I’m not quite sure I know how the SRM slope affects power measurements.
If your slope is lower in the PC-V than it “should” be, then you are over-reporting power, correct?
My slope was off by just about 1%. Does this mean my power output was off by 1%?
Yes, if your slope calc is lower than it should be, the power recorded will be slightly higher. I think that the percentage it will be off is roughly equivalent to the percent that the slope is off.
Remember that you can change the slope that was recorded with the file by change the slope in the file properties. When I did it on one of my files it changed the average power up and down.
What Darrell said…
If you what directions on calibrating your SRM, I can post tomorrow. It’s not rocket science and I built a lil Excel sheet to do the calcs. (for my own record keeping). Although, I’d imagine the new SRM has directions (I downloaded it, but haven’t read it yet).
I have the downloaded directions, but I would appreciate your little excel macro as well.
Cut and paste, ignore the highlights.
My Excel sheet isn’t any macro or fancy calcs, it’s simply the below (bottom) calcs in Excel so I don’t have to fumble with a calculator. Entire process takes less than 15 minutes once you got everything set up. I recommend putting your bike in a trainer (preferably on a table so you can lower the weight) so you can roll the wheel backwards to raise the weight.
And, if you don’t like your new slope, then use the old one. But, each of my SRMs have been set optimistically (too low) from the SRM Center…
<<<<<<<<<<<<<<
Version 1.4 May 2001
This protocol can be used on either an SRM ergometer or on a bicycle. If it is to be used on an ergometer, you will need to hold the flywheel still once the pedal is weighted, in order to hold the crank level. If it is to be used on a bike, it is best to secure the frame using a bike stand, although in the field it is possible to follow the protocol with one person holding the bike steady and another adding weights and checking the zero.
1.) Attach powercranks to bike, making sure that the sensor cable is correctly positioned. For existing installations check all mounting and chainring bolts for appropriate tension.
2.) Ensure that the bike has been in a stable temperature environment for more than 30 minutes. This is necessary to keep the zero torque offset stable during the calibration process.
3.) Stand hard on each pedal to “bed in” the cranks, and then re-check tension on both crank bolts, powermeter mounting bolts and chain-ring bolts. Do not over-tighten however. This should help stabilise the zero-torque offset. (The zero-torque offset represents the transmission frequency from an active powermeter when there is no strain on the cranks, and is subtracted from all strain readings in order to accurately measure torque. It has a critical impact on the accuracy of all power measurements and calibration calculations, and therefore must stable and accurate. The zero-torque offset is sensitive to both temperature and the tension of mounting bolts, hence the need to ensure these variables are stable.)
4.) Record the powermeter serial number and SRM reported slope.
5.) Record the crank length.
6.) Record the date and temperature.
7.) Connect the powercontrol box to the sensor cable.
8.) Obtain known weights that fit on the pedal blanks. Record the total mass to the nearest 50g.
9.) Insert pedal blanks into both crank arms. Hand-tighten only.
10.) Ensure that the bike is on a level surface, and is held perfectly vertical and still during all calibration readings.
11.) Obtain the zero torque offset from the powercontrol box by firstly activating the powermeter by rotating the cranks backwards for 5-6 revolutions, allowing them to settle horizontally. Make sure that there is NO LOAD on the cranks, then press the Mode and Set buttons simultaneously. The zero-torque offset is the number on the right. Wait until the number stabilises, and then record this number as right zero-torque offset. On repeated checks (before each measurement), this number should not change more than 10 units. If it does, re-check all mounting bolts and ensure temperature of environment has not altered.
12.) Position the right crank arm at 90 degrees so that it is perfectly horizontal when under load. If possible check with a spirit level. Hang the weights on the pedal blanks on the right crank.
13.) Ensure the cranks are level using a spirit level, and if necessary make small adjustments by rolling the bicycle forward or backwards.
14.) Rub a magnet across the surface of the powermeter to re-activate it by triggering the reed-switch (visible as a dark spot under the white plastic powermeter lid). Powermeters automatically turn off after 20 seconds of inactivity to conserve battery power so you will need to repeat this frequently to simulate the effect of crank rotation during a static calibration routine.
15.) Press the Mode and Set buttons simultaneously, and observe until you find the highest stable frequency reading that appears under the “Set” heading. Raise the crank arm very slightly above level, and continue observing the frequency under the “Set” heading. This number should peak and then drop off as continuing minor adjustments to the crank position are made. Record the peak as right 1.
16.) Unload the crank and repeat from steps 11-16 until you have 3 readings for each side, checking the zero-torque offset each time and recording the loaded frequency readings as right 2, right 3, left 1, left 2, and left 3. It is best to alternate sides for each reading, as any bedding in that occurs will be distributed evenly between the two sides.
17.) Take the average of all 6 zero-torque offset readings, and use in the calculations as the zero torque offset for both sides.
18.) Perform the calculations described below.
19.) If the Average Slope is less than 5% different than the SRM reported slope (or slope from the previous calibration if it has been calibrated by trained staff previously), accept the average slope as the new slope for that crank. Enter the new slope into the powercontrol. (BCF ONLY: Inform the Exercise Physiologist in writing about the change in slope.) If there is greater than 5% variation between calibration trials, try the calibration again the next day. If it’s still greater than 5% send the powermeter back to SRM to be checked.
Calculations:
1.) Mass (kg) x 9.81 x (crank length/1000) = Torque
2.) (Right 1 + Right 2 + Right 3)/3= Right Average
3.) (Left 1 + Left 2 + Left 3)/3 = Left Average
4.) Right Average - Right Zero = Right Difference
5.) Left Average - Left Zero = Left Difference
6.) Right Difference / Torque = Right Slope
7.) Left Difference / Torque = Left Slope
8.) (Right Slope + Left Slope) / 2 = Average Slope
9.) 100 - ((Right Slope / Left Slope) x 100) = % Variation between sides
100 - ((Average Slope / SRM Reported Slope) x100) = % Variation between methods