Accuracy and Calibration

The Wattbike is certainly an exceptional ergometer, and the excellent repeatability of testing is ideal for training facilities where athletes are tested regularly for fitness program effectiveness.

Tom Stanef, Senior Technical Officer, South Australian Institute of Sport, 7th April 2010

Accuracy

The Wattbike designed to measure power output independent of the air brake, magnetic brake settings and environmental conditions. As with other ergometers it is not possible to have the same accuracy over the full range (for the Wattbike Pro the range is 0-3760Watts, for the Wattbike Trainer the range is 0-2000Watts).
The mean accuracy of the Wattbike is typically within 2% (-0.42% +/- 1.21%) with retest variance typically better than 1% (0.42% +/- 0.46%) - South Australian Institute of Sport 31/03/2010.

Split down into intermediate ranges the Wattbike accuracy is typically:

Calibration

The Wattbike is factory calibrated and does not need recalibrating. It measures absolute mechanical power in Watts with the amount of power produced measured from the sum of all the forces applied to the chain through the cranks. The forces are measured by one load cell and sequencing of the applied force is calculated according to crank position (determined by the location of two magnetic sensors on the crank). The chain itself has no influence on measurement provided that it has not become rusty and stiff.Calibration is the final process before the covers are attached to complete the Wattbike construction.
A very precise process is used to calibrate each Wattbike individually to determine square polynomial coefficients using weights in 10 kg increments up to 100 kg. The production chainring/sprocket/crank set is an integral part of the calibration process. Each Wattbike has a unique Sensor ID which is stored in the OneWire memory. This ID can be read in the Wattbike Performance Computer together with the individual coefficients (from Main menu go to Setup then Properties). A calibration file for each Wattbike is saved in a factory central database and the coefficients loaded into the OneWire memory of the Wattbike. When calibration is complete, the circuit board potentiometers are sealed with an acrylic solution to prevent previously set values changing.
If, for any reason there is a need to reload the calibration coefficients Wattbike will provide the calibration file and instructions of how to reload the coefficients (email .(JavaScript must be enabled to view this email address) and quote the Sensor ID number of the Wattbike).

Testing the Accuracy of the Wattbike

To test and compare the accuracy of the Wattbike it is important not to replace any of the production parts used in the original calibration process. It is not possible to fit a SRM crank set, or any other crank lengths without factory recalibration of the Wattbike and update of the Wattbike Performance Computer firmware. With the correct equipment and test protocol the accuracy of the Wattbike can be tested using dynamic calibration rigs or by attaching a torque sensor (such as HBM or Vacuumed Calibrator), without affecting any of the production parts. This may involve considerable cost for test equipment but it is important to apply scientific methodology and rigour to ensure test accuracy. Wattbike are very happy to provide any assistance is required, particularly to coaches and sports scientist that may wish to validate the accuracy of the Wattbike power output and to assist in development of adapters for calibration equipment.
Please see the instructions on returning your Wattbike Performance Computer to Zero State to restore your monitor to it’s factory state.

To see a comparison of the Wattbike and SRM values please see below:

Warranty

The Wattbike is constructed and calibrated by experienced engineers, tampering with the Wattbike invalidates all warranties and Wattbike cannot guarantee accuracy if the Wattbike is taken apart or any of its production parts are replaced with non Wattbike approved materials.