Correct way to zero powertap before a trainer session
For you powertap users, how do you zero your powertaps before a trainer session?
I used to do it with the bike loosely in the trainer clamp but without the resistance unit applied to the rear wheel, then for some reason several weeks back I started zeroing it with the bike clamped in tightly and the resistance unit applied. Now I am starting to wonder if zeroing it with the bike fully clamped in has affected my power readings.
Searching the internet so far has revealed only a single comment that the resistance unit should not be appplied to the wheel when zeroing the unit.
What do people here do? 
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Comments
IMPORTANT: ZERO TORQUE
Frequently the torque must be zeroed to ensure the most accurate power
information is displayed. If the current power display is positive or
negative while coasting the the torque must be zeroed. This operation
must be done while stopped with no tension on the chain, no tension
placed on the pedals, and the transmission icon must be illuminated. To
zero torque:
1) Press [MODE] to scroll the cursor to the top line of the main display.
2) Press [SELECT] until current power function is displayed.
3) Hold [SELECT] until the word “WATTS” begins to flash to enter the torque
function.
4) Hold [SELECT] again until “0” is shown.
5) Press [SELECT] to return to current power. The current power function
will now read zero while coasting.
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@D: makes no mention of whether bike should be in trainer or not or clamped in or not. What do you do?
@K: powertap will not give negative readings, only a zero value, so seeing a zero reading doesn't imply that zero offset is ok. I believe the recommendation is to zero before every ride. Apparently also the autozero function (if enabled) will only work properly if you have manually zeroed prior to riding.
From the Joule User's guide:
1. Press and release the [MODE] button until Main mode is displayed.
2. Press [JOYSTICK] in and release to select Sensors.
3. Press [JOYSTICK] in and release to select Power Sensor.
4. Press [JOYSTICK] up/down to highlight the Active Sensor. Press [JOYSTICK] in
and release to select the Active Sensor. Note active sensor is designated by a
check mark next to the sensor name in the sensor list.
5. Make sure the power sensor is awake. For PowerTap and IC Power sensors
rotate the hub/wheel at least one revolution. For ANT+ sensors see that sensor’s
user guide for information on waking up the sensor.
6. Press [JOYSTICK] up/down to highlight Manual Zero. Press [JOYSTICK] in and
release to select Manual Zero.
7. The raw torque value, offset and fi rmware version will be shown. If the raw
value and offset value does not match a manual zero should be performed.
8. Press [JOYSTICK] up/down to highlight Manual Zero. Press [JOYSTICK] in and
release to perform Manual Zero.
IMHO Clamp/ing should not have an effect....it doesn't apply tension/torque to the drive train...thus should have not impact on power reading.
@JL: on *other forums* people sometimes attribute spurious powrtap readings to clamping the hub in too tightly in the trainer (rightly or wrongly, I'm not sure) , and some have also implied that you can ruin the hub if its clamped in too tightly (yikes!). I should probably email saris to get a straight answer, but I was hoping someone here had already done that.
@Satish. Thanks that worked. Not sure what it did but said calibration successful
Just recently became the proud owner of a PT G3. Had been using TR for "virtual power" for my weekly FTP and VO2 workouts. Thought my FTP was 250 (per 20/20 and 5/20 tests). However, when I got the PT, tests revealed my FTP is at best 200 (nearly died for 20 mins with Pnorm of 212). So, I was rather bummed and began investigating possible explanations (other than my own weakness!). So, I've scoured the forums and cyclops site and found same info others have provided. Spin wheel to wake it up. Wait until it stops. Calibrate. It should tell you "successful" and I usually see some number in the 500s.
Still convinced my FTP was not really in the mid 200's, I went so far as to do the formal calibration test using a heavy weight on the pedal with the back wheel clamped by the brake. Found with this tedious test that my PT is slightly off (should be within +/- 2.1%, and it is +/- 2.6%. I accepted that as good enough for me (middle of the pack AG'r). If you want to do this, PM me and I'll find the link.
It is a little frustrating that there is not much out there on how to do this. Seems like if you have a Garmin (like me) and a cyclops/PT PM, no one wants to take the onus. Cyclops says consult your computer maker and computer makers say consult your PM maker!