In Reply To:
I got a new Powertap recently and am concerned that it reads low. I have used an SRM for a few years and have an idea of what power levels feel like at different intensities (for me). It seems to me that the PT could be 25% low. I have zeroed the torque a few times on every ride. Before I send it back to Saris to get checked out I tried the calibration test as described here below. My numbers are 167lbs x 175 x 1in/25.4mm x 26/34 = 880. The PT CPU shows 570 so a difference of 35%.
Anyone tried this test and come up with this kind of variance?
"Technically, the PowerTap cannot be user-calibrated, but its accuracy can be checked using a simple test that is similar to the SRM calibration check. First, check that the transmission icon is on, and if not, give the rear wheel a spin. Then, enter the torque mode by holding the “Select” button down for 2 seconds or longer (the “WATTS” designation will disappear from the top line.) Apply the rear brake sufficiently to lock up the rear wheel. Now, measure torque as follows: with the cranks exactly horizontal (right crank at 3 o’clock), hang a known weight of at least 50 lbs from the right crank, or simply stand on it – hence the name ‘stomp test’! Measured torque = (weight in lbs) × (crank length in mm) × (1 in/25.4 mm) × (cog teeth/chainring teeth). For a 159 lb rider standing on a 175 mm crank, with the chain on the 39 tooth ring and the 23 tooth cog, 159 lbs × 175 mm × 1 in/25.4 mm × 23/39 = 646 in-lbs. Compare this to the displayed value by calculating % error as (measured torque - displayed torque)/measured torque."
Not only have I tried that test, I invented it about 10 y ago.* :-)
For some reason, though, many others can't balance on a single pedal well enough to obtain a valid reading, so first thing you might try is using some heavy weights (and/or repeat the test making absolutely certain that
ALL of your weight is on the pedal). If it still seems off, then you should probably give Saris a call (miscalibrated PowerTaps are rare - especially by such a large margin - but I've heard of one or two).
BTW, I wouldn't necessarily put stock in the numbers from the SRM unless it, too, has been statically calibrated using a similar approach...factory-miscalibrated SRMs don't seem to be as common as they once were, but historically there have been a lot more of them than miscalibrated PowerTaps.
*I just checked: it will be 10 y exactly a week from today.