I love the Powertap for it's ease of use, simplicity, and industry-leading price point for an accurate power unit. Powertap units, certainly the third generation of them, just work. They are robust, durable, and accurate.
As a Powertap retailer, the first question I invariably get from a customer who has just purchased a unit. The Powertap web site and documentation is surprisingly barren of any procedure for how it should be done on a Garmin, and in fact the data that the Garmin presents the user has changed recently in newer Garmin firmware, just because Garmin made it so. Powertap is a completely distinct company and has limited input in just how Garmin sets up its firmware and its calibration functions.
Here's my take on Calibrating a Powertap G3:
When you "calibrate" a Powertap on a Garmin, you are not in fact calibrating anything, you are just forcing the Garmin unit to recognize what a zero load condition looks like, i.e. manual zero, or zeroing the torque.
So the procedure is:
- spin your pedals backwards to wake up the hub
- run the calibrate routine in your Garmin (while not sitting on the bike or applying weight to the pedals)
- then go riding
Previous generations of Garmin firmware simply read the sensor ID rather than the calibration value. Newer Garmin firmwares give you the calibration number (usually around zero, see how its determined here), and the torque, which will be zero since you have no weight on the bike or pedals. The calibration number varies with temperature.
Now, Garmin units let you turn on a function called "autozero" which zeros the torque whenever you are coasting. If you have this set (which I recommend) you don't really need to "calibrate" since the unit is always resetting the torque to zero when you are coasting down hills.
So you would only need to calibrate the unit if the temperature changed a lot since the last "calibration" and you wanted to make sure the power was really accurate before the first time you coasted down a hill that day, which is when it would automatically autozero for the first time. So in reality, you never need to calibrate if you have autozero set, which is the beauty of PT, but I do just to check the calibration number is reasonable, maybe once a week.
Also, when I am training I have my Garmin screen set to show me 3 sec power (I find anything less is just too jumpy, that's the nature of power). When I am coasting downhill I check every now and then that it does indeed read zero (after 3 seconds obviously). Incidentally I also like to track Lap Power, Last Lap Power, and Max Power, if I'm doing intervals. The Garmin also lets you choose other time average intervals, like 5 minutes, which some people find useful.
If you're really keen and curious you can actually truly calibrate the unit (i.e. check the accuracy and precision) yourself using that torque value it gives you on the Garmin calibration screen.
Two very similar procedures to accomplish this are given here and here (this is also called a stomp test as it involves stomping on, or putting a weight on, the pedals).
For a ridiculous amount of information well beyond what an average user would need, you can always refer to the Slowtwitch Official Powertap thread. Alex Simmons has also posted some interesting information at his blog here though it is not specific to using a Powertap with a Garmin head unit (which is what most people seem to use these days) but rather to the old Powertap "Little Yellow Computer," or LYC.
Edited 2 May 2017 with new link to Powertap calibration offset values.
For a ridiculous amount of information well beyond what an average user would need, you can always refer to the Slowtwitch Official Powertap thread. Alex Simmons has also posted some interesting information at his blog here though it is not specific to using a Powertap with a Garmin head unit (which is what most people seem to use these days) but rather to the old Powertap "Little Yellow Computer," or LYC.
Edited 2 May 2017 with new link to Powertap calibration offset values.
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