ANT Forum

When receiving power from my SRM power meter from my phone with ANT+ Plugin Sampler Power app, "Calculated Power" is usually higher(more than 100W) than the value I receive from 910xt.(Both are set next by each other.)

Any suggestion is welcome.      

Hi,

We're unable to replicate the issue here using the ANT+ Plugin Sampler. To assist in debugging, could you use SimulANT+ to simulate a bike power meter and report the results? This will allow you to transmit a constant power to both devices using an ANT USB Stick.

Thanks,
Harrison      

Hi,
I tried Ant+ Simulator. It will recieve the same power for the two devices.
At the same time, I figured out the default Crank length is 172.5mm.
My crank length is 167.5mm, is it possible it differs because of Crank length configuration?

Thanks.      

Hi,

Did you emulate a crank torque power meter? Power only pages differ from crank torque pages in that they send only power data as a single value rather than crank torque data (which power is calculated from).

It is possible the 910XT may be influencing the value based on crank length, but the ANT+ Bike Power Device Profile does call for bike power to be calculated only from the data fields on the relevant main bike power page...You may be able to find an answer for this question from Garmin support instead.

If you'd like to continue your own investigation, you could use another ANT stick and simulate a Bike Power Display. SimulANT+ can be used as a benchmark to simulate displays and sensors against.

Best regards,
Harrison      

Yes, I emulated a crank torque power meter.
After I click "Request Manual Calibration", the display value on Ant+ sampler is eqal to 910xt display value.
Could you explain how "Request Manual Calibration" functions when it is called?
After I check http://www.thisisant.com/developer/ant-plus/device-profiles it se,ems the power meter sends the power to the phone and the watch?
So, how does "Manual Calibration" affect the power meter's device output?

Thanks,

Best regards,
Jirong      

Hi Jirong,

Your power meter probably uses a data type called CTF or crank torque frequency which you can confirm using the Plugin Sampler. CTF meters use a value called the zero offset to influence the power calculation done by the display. Typically a CTF power meter automatically transmits a new value while it is coasting, but it sounds like the Plugin Sampler may have received or was using a different cached value from the one your 910XT was using. By sending a manual calibration command, you force the power meter to send a new calibration value, which is now the same one your 910XT is using. I believe SRM recommends doing a manual calibration once per day at the start of the day to account for temperature changes and the sort. The plugin sampler does not do this automatically, but it does store the value between the times it receives a new value.

Best regards,
Harrison