Reconciling calories, joules, watts

[Andrew Gabriel]

I've been trying to reconcile the figures that my exercise bike gives me, but they don't seem to make sense, so I think I'm doing something silly.

As an example, I cycle until it says I've used 250 calories. This takes 33 minutes, and the instantaneous power output varied between

80W and 110W. So I thought I'd work out the average power output...

The calories are probably kCal or food calories (same thing).

1kCal = 4186.8J, so 250kCal = 1046700J 33 minutes = 1980 seconds 1046700 / 1980 = 528W

That's too good to be true, and doesn't tie up with the 80-110W the bike claims I was doing (which is believable).

Any idea what's wrong?

[Bob Minchin]

I expect they will have assumed a factor for your efficiency. Not only have to put work into th bike but you will have heated up your weight by a few degrees, vaporised a certain amount of sweat etc

Does that help?

Bob

[Andrew Gabriel]

Possibly. The watts figure is my power output. It doesn't say what the calorie figure represents, but as it's probably there for diet purposes, it's probably an estimate of my energy input/consumption, in which case it is factoring in my efficiency.

In that case, it's rating my efficiency as around 20%. I wonder where it gets this from - just a burned-in number, or basing it on some measurement it's doing? I would have thought evolution would have got us better than that, although it's obviously not 100% or I wouldn't be getting hot doing it. OTOH, we didn't evolve to do 30 minute cycle rides.

[Andy Dingley]

I think Eddie Merckx set the record here at something like 550W, so you're either fitter than you think, or the other figure is more credible.

I guess the exercise machine is assuming a "mechanical" calorie requires more than this in "food" calories to supply it, and it's converting for you.

[whisky-dave]

s:

I certainly haven;'t evolved to do that ;-) But we also need a cerain number of caloroes to do 'nothing' I think it's 2500 a day for a male, so that works out roughly at 50 caleries per half hour just sitting, breathing and maintaining life support' etc.... Not sure how you'd factor such things in, I've heard that 25% of our calories are used to 'power' our brain. Maybe that's why playing World of warcaft means eating lots of chocolate biscuits

[root]

I'd guess that it's a made up "power output" - probably based on soemthing like a person's ability to light a bulb. As that's what they usually show people doing to illustrate cycling->electricity conversion. In which case 20% isn't too bad - considering what a coal-fired power station has for it's efficiency. I'd guess most of the energy consumed would be wasted: heating you up and moving the mass of your legs up and down. Couple that with the inefficiencies of electrical generation and 20% sounds pretty good!

[Chris Hogg]

It's a sales ploy? Your average Joe knows SFA about watts, let alone Joules, ergs, mechanical equivalent of heat or whatever, but may have just about heard of calories and that they're the things you want to get rid of. So if he's buying an exercise bike and after peddling for

30 min one tells him he's burnt off 250 calories, but another tells him he's only burnt off 50 calories for similar effort, he's going to buy the first. Or am I just too cynical?

[Chris Hogg]

Finger trouble again Harry! Try 34.4 diet calories. Food calories (I assume that's what you mean) are usually kilocalories or kcal aka Calories or Cal (with a big 'c'), aka kilogram calories, i.e. all of them calories/1000

[Andrew Gabriel]

You apparently also failed to notice I had converted energy to power, and no, you don't need to be superfit to generate 80W output power.