Why are wattages so different for same ride?

[GWPOS]

I have been doing some 2x20's on a climb and recently my numbers were 20-30 watts lower than before with about the same speed. If anyone could tell me why I would appreciate it. I have attached the 2 different ride files. And if possible which one is showing the correct numbers. Thanks

iBike_03_18_2011_284W_291W.csv

(1.31 MiB) Downloaded 296 times

iBike_05_03_2011_266W_264W.csv

(1.77 MiB) Downloaded 292 times

[Velocomp]

If you look at the ride files carefully you'll notice 2 things:

1) In the first interval you did not start at the same place. Though your speed was essentially the same 10.0 vs 9.9 mph), the average slope for the 5/3 ride was about 0.7% higher than the 3/18 ride, due to the different starting place. Your iBike correctly reports a higher wattage for the steeper section you climbed on 5/3.

2) in the second interval, you rode exactly the same stretch of road (you can tell by the slope profile), but your average speed on the 5/3 ride was 9.2 mph, whereas on the 3/18 ride your average speed was 9.9 mph. Your iBike correctly reports that your wattage was higher on the 3/18 ride.

Note that the hill slope measurement of the iBike makes it possible to identify EXACTLY where you were riding. No other PM can do this...

Your iBike is working perfectly and both files are correct.