Watt readings inconsistent

Discussion in 'Power Training' started by lookatmy595, Oct 24, 2007.

  1. lookatmy595

    lookatmy595 New Member

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    Hello all, I'm new to this forum and new to power training, i recently purchased a pt 2.4 sl, today when doing my turbo session (1hr at 170-190 watts) the reading was very up and down when maintaining a constant cadence and speed. Its not so much a problem now as my average at the end of the session was 179 watts, but surely when i attempt an interval session where the constant power needs to be held at a more accurate level this is going to be a problem. Have any of you the same problem? Thanks in advance.
     
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  2. rule62

    rule62 New Member

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    It just does that. The swings will be even more pronounced out on the road. As long as maintain your interval average within the target range, you shouldn't worry too much about the ups and downs. With some practice you can get better at smoothing things out on the trainer but that's about it. Out on the road there are too many variables with wind, terrain, and such to try and flat line your power output stroke by stroke.

    What works best for me is to go into interval mode on the PowerTap, dial in my effort to the proper range, then watch the average watts. As long as I am within my target range, I am good to go. Across a workout of multiple intervals, they will fluctuate a bit from one to the next too. As long as they all fall within the target range I know that I will end up meeting my workout goal.

    Hope this helps. ;)
     
  3. Spunout

    Spunout New Member

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    Go into the setup menu in the PT head unit and choose a larger averaging factor for the watts display. This will smooth the display.

    Anything over 3 seconds usually works for most.
     
  4. Alex Simmons

    Alex Simmons Member

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    What was your cadence?
     
  5. lookatmy595

    lookatmy595 New Member

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    Thanks for the responses folks. Spunout: My PT only seems to have 1 or 2 sec average recording settings. Alex: My average cadence for the hour was 80.
     
  6. Terry Ferguson

    Terry Ferguson New Member

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    Not the recording setting (leave that at 1sec), the display setting. Should be something like 1,2,3 5,10,30 seconds as the options. It will also have these options to smooth the Cadence and Speed as well as a 3 different options (something like Normal, More & Less) for the HR display. - TF
     
  7. Alex Simmons

    Alex Simmons Member

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    OK, I guessed as much. There is an artifact of the methodology the PT uses in determining power for each data point, known as aliasing, which would explain the choppy nature of the power reading. If you repeated the exercise at ~ 95rpm, I bet you could make the display read much more smoothly.

    Aliasing with a PT:

    The PT records a power data point every 1.26 seconds and uses the average of the hub torque readings (sampled at 61Hz) to determine the power for that 1.26 seconds.

    When we pedal, we provide torque to the hub which pulses in a sinusoidal manner, with two torque peaks per pedal revolution (one from each leg).

    So at a cadence of 43, 95 or 143 rpm the number of torque pulses at the hub will be equal for each 1.26 second time period at 2, 4 and 6 pulses per 1.26 seconds respectively.

    It is also a even number of pulses per 1.26 secs at cadences of 71, 119 and 167 rpm but with an odd number of pulses per 1.26 secs (3,5, & 7) - so as long as there isn't much of a leg imbalance, then the power reading from one 1.26 sec interval to another will be consistent.

    The further you deviate from those cadences the likelyhood that consecutive 1.26 sample periods will contain different number or fraction of torque pulses (which will raise or lower the average torque recorded for that interval). Since power = torque x cadence, if cadence is fixed then your power reading will vary in step with the torque measurement.

    So at 80 rpm (approx mid way between 71 and 95 rpm) the number torque pulses will vary for each consecutive 1.26 second time period and hence the instantaneous power reading will naturally move up and down with each time period.

    As you say, it makes no difference when averaged over a longer duration and the solution is to lengthen the power display rolling average interval. You would need the Pro CPU to choose longer display averaging intervals. 3 or 5 secs works well.
     
  8. rmur17

    rmur17 New Member

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    dang, that's a nice explanation Alex. I know the theory but have never bothered to work out the gory details. It just jumps around - disconcerting at first but then you get used to it.

    I prefer a 5-sample display setting .... anyhow I believe it's samples of 1.26 sec period rather than 5-seconds?
     
  9. lookatmy595

    lookatmy595 New Member

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    Thanks again guys!! Will try all suggestions. Alex thats an encyclopedic knowledge you have of these machines. Great forum.
     
  10. Spunout

    Spunout New Member

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    1) Either pedal at 143rpm or

    2) Use 5 second averaging, then you have 3 observations in your averaging data. 3 second smoothing only gets 2.
     
  11. Alex Simmons

    Alex Simmons Member

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    Naturally, the longer the display averaging is set, the less an extra whole or fractional torque pulse will impact the displayed power.

    It is worth remembering that while the CPU display averaging can be set like this, it does not affect the recording of the data, hence the choppiness of the data can be seen in the charts from the downloaded data.

    Finally, unless you really have to due to memory constraints (like a week or so away and unable to download), then always record using the 1 sec (1.26 sec) recording interval. If you choose longer sample periods, such as 2 sec (2.52 sec) the PT actually discards every other data point, rather than averaging over the whole 2.52 seconds.
     
  12. Alex Simmons

    Alex Simmons Member

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    Actually I'm not sure on that. 4 samples of 1.26 secs would be almost exactly 5 secs (5.04). Not that big a deal really.
     
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