Watt readings inconsistent

Originally Posted by lookatmy595

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.

Originally Posted by lookatmy595

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.

Originally Posted by lookatmy595

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.

Originally Posted by Alex Simmons

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.

Originally Posted by Spunout

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.