The vast majority of power meters should leave the factory properly calibrated, in other words they should be configured to report power output within the limits of the manufacturers
stated accuracy (eg +/-2%) and on average they will not read too high or too low. But not everybody will be so lucky, some power meters will over or under read and for this reason it´s
advisable to calibrate them (or at least check their calibration in the case of units that cannot be self calibrated) at intervals recommended by the manufacturer.
Before considering the specifics of calibration it is insightful to summarise just how the power reporting technology works and why this necessitates calibration. Most designs detect
torque via a number of strain gauges which transmit, electronically, a frequency indicating the degree of torque they are presently experiencing. They measure torque because Torque x RPM
(e.g. cadence) = Power, so power is a calculated value, not something that is measured directly. This further explains why power meters need some way to detect cadence (at the cranks)
or RPM (at the rear wheel).
The implication of measuring torque via frequency information transmitted from strain gauges is that the computer in charge of displaying power output needs to know
- The frequency coming from the strain gauges in a no-load situation (zero torque) and
- How an increase in that frequency relates to a loaded situation when experiencing a real amount of torque.
The no-load frequency is something the rider can reset at the beginning of every ride, SRM for example call this adjusting the “zero offset”, and it´s advisable because the strain gauges
are sensitive enough to be affected by changes in ambient temperature and
chainring bolt de-torquing. Setting the relationship between changes in the frequency and actual torque (SRM call it the “slope” of the power meter)
is what constitutes the more infrequent calibration requirement.
The Calibration Process
All calibration checks involve subjecting the power meter to an amount of torque that is known and then either comparing this with the torque implied by the power meters frequency reading,
or in the case of the PowerTap simply reading the value shown in “torque mode”. Since torque can be defined as a force around an axis, applied at a particular radius, we can create a
known amount of torque by calculating it from a known mass (weight) hung under gravity on a horizontal crank arm of a known length.
The calculators on this page provide the essential maths required to carry out calibration or calibration checks on the 3 most popular power meters - SRM, Quarq, PowerTap - and their usage
is explained with instructions for each of the relevant processes.