Garmin 520 With Power Meter (PowerTap G3)

LockyHK

Hi All,

Just wanted to ask if anyone here has a PM paired with the 520 and whether they can offer any insight to the calorie count feature and it's accuracy. I have read online that;

"Unfortunately, Garmin makes the mistake of simply converting the KJ into Calories. It seems absurd that they would make that mistake, when all it would take to make a more accurate calorie count would be to multiply KJ by 1.15 to get a much more accurate calorie count."

Does this still hold true or have they updated this?

Also, does the calorie count on the main screen (if chosen to display) get its information from the power meter (if installed), or does is it still use heart rate with the beat to beat technology, and the power meter derived calorie burn is on the power meter display page?

Thanks.

Tom Dean

It should be easy to tell from your files

Man Of Lard

Somehow there has to an assumption made in how efficient your body is at turning calories (pies) into kilojoules (work).

Depending on your athletic capability, genetics, the temperature, hydration, humidity,... (you get the picture - there are lots of variables) the accepted efficiency of the human body in converting pies into work is in the range of 20-25%.

By pure coincidence 1 calorie is 4.184 joules (1000 calories = 1 (nutritional) Calorie is 4.184kilojoules)

Turn that around - 1kJ = 0.239 Calories - so straight conversion of the kJ number into Calories assumes 23.9% efficiency.

Is 23.9% a reasonable estimate for most people given the accepted range of 20-25%? Personally I think it's not a bad proxy.

HOWEVER: On my last turbo ride (with power):

Calories expended: 332 C
Work done: 339 kJ

So it isn't using a straight conversion anyway and the point is moot.

None of this takes into account that the measured power is going to be affected depending on where in the drivetrain it's measured either (losses exist between pedal axle & pedal body, crank & bottom bracket, chainring and chain, chain & cassette, cassette & hub, hub & spokes, tyre & road)

So you could put 100W in at the pedal and only get 90W out at the tyre/road interface... or with a well-tuned drivetrain 98W, with a cruddy one 75W....

amaferanga

Garmin calories and kJ differ by a small amount when you have a ride with lots of laps due to rounding errors.

LockyHK

Man Of Lard et al thanks for the detailed responses, very helpful.

pastryboy

Even if you monitor exactly how many calories you eat, you can't be sure you're going to digest them perfectly and get all the nutrients. Then there's the variability of how many calories you'll be burn at rest depending on a ton of factors. I don't think the exact rounding of calories used on the bike will matter much in the grand scheme of things.