I got to thinking about the accuracy of the PT because all measuring devices have a tolerance. I know the speed sensing of mine is very close by comparing it with others when I ride but I wasnt sure about strain gauge accuracy for torque readings (and therefore power). So I did the "stomp test" by locking the wheel and standing on the pedal. It was difficult to do very well cause I was using a hand on the bars and the other hand on my workbench to balance and any weight which I was bearing with my hands would reduce the torque. But experimenting around I came to the conclusion that the torque was being under measured by about 5%! This meant my power was 5% higher than I thought. Sweet! Well, then I got a little more methodical and just hung a weight from the pedal and wouldn't you know it was reading spot on. I guess there truly is no free lunch.
Has every one else gone through this, or are we all confident that the power readings are right?
I got to thinking about the accuracy of the PT because all measuring devices have a tolerance. I know the speed sensing of mine is very close by comparing it with others when I ride but I wasnt sure about strain gauge accuracy for torque readings (and therefore power). So I did the "stomp test" by locking the wheel and standing on the pedal. It was difficult to do very well cause I was using a hand on the bars and the other hand on my workbench to balance and any weight which I was bearing with my hands would reduce the torque. But experimenting around I came to the conclusion that the torque was being under measured by about 5%! This meant my power was 5% higher than I thought. Sweet! Well, then I got a little more methodical and just hung a weight from the pedal and wouldn't you know it was reading spot on. I guess there truly is no free lunch.
Has every one else gone through this, or are we all confident that the power readings are right?
I've tested the accuracy of every powermeter and ergometer I and/or my wife have owned, which by now includes two Velodynes, two SRMs, and too many PowerTaps to recall (since several have died, and for a while I was rescuing "orphan" systems from eBay and passing them on to friends at cost after getting them working again). I always recommend that others do the same.
I've tested the accuracy of every powermeter and ergometer I and/or my wife have owned, which by now includes two Velodynes, two SRMs, and too many PowerTaps to recall (since several have died, and for a while I was rescuing "orphan" systems from eBay and passing them on to friends at cost after getting them working again). I always recommend that others do the same.
I've tested 4 or 5 SRMs, 1 Ergomo, 1 Polar, and more PTs than i can recall. I've yet to find a PT that isn't accurate and working. If the torque tube is ok, they seem to be spot on fine, or within a very small margin of error (see the Gardner paper -- for the benefit of everyone else and not Andy!). Based on my experience and that research I'm confident that all PTs are accurate (or you'd have to be very unlucky to find one that wasn't accurate -- i recall several years ago that Andy did find a bad PT that was new).
i recall several years ago that Andy did find a bad PT that was new).
It was 3% low (albeit at only one torque), when they claim +/-2% accuracy. When I mentioned this to Saris almost in passing, they offered to replace it, no questions asked.
It was 3% low (albeit at only one torque), when they claim +/-2% accuracy. When I mentioned this to Saris almost in passing, they offered to replace it, no questions asked.
I've been very happy with my PTs, having used all their models (from being a beta tester for Tune). Their customer service is excellent.
