The terms ‘power meter calibration’ and ‘zero-offset’ often get used interchangeably. However in reality, they are completely different procedures. In fact, many bicycle computers actually use the words ‘calibrate’ when they should really be using the term ‘zero-offset’ instead. In this post, we’ll try to explain exactly how these terms differ. We’ll also tell you what you need to know in order to ensure you get accurate and consistent results from your power meter.
Power Meter Calibration vs. Zero-Offset
Calibration is a one-time procedure that happens at the factory when your power meter is manufactured. The calibration process essentially sets the relationship between the force measurement input and output. The factory calibrates your power meter by setting what is called the ‘slope’. (See the blue line in the graph below. We’ll get to the red line in a minute). The slope is a multiplier that is used by the software within the power meter. The software uses the slope to convert the pedaling force into torque and ultimately into watts (which is what you see on your head unit when you’re riding). Slope is given in Hz/Nm and is often written somewhere on the power meter. It should also be included in the paperwork you receive when you purchased your power meter.
Slope Graphic Provided by PowerTap
The slope of your power meter should not change to any significant degree, nor is it a number you need to worry about. Every power meter has its own unique slope. You can almost think of it as a serial number. Having said this, if you feel for any reason that your power meter is no longer measuring accurately, you can send it back to the factory where they can check the slope. It’s not common, but over time, it’s possible for the slope to change slightly. Any change in the slope would negatively affect the accuracy of your power meter.
Zero-offsetting or zeroing is similar to setting the tare on a scale. This is the process of setting the scale to zero before adding weight. It removes any residual weight that might remain in the scale that could affect its accuracy. The same concept applies with a power meter. A certain amount of ‘residual torque’ can remain in the system after your last ride. See the red line in the graph above. The slope has effectively moved off its correct zero point due to this residual torque. This is usually due to ambient temperature or air pressure changes. Atmospheric conditions can affect the strain gauges that measure your power output. In addition, mechanical changes such as installing the power meter on a new bike or changing some hardware near the power meter can also affect things.
When you perform a zero-offset on your power meter, the power meter zeros out this residual torque. Or more specifically, measures the value at zero load (resets the zero point) and then records this value as a baseline for new power measurement on your next ride. To ensure maximum accuracy, you should perform a zero-offset before every ride. You can zero your power meter using your bicycle computer. It’s a 15 second task, so you have no excuses not to zero. Now go enjoy a ride and rest assured you’re getting accurate data out of your power meter!
3 Key Takeaways
- Power meter calibration is a one-time procedure that happens at the factory. It’s not something you need to worry much about.
- When bicycle computers say “Calibration”, they really mean zero-offset.
- You should perform a zero-offset before every ride in order to ensure maximum accuracy.