How to compare two power meters

1. Record two independent sources

For comparison, two meters on the same bike or a meter and a trainer will do. Record them. simultaneously: for example, a cycling computer receives power from the pedals, while a training app receives it from a trainer.

Before the start, check the selected power source on each device. If both recordings receive watts from the same pedals, you will be comparing the recording methods of one sensor. Check the settings for substituting the trainer's power with an external sensor: the names and behavior depend on the app and model.

• Check the charge of the sensors and the mounting according to the manufacturer's instructions.
• Perform warm-up and calibration or zero setting, if they are provided for your model. There is no universal requirement to "make a spindown to all machines": some models are calibrated automatically.
• If possible, set the same recording and auto-pause settings, start and stop both recordings together.

Calibration and differences between models describes Wahoo instruction on power discrepancies. For another brand, use its manual.

2. Make a record that is convenient to compare

For an initial comparison, a ride on a trainer with several flat sections is convenient. After preparing the equipment, record a few minutes of steady pedaling, then a few minutes at a different comfortable load. This is an example of equipment testing, not a training task.

At the beginning, you can stop spinning for a while and start again: a noticeable transition will help to combine the charts. Make the main conclusion on even pedaling, and not on a single peak.

Save the original FIT file From each recording device or app. A screenshot and regular GPX with the route are not suitable for this tool. Name the files so that it is clear where the pedals are and where the trainer is.

3. Upload files to "Compare Watts"

1. Open "Compare watts".
2. Add entries to "File A" and "File B" or select both via "Quick Upload of two files".
3. Give clear names to the series: for example, A — 'Pedals', B — 'Trainer'.
4. Make sure both files are uploaded and the metric 'Power (W)' is selected.

For percentages, file B serves as a comparison base. This is a conditional choice, it does not make device B a reference.

4. Match records by time

Find a common transition — the beginning of pedaling or a change in load. If it occurs in different places on the graph, adjust "Shift A" or "Shift B". The slider and the numeric field allow you to shift the record within ± 120 seconds.

You can start with the "Auto-shift B" button, and then check the result with your eyes. Automatic selection is especially important to double-check on recordings at repeating intervals. The match must be maintained at the beginning, middle and end of the trip.

If the beginnings coincided, and by the end the charts diverged in time, a constant shift is not enough. Check for recording pauses and gaps. Do not adjust each section just for the sake of a smaller discrepancy in watts.

"Smoothing" makes the graph calmer. To check the transitions, reduce it, and return a few seconds for a review. It changes the appearance of the line, and does not correct the sensor readings.

5. Compare the same segment

Increase the flat section with the buttons 'Increase' and 'Window 1m' or 'Window 5m', move with the buttons 'Back' and 'Forward'. The 'Statistics of the current segment' will be updated under the graph. Choose a window that has data from both records.

First, look at the average power, then at the nature of the discrepancy: whether it persists at a different load level, whether it changes over time, whether there are failures at only one source.

Consider the method of counting. In the current tool, segment statistics take into account only positive values, and long records are thinned out for the graph. Therefore, segments with a roll, zeros and gaps, as well as short peaks are not suitable for strict accuracy assessment. The general summary and the selected segment may give different averages.

6. Analyze the reason for the discrepancy

Two devices can measure power in different places. Pedals and cranks measure before transmission losses, while an indoor trainer measures after. The result is also influenced by the conditions and error of each device. More details — Wahoo's explanation of the difference between a meter and a trainer.

The difference is approximately constant on flat areas

Compare it with the accuracy stated for both models and the place of measurement. There is no universal allowable percentage for any pair.

There are large discrepancies only on transitions

First check the time shift and smoothing. Do not make a conclusion on a single maximum value.

Failures occur only at one source

Check the power, communication and original recording. Data skipping and real zero power are different things.

The difference grows by the end of the trip

Repeat the check after preparation according to the manufacturer's instructions and check the time alignment at the end of the recording.

According to two divergent records, it is impossible to reliably determine which device shows the true power. If the problem persists, save both FIT files, models, firmware versions, and a description of the preparation — this is more useful for support than a photo of two different numbers.

Before making a conclusion

• The recordings were made simultaneously and from different power sources.
• Preparation of the equipment is carried out according to the instructions of its manufacturers.
• The graphs are combined, the beginning and the end are checked.
• The same section of smooth pedaling without gaps is compared.
• Percentages are read relative to B; selecting B does not mean reference accuracy.
• The result was checked at several sites and, if necessary, re-recorded.

Compare your FIT files