Ironman with a power meter (zeroing / calibrating)

What is this?

"The slope is a multiplier used to convert pedaling force into newton meters of torque and, ultimately, into the watts you see on your head unit. When Quarq says "calibrate" we mean a routine that sets the slope. The Calibrate command in Garmin head units sets the zero offset and should be called Zero Offset or User Zero Offset. Think of the zero offset as a "tare" - like zeroing a kitchen or bathroom scale. Setting the zero offset does not set or change the power meter's slope."

Unfortunately, these $1500 devices aren't quite "set it and forget it".  It's more like:

Make sure your bike adjusts to ambient temperature (if kept inside, take outside for 15-20 minutes before riding).

Put the drive-side crank at 6-oclock, be unclipped, and select "calibrate" from your head unit.

If possible, do the previous step again 15-20 minutes into your ride.

Periodically during the ride, zero it again while riding.  This is different for different units.  Quarq requires 4-5 backward revolutions of the crank.  Power2Max requires a few seconds of coasting with no pedaling.

I hadn't heard that - the rest I am aware of, but hadn't heard that one.  Thanks.

I think theoretically these do the same thing but this version is more reliable than the "in-flight" version.  Plus, with this one you get a number that you can keep track of vs. the "in-flight" where you get no confirmation or feedback that it was done.

I've read that you should keep records of this number pre-ride and post-ride to see how much it "drifts".  If it drifts "a lot" it could be an issue with your power meter.

I calibrated my Quarq in transition the morning of my last IM and then left the Garmin on.  I did the math and knew that even if I went all the way to the midnight finish my Garmin would still last.  I also tried to remember to "back pedal" fairly early in the race (and a few more times).

My "low battery" signal went off on my Garmin about a hundred feet before the finish chute. :D

Thanks for giving me one more thing to worry about!

I haven't, but that could be because I haven't purchased a power meter yet.    I'm just doing my research at this point.  From everything I've read, and not specific to Quarq, is that these zero/calibration steps are pretty important.

They are not as important as you think or it's made to be.  I've calibrated before, not, rode from the house, from the car, etc and there has been little difference in wattage readings.  If 1-3 watts makes that much difference to you, then be my guest.

I did keep track of the calibration number and it's been in the 200's for a straight month. 

AKA:  Don't sweat the small stuff.

I hope that's the case.  I'm excited to get one and get it installed so I can see how the numbers come out.  Been training with VirtualPower (TrainerRoad) and excited to see how close those stack up as well.

The only time I've seen a noticeable difference is after changing batteries. I have two PT's and after zeroing once, I'm set until it's time change the batteries again.

Well, the low battery light should be close to 40-60' before the thing actually shuts off. :D

Not only that, but that's across rides.  If you calibrate the morning of the race, it's not suddenly going to uncalibrate itself sitting there unused while you're doing the swim.

I guess I was more concerned with temperature changes between ~6am and ~9am when I would be done with the swim.  I haven't gotten my power meter yet so I don't have a feel for how much 5, 10, 20 degrees of temperature will drift the zero value.

I've read temperature is the biggest factor in calibration, however...

At Vegas last September, I had calibrated (really zero-set) my PT to the Garmin Edge 500, the day before.  I think it was still late morning at that point.

Coming out of T1, the Edge was going CRAZY, and it kept posting "multiple power meters detected".  It was a real distraction, especially since I can't read fine print without reading glasses.  I never did figure out what it was, and it finally settled down, but I don't think it recorded anything after that.

Fortunately I had the 310xt on as well, and it was fine the whole time, so I went by those numbers.

The Edge recorded everything else OK

One lesson learned was NOT to try and pair anything in transition; you're surrounded by ant+ devices!

You are going to be in for a surprise...and most likely not a good one.  When I got my Quarq, my FTP from virtual power to real power went down by 30w.  From what I've heard, I'm at the low end of the variances...one guy saw a 100w drop.

VP isn't very accurate and can really only be used to compare against other VP rides.  The guys at TR have said that there really isn't a very good comparison to VP and real power.  In other words...don't be that guy that quotes a FTP of 340 and then quietly says VP afterwards

I'm definitely not that guy.  At 150 lbs, so I'd rather talk about watts/Kg than absolute watts anyway.  I do suspect I'm relatively close though.  I use a Kurt Kinetic Road Machine, I air up my tire to 110psi every ride, and I tighten down the roller 4 full turns after initial contact with the tire.  There was a blog post by the TrainerRoad guys where they used this same setup to get results that matched a Powertap very closely.  

But I know it won't be exact...

I used the exact same set up before getting my Quarq.  I wanted to cry when I saw the numbers after the FTP test.  I thought I was doing pretty well in comparison to some of the numbers I had seen posted, had read what the TR guys were saying and was thinking that I wouldn't be too different either.  Hopefully you will be one the lucky ones that is not too far off...but that would certainly make you the exception to what I've heard from others.  

In any event, enjoy the wonderful/miserable world of a PM if you go down that road.