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. (Take the Garmin Edge bicycle computers for example.) Below, we’ll break these terms down in their true definitions. In addition, we’ll 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 factory sets what is called the ‘slope’. 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. Watts are the power figure that is displayed on your head unit. 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. The slope of your power meter should not change to any significant degree.
The slope isn’t a number you need to worry about. Every power meter has its own unique slope. This isn’t a number that you need to remember or a number that will in any way impact how you use your power meter. 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 effect the accuracy of your power meter.
Slope Graphic Provided by SRM
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 effect the accuracy of the scale. The same concept applies with a power meter. A certain amount of ‘residual torque’ can remain in the system after your last ride. This is usually due to ambient temperature or air pressure changes. Atmospheric conditions 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 and then records this value as a baseline for new power measurement on your ride. To ensure maximum accuracy, you should perform a zero-offset before every ride. You can zero your power meter using your bicycle computer and it only takes about 30 seconds to do. (Note that there are a select few power meters, such as the Verve InfoCrank, that don’t need to be zero’d.) Then, as you pedal, the power meter can accurately measure your power (aka torque), and convert this figure into watts.
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.