there's no denying it.

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