Monday, November 10, 2008

Method for Calibrating Ergomo Power Meter using Computrainer

This describes how to calibrate an Ergomo Power Meter using a Lab Computrainer.

It is hard to verify that the Computrainer is more or less accurate than any particular Ergomo. But using a simple ramp test we can confirm two things. First how the two devices diverge. And second get a reasonable idea on how reproducible the two report.

Finally, if you have more than one Power Meter for different bikes, then using this method, you can re-calibrate them so that they agree. And that they agree with your trainer (assuming you are using a CT for your trainer.) 

I have three different Ergomo BB's (road, TT and track) and train indoors on the CT. Prior to developing this procedure I was getting fairly divergent numbers. Large enough to make a difference to CTL/ATL/TSB numbers in WKO+. Getting them all to agree was the reason for doing this.

1. Setup

Setup is simple. You just need to get your bike with PM installed and setup on the Computrainer. 

Tire pressure should be about 110PSI. 

The roller on the CT should be cleaned.

Do the initial re-calibrate for the CT. Typically I try for about 3.00 at this point. Adjust the roller appropriately.

2. Warm up

Do a simple warmup. About 100-200 watts. Do a re-calibrate every 3-4 minutes. If the number falls below 2.5, increase the roller clamp pressure.

Once you have a stable number over 2.5, seen in two or more re-calibrations a couple of minutes apart proceed to step 3.

3. Ramp Test

Start the coaching software, using the following erg file:

DESCRIPTION = Power Meter Calibration 60w per min
FILE NAME = PowerMeterCalibration20.erg
0.0     100
8.0    580
This is a simple ramp test, starting at 100W and increasing 60W every minute. 

4. Test

To ensure that the CT didn't cool down, do another few minutes of warmup with the coaching software running, the test loaded, but not started.

Then start the test, ensure that your Power Meter is also recording.

Try to maintain a fairly constant and comfortable cadence. If possible over 80RPM. 

The first few minutes are easy... the last minute is hell... hang in as long as you can.

5. Results

Copy the CDF.txt file produced from the Coaching file into WKO+. Also import your PM data.

Inspect both files and cut out the parts that do not pertain to the test. You should end up with about 4-6 minutes of data from both the CT and your PM.

Use the Export facility to save these into a CSV file.

6. Excel

Open the two CSV files in Excel, and also create a new empty workbook. Cut and paste the Minutes and Watts columns from the CT and PM CSV files into the workbook. You can use Excel tools to compare your results.

7. Sample one - Soloist Ergomo with K-factor 184

This is from a Soloist with Ergomo BB and the factory K-Factor setting.

The two time series are plotted as an X/Y scatter plot (as lines) with trendlines added. Finally a difference column was added and plotted.  

This shows that there is a substantial difference, with the Ergomo reading much higher. The difference does not quite scale linearly. It increases slightly as can be seen in the Diff plot (which uses the right axis).

8. Sample two- Soloist Ergomo with K-factor 164

This test is for the same bike with the K-factor adjusted down from 184 to 164. 

Note that while not an exact match, it is reasonably close now.  

The Diff line is still not exactly linear and does show that the Ergomo is more variable (again Diff is show using right axis.)

9. Conclusions

As can be seen, it is reasonably easy to get the Ergomo to match the Computrainer by adjusting the K-Factor appropriately. 

While this does not guarantee accuracy, it does provide a way to ensure that multiple devices are all setup to produce similiar results.

The (presumably) best results for this procedure would be with a reasonably new Lab Computrainer. But it is possible that acceptable results could be obtained with the non Lab version or possibly a Tacx or other trainer that can control the watts in "Erg" mode (i.e. set the load level absolutely, regardless of tire speed, rider cadence etc.) and can record the results.