powertap zero offset question

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xixang
Posts: 275
Joined: Thu Sep 08, 2011 18:18 pm

powertap zero offset question

Postby xixang » Mon Feb 10, 2014 22:29 pm

ok, I've got garmin 500 which I use with pro+ powertap. the PT is an older one (G2?) so I cannot see the torque offset on the garmin, only the sensor ID. I've updated the 500 to the latest firmware and calibrate/zero every time when prompted at powerup. Question is - how important or otherwise is knowing the PT offset (the magic 512-ish number)? My thoughts are that if I'm doing a manual zero every time with the garmin what benefit is there in knowing the offset as well? Would it indicate a problem or need for a service if it was off?

I've also done a stomp test which showed it to be out by 3.3% but taking "weight precision" into account reduces to 2.1%, a little outside the +/-1.5% quoted.

To be able to read the offset I believe I'll need either a joule or LYC. Is it worth getting either?

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Alex_Simmons/RST
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Joined: Fri Sep 14, 2007 08:19 am

Re: powertap zero offset question

Postby Alex_Simmons/RST » Tue Feb 11, 2014 00:03 am

If the torque zero number is out of spec, there's a fair chance you have a problem. It usually also manifests as a T0 that drifts about a lot.

Keep in mind that the torque units on a PT are roughly 10 times those on a Quarq or SRM. So a drift of a small value on a PT is still quite a substantial drift.

xixang
Posts: 275
Joined: Thu Sep 08, 2011 18:18 pm

Re: powertap zero offset question

Postby xixang » Tue Feb 11, 2014 11:27 am

thanks for that.

So you'd suggest I hunt down a LYC/joule to keep an eye on the TO?

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Alex_Simmons/RST
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Joined: Fri Sep 14, 2007 08:19 am

Re: powertap zero offset question

Postby Alex_Simmons/RST » Tue Feb 11, 2014 20:56 pm

xixang wrote:thanks for that.

So you'd suggest I hunt down a LYC/joule to keep an eye on the TO?

Well being able to see the T0 means you can check things like this as well as do a slope calibration check. It's handy IMO to be able to do this with any meter. Whether it's worth it to you is up to you. You might have a mate who can lend one at times.


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