I got the PowerCal to be able to ride in Zwift on Spin bikes while traveling. However, the PowerCal does not produce reliable enough power numbers to be used in Zwift. For a given heart rate the watts will vary from 0 to 150 to 250 in a given 10 second period. In Zwift this means you are stopping and starting constantly.
In working with PowerTap support I did some testing on a trainer to compare the PowerCal to a Wahoo. I recorded a single session 2 ways.
- Using Zwift I recorded the power from the Wahoo and the HR from the PowerCal
- Using a Garmin 820 I recorded the power from the PowerCal and the HR from the Powercal
During the test session I could not determine any pattern to the power output from the PowerCal. At any given HR the power jumps around randomly from 0 to 100 to 150 to even 600 watts. I would have expected that for given HR there would be a given consistant wattage.
Here is the Strava data from the session
You can see that the HR data is nearly identical as you would expect but the power data is significantly different. A note the difference in distance is due to Zwift calculating speed based on terrain and Watts/Kg while the garmin is using only wheel speed.
I found this problem as well with the stock strap that came with mine—the dualband PowerCal I bought came with newer type of strap with a metal hook closure. This strap gave me stability and heart rate accuracy problems. I had a previous, BTLE-only PowerCal before that worked fine, and it used a different strap, identical to the Wahoo TICKR HRM straps. I have spare ones of that type laying around, so I switched the new PowerCal unit onto one of those straps and all of my stability and accuracy problems disappeared. The new, metal-hook closure straps don’t work well. The older type that closed by snapping the buttons onto the unit work correctly.
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