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Discussion Starter · #1 ·
Hi,
I haven't calibrated my powertap in like a month and a half- oops!
So I'm wondering, how much could my wattage have changed? By like 1-2% or like 10%?
Is it possible that the wattage shown on your powertap could be higher than what it should be because you haven't calibrated?
Powertap geeks- help!
Thanks,
:)
 

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When you say "calibrate", I'm assuming you mean zero'ed the torque. If you want accurate information, then I would get in the habit of zeroing everytime you ride. It might have been fine the whole time, or off by a lot--it is not consistent from situation to situation.

You can check whether a ride was zero'ed or not by opening the ride file in Excel. Each line in the file is a record (records every 1.26 seconds unless you change recording rate). I believe the second column is torque--you should see some zeros every so often at the same time there are zeros in cadence (last column), but the speed (third column) is not zero.

There is also a way to correct the torque in ride file in Excel. After correction, you can reload it into your analysis software and it'll be all fized. I've never done a zeroing torque correction myself, but it's explained very well and in great detail here (just the answer you need):
http://groups.google.com/group/wattage/web/ptnonzero
BTW--The Google Group "Wattage" is a great resource--that link is a permanent page in the group pages.
 

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Discussion Starter · #3 ·
so by far off, do you mean like twenty watts?

I did what you said, I opened the file in excel and during the times I was coasting, the torque read zero. Does that mean that it wasn't that far off?
thanks!
 

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Unless you turned it off the PT is set up to auto-zero when you coast.

I have only had to manually zero it when I had the hub worked on by Saris.
 

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Discussion Starter · #5 ·
So can I assume that the whole time, it wasn't that inaccurate/far off the real wattage? I did a power test haven't done one in a long time) and my wattage had gone up a bit :) which it should've due to training, but I just want to cover bases and make sure it wasn't due to an inaccuracy!!!
 

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ericm979 said:
Unless you turned it off the PT is set up to auto-zero when you coast.

I have only had to manually zero it when I had the hub worked on by Saris.
While the auto zero function usually does the trick, it doesn't work if the torque zero is off by a large margin (> 8 ft-lbs IIRC) which can happen for instance if the temperature changes quickly (e.g. bike kept indoors but it's really cold outside when you ride) or you swap a PT wheel for another but use the same CPU (or vice versa). I have seen fluctuations if the bike has been on roof of car on a cool day.

It is always a very good idea to do a manual zero before you start your workout, and if there is a temp change, then do another torque zero after 15-min or before you do the main elements of your workout that day. It only takes a few seconds to do.


As for calibration, the PT's calibration can be checked but it cannot be changed.
 

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Discussion Starter · #8 ·
Alex_Simmons/RST said:
While the auto zero function usually does the trick, it doesn't work if the torque zero is off by a large margin (> 8 ft-lbs IIRC) which can happen for instance if the temperature changes quickly (e.g. bike kept indoors but it's really cold outside when you ride) or you swap a PT wheel for another but use the same CPU (or vice versa). I have seen fluctuations if the bike has been on roof of car on a cool day.

It is always a very good idea to do a manual zero before you start your workout, and if there is a temp change, then do another torque zero after 15-min or before you do the main elements of your workout that day. It only takes a few seconds to do.


As for calibration, the PT's calibration can be checked but it cannot be changed.
When I coast, watts always goes to zero, which should mean that the torque is zero'd- correct?
Also, about torque: sometimes I will have a higher avg. power with lower torque than other times when I've had a lower higher avg. power but a higher torque. That can happen, right?
 

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kreuzberg said:
When I coast, watts always goes to zero, which should mean that the torque is zero'd- correct?
Usually, but not always. The PT does not, for instance, show negative torque values, it displays these as zero. So if auto-zero is not on, or if the offset is out by more than the auto-zero can correct, then it can still be wrong.

Why is it so hard to do a two second manual torque zero to be sure you are getting good data?

kreuzberg said:
Also, about torque: sometimes I will have a higher avg. power with lower torque than other times when I've had a lower higher avg. power but a higher torque. That can happen, right?
The PT torque values are pretty meaningless, in the sense that they represent torque at the hub and that depends on what gear you are in. To make any sense out of torque data, you need calculate what the effective torque at the crank is.
 

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kreuzberg said:
When I coast, watts always goes to zero, which should mean that the torque is zero'd- correct?
Not at all. It just means you aren't pedaling. In fact, you could be producing substantial amounts of torque and producing zero power.

If you aren't moving anything (i.e., your pedals), you aren't doing any work, and thus aren't producing any power.
 
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