I use a CompuTrainer Lab Pro indoors and a PowerTap 2.4 outdoors. In the past I've been involved in discussions about the calibration procedures when using a CompuTrainer. I've always taken great pains to ensure that I follow the recommended procedures and warm up the tyre and drum properly for at least 10 minutes @ 175W before calibrating and starting my training session.
Anyway, I have been fortunate enough to be given the use of a PowerTap hub to use on my CompuTrainer until next Friday as I wanted to do some comparisons just out of interest. I have the hub on a one week loan from the ever generous and ever helpful Breeze Bikes in Amble, Northumberland.
I installed the wheel onto the CompuTrainer today and have done the first of my tests, simply to prove the necessity of proper unit calibration. For this test I simply got onto the trainer and rode in ergo mode for the first 2 minutes @ 100-125W to get going and from then on from minute 2 to the end of approximately a 10 minute period @ 175W.
I then downloaded the PowerTap and copied the file off the CompuTrainer and graphed the power outputs recorded at identical time points. The results are shown below, there are no surprises. There is however clear confirmation that if you don't calibrate your trainer properly after fully warming it up you will think you are about 20W more powerful than you actually are. More comparisons will follow, done with a properly warmed up and calibrated unit!