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@Dammit
55 minutes of zone 2 on the turbo:
I set the zero offset, span for 10 minutes then reset the zero offset- changed from 286 to 280 in that time. What is interesting is that the turbo clearly warms up during that time- power rises whilst speed falls slightly over the 10 mins.
Just to look super-dorky I wore both my Garmin HR strap AND my PowerCal HR strap, giving the following:
The SRM reckons I averaged 240 watts for the 10 minute warmup, the PowerCal reckons 194 watts.
Moving onto the longer session we have the following:
The SRM gives an average of 220 watts for the 45 minutes, the PowerCal says 191 watts.
Now the PowerCal works by tracking change in HR - and it may also rely on wheel speed, which it didn't have here- I'm not sure about that part.
Anyway, it's possible that my fairly constant HR isn't ideal for the algorithm- I simply don't have enough data yet.
I'll go out dorked to the max at the weekend and see how the two power devices track each other when on the road.