One of the most common myths surrounding power meters is that you have to perform calibration before every ride.
Well, the truth is that the process is correctly called setting zero-offset (or zeroing).
The difference between power meter calibration and zero-offset is that calibration is a one-time process done in the factory. Zero-offsetting is a process done by the rider that updates the’ zero-offset value’ of a power meter to ensure power data accuracy and consistency.
Below, I explain them more in detail and answer some frequently asked questions.
What Is Calibration?
Power meter calibration is a one-time process done in a factory by the manufacturer. They set the so-called slope (given in Hz/Nm).
The slope is a multiplier that is used by the software within the power meter. The software uses it to convert the pedaling force into torque and ultimately into watts. (Source) In other words, slope indicates how much the measured value changes as torque increases. (Source)
You don’t need to worry about the slope unless you feel that your power meter starts to provide you with wrong data (usually after several years). Then you can consider sending it to the manufacturer for calibration and double-checking the slope.
The following chart shows two uncalibrated slopes and one calibrated one. If an actual reading is 50, the reported reading should also be 50. If the slope is wrongly calibrated, a different number will be reported (in this case 40 or 60).
What Is Zero-Offsetting?
Zero-offsetting is a process that updates ‘zero-offset value’ of a power meter. This means a state when there is no load on the power meter.
The zero-offset process may not return zero value, but rather a value measured by the sensor. It is then used as a baseline for future power measurements. (Source)
KITCHEN SCALE EXAMPLE
You can think of this process as when offsetting a kitchen scale. You mostly want to start at 0 and then measure the weight of ingredients. But the offset may change to +5g, for example. So if you weigh 50g, the wrong offset causes the measured weight will be different (25 or 75).
How Often Should You Perform Zero-Offset?
Most power meter manufacturers recommend performing zero-offset (incorrectly called calibration) before each ride.
You don’t have to set zero-offset on power meters with an automatic zero-offset feature.
How to Perform Zero-Offset?
To perform zero-offset, follow the instructions of your power meter manufacturer because the exact process may differ.
Usually, you need to download the manufacturer’s mobile app and use its in-build zero-offset feature, or use your head unit.
I recommend following these best practices before you do the zero-offset.
- Update your power meter and head unit firmware.
- Place your bike on a level ground.
- Make sure you don’t put any force on pedals.
You don’t have to worry about this if you have a power meter with the automatic zero-offset feature. These include power meters from FSA, power2max, Quarq, etc.
Why Is Zero-Offsetting Important?
Zero-offsetting your power meter is important to ensure you get accurate and consistent data.
When you train, you need data that you can rely on. If you get different results each time you ride a bike, they will be useless. It is like with the kitchen scale example above.
To learn more, read my article on power meter accuracy vs. consistency where I explain their differences and importance.
Power Meter Calibration vs. Zero-Offset FAQ
I hope you now better understand the differences between zero-offset and calibration.
The main conclusion is that calibration is a one-time process done in a factory while zero-offset should be performed before each ride.
In case of any questions, feel free to add a comment below.
Preview picture source: power2max