How a C2 actually measures energy expenditure

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How a C2 actually measures energy expenditure

Postby mdduff » January 6th, 2018, 1:45 pm

Does anyone know how the C2 actually measures the energy expended while rowing? Does the computer average the number of revolutions of the flywheel over time, then calculating by some known quantity based on the weight/friction of turning the wheel? Or does it measure changes in angular velocity of the wheel? This would come in to play when trying to maximize certain scores on the erg. (Distance vs. wattage vs. calories).

At this point in my life, my rowing is confined to ergs. So this question stems from "bro science" advice from a Crossfit gym. The popular wisdom there and on the internet is that, when rowing for meters, choose a higher SPM rate to keep the wheel spinning. But when rowing for a set number of calories, go with a low SPM and let the wheel slow between catches. In today's workout, we had intervals that included "max calories in 1 minute" on the erg. Some of the guys insisted that they racked up more calories at 1250 cal/hr by pulling hard at 24 SPM than I did at 1600 cal/hr at 36 SPM. I don't see how that could be possible. Hence my question about the actual mechanics of how the C2 gets it measurements.
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Re: How a C2 actually measures energy expenditure

Postby Citroen » January 6th, 2018, 2:22 pm

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Re: How a C2 actually measures energy expenditure

Postby hjs » January 6th, 2018, 4:23 pm

mdduff wrote:Does anyone know how the C2 actually measures the energy expended while rowing? Does the computer average the number of revolutions of the flywheel over time, then calculating by some known quantity based on the weight/friction of turning the wheel? Or does it measure changes in angular velocity of the wheel? This would come in to play when trying to maximize certain scores on the erg. (Distance vs. wattage vs. calories).

At this point in my life, my rowing is confined to ergs. So this question stems from "bro science" advice from a Crossfit gym. The popular wisdom there and on the internet is that, when rowing for meters, choose a higher SPM rate to keep the wheel spinning. But when rowing for a set number of calories, go with a low SPM and let the wheel slow between catches. In today's workout, we had intervals that included "max calories in 1 minute" on the erg. Some of the guys insisted that they racked up more calories at 1250 cal/hr by pulling hard at 24 SPM than I did at 1600 cal/hr at 36 SPM. I don't see how that could be possible. Hence my question about the actual mechanics of how the C2 gets it measurements.


Partly true, everybody has an optimal spm for a given pace, the pm can,t measure that though, it only "knows" how much energy you put in the chain.
Above and below that optimal spm you use more enery.

Our body knows best, the higher our heartrate the more energy we use.
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Re: How a C2 actually measures energy expenditure

Postby mdduff » January 6th, 2018, 4:34 pm

Thanks, Citroen. The specifics of the mathematics were complex; but after a couple of hours, I think I understand the equations. What I couldn't figure out, though, was what kind of mathematical gymnastics the erg computer performs in measuring the power supplied (point 8 in the article). Does it measure the torque by measuring the absolute change in momentum of the wheel every stroke, or the average number of turns of the wheel in the stroke? If the former, it would appear one could score higher by allowing the wheel to slow, then applying greater force in order to take the wheel through a more exaggerated acceleration each stroke.

If the latter, then keeping the wheel spinning by increasing one's SPM would be the best approach, as acceleration of any object requires more energy than maintaining its velocity.

Any other thoughts out there? (This question also relates to damper settings...)
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Re: How a C2 actually measures energy expenditure

Postby Citroen » January 6th, 2018, 7:44 pm

The flywheel has three magnets, the fan cage has a hall effect sensor. Nothing more.

So all the monitor is doing is counting tachometer pulses.

It knows the mass of the flywheel, it knows the moment of inertia. It can do the maths.

It sees the start of the stroke as a rising saw-tooth edge on the tachometer signal. It sees the end of the stroke as the flywheel decelerates.
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Re: How a C2 actually measures energy expenditure

Postby Ombrax » January 6th, 2018, 9:25 pm

mdduff wrote:At this point in my life, my rowing is confined to ergs. So this question stems from "bro science" advice from a Crossfit gym. The popular wisdom there and on the internet is that, when rowing for meters, choose a higher SPM rate to keep the wheel spinning. But when rowing for a set number of calories, go with a low SPM and let the wheel slow between catches. In today's workout, we had intervals that included "max calories in 1 minute" on the erg. Some of the guys insisted that they racked up more calories at 1250 cal/hr by pulling hard at 24 SPM than I did at 1600 cal/hr at 36 SPM. I don't see how that could be possible. Hence my question about the actual mechanics of how the C2 gets it measurements.


I'm no rowing expert, so take the following with a grain of salt:

I think you're mixing two separate issues:

1) How the C2 erg measures things, and

2) How the human body behaves and responds to different types of exertion / exercise, specifically in this case on the erg.

In the context of your question I think we can assume that as long as C2 had done their math and physics properly, the answer to Q #1 really doesn't matter. You're doing work by applying force to the chain and the erg can figure out exactly what you must be doing, given the behavior of the flywheel for a given damping level.

You can't trick the erg into giving you more "credit" than what you actually do, so the only questions then are a) What are your training goals, and b) What's the best way to achieve them, assuming you're using an erg to do it.
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Re: How a C2 actually measures energy expenditure

Postby jamesg » January 7th, 2018, 3:17 am

how the C2 actually measures the energy expended

The flywheel does work on the atmosphere during our recovery, and this work is the difference in energy (∆ Iω²/2) in the wheel at release and catch (Newton), then extrapolated to the complete stroke. The PM gets the catch and release times and speeds from the impulse train, and Work/Time = Power.

All the other numbers derive from Power or Work: speed from power with a cube law (W=2.8*V³), kCal by multiplication (1 kWh = 860 kCal) plus 300 kCal/h free of charge courtesy of C2.

These free 300/h have some effect on strategy needed to make the kCals look better. 300 kCal/h for 2 hours (600) is alone a lot more than 1600/h for 5 minutes (130).
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Re: How a C2 actually measures energy expenditure

Postby Pie Man » January 7th, 2018, 4:30 am

If you have access to the ergs they used have a quick look in the memory. Certainly on a pm5 it will show stroke rate and calories generated for the one minute row.
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Re: How a C2 actually measures energy expenditure

Postby Edward4492 » January 7th, 2018, 8:10 pm

In one minute, 1600 cals is more than 1250 cals. Period. Doesn't really matter if you did it at low rates and harder pulls or high rates and softer pulls. Just like drag factor.
Calories, watts, and 500m pace are all correlated, although the relationship is not linear; it is causal.

More watts= more calories= lower 500m splits. Rate and drag, irrelevant.

The wattage output does have an exponential relationship to pace. Going from 2:00 pace down to 1:45 requires going from 200w to 300w. That is, a fairly modest 8:00min 2k to a fairly aggressive (for many people posting here) 7:00min 2k.

Not sure how anyone reading a screen that says 1250 cal/hour can somehow determine that it's more than 1600 cal/hour.
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Re: How a C2 actually measures energy expenditure

Postby exerscreen » January 12th, 2018, 7:48 pm

Citroen wrote:http://eodg.atm.ox.ac.uk/user/dudhia/rowing/physics/ergometer.html


This. Regarding the efficiency of stroke ratings, make sure and look at section 13. The erg does not measure the energy required to move your body mass, so, all things being equal (they arn't), output of the same watts at a lower rating will get a better erg score since the scores are only based on what is going into the flywheel. And lighter rowers have a built-in advantage for erg tests on static ergs. I think I saw somewhere that the inertial moment of the flywheel is 1.001KgM^2. That's the only part of the system that really has to be calibrated.
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