Search the Wattage Google group, and the Bikeradar blog for extensive discussions on P2M. It seems to be a good system in many ways but some devices have had problems with temperature instability and drift. If you are particularly fastidious about your Tacx Flow calibration/coast down, tyre pressure etc. then that should tell you something, if you aren't very consistent then forget the flow. Are you building in extensive periods of coasting on the turbo to allow the P2M to zero?
Worth noting that turbo use, with no freewheeling, is a pretty stern test of any powermeter's stability. Can you use the P2M outside, perhaps with a known good Powertap out back, to test it a bit more?
Very true, there are extensive discussions about its use, drift, freewheel to zero etc, but no one has actually said they have done a back to back with a result of around 80watts difference in any discussion.
I am pretty serious about the turbo, tyre pressure, ride till warmed up, reset the calibration coast down etc and have always had around 10watts difference max. Im convinced I have a problem, its on its way back to Germany for checking so will have to wait and see.
Just curious if anyone out there have had similar with their unit yet? Temperature does affect it, but this is a massive difference in watts, now making a tempo session become a threshold!