iPod Chronicles - Part 8: Nike+ Accelerometer Calibration

I never bothered calibrating the accelerometer when I first got it. In the manual, it suggested calibrating the unit, but it also said that the default settings were likely fine for most users. I had measured out a variety of distances from my house in half-mile increments, so I figured I'd compare the default calibration with my already known distances and see how they compare. If the accelerometer wasn't too far off, I wouldn't bother.

For the first few weeks, the calibration seemed to mostly match my measurements, with the accelerometer being short about .1 miles after 3-5 miles (ie, I had to run 5.1-5.2 miles in order for the accelerometer to register 5 miles.

In the first week of February, I finally had to get a new pair of shoes - the cushioning in my old shoes was totally shot and my knees were suffering. I bought the same shoes, only in the newer version. Again, I cut a small slit in the underside of the tongue and inserted the transmitter, making sure to place it under where the laces cross, just as I had with my old shoes (I still hadn't figured out the thing was an accelerometer yet).

From the very first run (3 miles, if I remember correctly), the accelerometer seemed a little more off than before. Noticeably. It wasn't until I put in about 7 miles and hit the 7.5 mile mark that I finally decided that I needed to do a calibration.

The calibration process was very simple. A basic calibration can be done with a run or a walk of some distance no less than .25 miles (400 metres). A more accurate calibration is achieved by doing the distance as a walk and as a run. Wanting to be as accurate as possible, I decided to do both.

I measured out .5 miles (I should have done 1 mile, which would likely give a more accurate calibration, but the walk would have taken a while and I was pressed for time) and ran it out. I set it up again and walked the distance back to my original starting point. Good thing I did - it turns out that the calibration was off by about .07 miles PER MILE. That's not a whole lot at a short distance, but when you run 10 miles, it's going to matter.

I've done plenty of runs since the calibration, and the accelerometer seems to pretty much match my measured distances bang on. I read somewhere (website, manual? I forget now) that frequent calibration is encouraged. Now that I see how easy it is for the calibration to be off, I'll likely perform one every month.