Powermeter calibration question

MattAaronson

Sort of a random question, but what is the nature of the communication between the PM and the head unit and where is the calibration (offset / zero) number used? In other words:

- does the Quarq/Powertap send a "calibrated number" to the Garmin which just displays it on the screen, or does the PM send some sort of raw number which the head unit adjusts based on the last calibration number generated during the manual-zeroing process?

- when you do a backpedal-zeroing of a quarq, does it "transmit" the new calibration number to the head unit, or does it just start sending data that has a new offset?

I have always assumed the number was within the PM and the head unit zeroing process just sent a signal to the PM to get a new number. But that the head unit just displays whatever it receives. I.e. if I connected two head units to the same PM and didn't have one powered up during the calibration process, both would show identical numbers during the ride. But I'm not sure how it works and would love to know.

Thanks!

Ralph Moore

Don't know the actual answer to the question but do know that I often use a 920 and 510 Garmin at the same time connecting to my Powertap G3 and they do have identical numbers.

Evan O'Regan

My understanding is the calibration offset number is usually calculated/stored in the PM ANT+ Service Plugin; the head unit only receives the values from the PM. The PM stores the offset value until a new calibration is performed; this also allows the PM to dynamically adjust the power data transmitted if required e.g. in the case of coasting calibration or temperature/barometric variations detected by the PM (if the PM has that functionality), etc. The head unit only sends the command to the PM to calibrate.

i.e. a PM connected to two ANT+ head units (say, your Garmin bike computer and TrainerRoad on your laptop), the power values should be the same, even if you only used one of the computers to calibrate the PM.

Dawn Cass

Matt, I don't know for certain either however my understanding until now was that the PM transmits to the receiver head. This would make sense if you have tried a couple different receivers and they all say the same. The ne thing of interest to me is that with the latest firmware upgrade of the 520, the first thing it does is to look for calibration. Wonder if that eliminates the need for back pedal?

Tom Glynn

I picked this up from the Stages website which would leave me to believe that the calibration rests with the PM, not the head unit:
======================================================
Zero offset calibration

There are physical and environmental conditions that may aff­ect the zero offset value and there are methods both manual and automatic that will adjust this value to accommodate for the changing physical and environmental condition. The zero offset of the power meter is essentially the sensor reading or values measured when the power meter has no pedaling load (torque) applied.

The act of calibrating the zero offset causes the power meter to measure the value at zero load and then records this value as the baseline for power measurement. Loads applied while pedaling will then be measured as torque and used by the sensor to determine power in Watts. The zero offset value can be a­ffected by the installation of the crank arm and the tightening of the securing hardware. The torque applied to the securing hardware can impart some strain into the crank material that is easily accounted for by manually calibrating the zero offset.

Any time the power meter is removed from the bike and reinstalled the zero offset should be calibrated. Ambient temperature shifts can also aff­ect the zero offset to some extent. To ensure maximum accuracy it is advisable to manually calibrate the zero offset before each ride. The power meter utilizes automatic temperature sensors to compensate for temperature changes that take place during the ride. This is done automatically while you ride and you need not take any further steps to calibrate the sensor during the ride.

Dino Sarti

One note I got from Rich Sawaris at WheelBuilder.com is that the zero off set should "drift" minimally when on the same bike and the mounting hardware has not changed.

He said drift of +/- 50 from a usual number is OK, but drift of more than that might be suspect. Meaning, on my Calfee the Quarq Elsa usually reads around 315 when I calibrate in my garage before a ride. It's always within a close range of 315. The newer Elsa SR on my P5 always reads about 15-20. There is no concern that they vary from each other. There should only be concern when they suddenly change by a large amount without any other mechanical changes taking place (like uninstalling and then reinstalling the crank).