# Garmin 705 & Power Tap Accuracy?



## Donzo98 (Oct 1, 2008)

I have been using the Garmin 705 for the last few years... I use it in place of the Power Tap head. I was recently told that the 705 is not as accurate as the Power tap head. What is your experience with this??

I regularly calibrate the Garmin... but is this enough?


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## Borti (Aug 1, 2008)

interesting.... how inaccurate can the garmin be? Isn't it receiving the data from the powertap and merely relaying it (rather than interpreting it or converting it)?


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## chase196126 (Jan 4, 2008)

If I remember correctly, the GARMIN units use "smart recording". I believe that means that the Garmin takes an average over a period of time and records that, rather than recording .5 or 1 second data points. If anyone knows how to explain that better please do.... 

Basically the problem is that you are getting less accurate data do to the weird averaging system that the GARMIN uses.


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## Donzo98 (Oct 1, 2008)

chase196126 said:


> If I remember correctly, the GARMIN units use "smart recording". I believe that means that the Garmin takes an average over a period of time and records that, rather than recording .5 or 1 second data points. If anyone knows how to explain that better please do....
> 
> Basically the problem is that you are getting less accurate data do to the weird averaging system that the GARMIN uses.



I have smart recording turned off... I am aware of that. I was just asking if any of you have heard that the Garmin is less accurate or more prone to drift.


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## Alex_Simmons/RST (Jan 12, 2008)

Donzo98 said:


> I have smart recording turned off... I am aware of that. I was just asking if any of you have heard that the Garmin is less accurate or more prone to drift.


The drift, if any, would occur in the power meter not the CPU.

However if you can't validate and set the torque zero with the Garmin, then there is every chance your data is suspect.

"Smart" (dumb) recording is another issue but you seem to be aware of that.

I'm not sure about Garmins but also make sure you are recording power including zeros.


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## Mega311 (Feb 10, 2010)

I have a question that relates to this somewhat. I have a Garmin 705 and a friend lent me his Ptap SL+ for the week. I used it today and it was reading 300+ watts @ ~17mph and 700+ watts when in the front pulling @ 28+mph. I know this data cannot be accurate. I was told that they torque must be reset or "zeroed out" but I am not sure if I am doing it right or if it is working with my Garmin 705. I hit the button on the 705 that says re-calibrate and then it goes back to the previous screen and my data is still higher than what it should be. I did this with the bike moving but not pedaling, while pedaling, and not on the bike at all. Still seems to show error. Can anyone recommend a solution to my problem? I would like to get to use this thing. I do not have his powertap head so I can't reset it that way. 

Thanks for any help anyone may have. Also, he said it had been acting up before he gave it to me for about a week, but forgot to zero it and he is in Canada for 9 days.

Thanks again.


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## djg21 (Oct 25, 2003)

Alex_Simmons/RST said:


> The drift, if any, would occur in the power meter not the CPU.
> 
> However if you can't validate and set the torque zero with the Garmin, then there is every chance your data is suspect.
> 
> ...


The 705 has an "autozero" function. It works fine.


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## Alex_Simmons/RST (Jan 12, 2008)

djg21 said:


> The 705 has an "autozero" function. It works fine.


au contraire - even the auto torque zero with a Powertap head can be readily fooled by the zero point being more than a little out. The auto torque zero only works if the torque is a just bit out. Once it is out by a larger amount (>8 IIRC), the auto torque zero function no longer works and won't zero the torque.

Always perform a manual torque zero. Then autozero can do it's stuff.


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