Not so fast tiger.
Calibration is slightly differeent depending on which model of powertap your are using iirc. There's a difference between wired and wireless modles. Best do some reading perhaps.
I'm not so sure that the newer wireless ones need this calibration at the start of every ride. I do it mostly anyway, but when I haven't bothered I've SEEN no differences. Auto zero should pick up problems anyway unless the zeroing is way out. :?:
Ok, guys... I'll zero the Powertap directly before I start my rides.
I wasn't thinking of any other outside interferences.
Oh... When you say 'zero', you're referring to 'clearing' the powertap: