I recalibrated ths AM and its gone from 533 to 515.. not a significant change in my opinion... I guess its just settling in after its been tqd down on the bike...
I'd still consider a manual calibration with a known weight.
Otherwise, PAGING GEOFF!!!
The offset is not the same as calibration and will often change whenever you make major changes to your PowerMeter requiring a re-calibration. Generally speaking, the PowerMeter will need to be re-calibrated whenever you modify any interface between the PowerMeter chassis and any of the other crank components. This would include replacing the battery or changing the crankarms.
The interface between the PowerController chassis and the crankarms is particularly robust on the Si version. Because of that, it is the most accurate of all the PowerMeters. Notwithstanding that, if you change the crank arms, remove and replace the crank arms, or re-torque the crankarms, you should re-calibrate the PowerMeter. Fortunately, you can re-calibrate it yourself.
My SRM was pretty consistent from September onward in the 43-460 are temperature pending.. at that time of year in Winnipeg we've entered the "anything goes" time of year... I was recently in Maui for 3.5 weeks and was consistently getting 425-435 values every morning... the house temp is around 68 or so where I was doing the offset.. I just installed and did an offset and got the 500+ value... after actually riding the Computrainer now, I guess things have meshed so to say and its back to the normal 430ish value I was always getting before.. I guess thing just needed to seat in... that computrainer watts and SRM watts were close enough that I can see everything is OK...
I have never done the calibration test.. is this what you are referring to??
1. Look for a weight of about 30 to 40 kg. Hang it on a cable of about 25 cm, so that the weight does not touch the floor when it is hanging on the horizontal crank pedal, or take a longer cable and put the bike on a table.
2. Calculate the weight in Newton. E.g. 30 kg = 30 * 9.81 = 294.3 N 3.
3. Calculate the torque you get when the weight hangs on the horizontal pedal crank.
E.g. at 172.5 mm cranks : 0.1725m * 294.3 N = 50.77 Nm (Newton meter).
4. Switch the Powermeter on by pedalling backwards. Do this with a middle gear so that the chain is in a line, e.g. 53/15.
5. Notice zero of Powermeter (MODE + SET, right number)
e.g. F0=500 Hz.
6. Bring crank in horizontal position and hang the weight on the left pedal.
7. Notice frequency output-left of Powermeter (MODE + SET, right number)
e.g. Fleft=1450 Hz.
8. Bring crank in horizontal position and hang the weight on the right pedal.
9. Notice frequency output-right of Powermeter (MODE + SET, right number)
e.g. Fright=1550 Hz.
10. Calculate frequency change of Powermeter with this weight as (Fleft+Fright)/2 -Fo
e.g. (1450+1550)/2 Hz - 500 Hz = 1000 Hz at a torque of 50.77 Nm.
11. Calculate average slope of Powermeter.
Slope = 1000 Hz/50.77 Nm = 19.90 H z/Nm.
12. This slope is the calibration of Powermeter and must be set in Powercontrol.
We recommend to check the slope monthly.
Its handy and I'd do it at peast once a year. I sent my srm to colorado for service in october 2011 and calibrated it 2 weeks ago. The srm factory slope was 1,35% off of what I got using a known weight.
Guess I should recalibrate the slope soon then..
What are you guys using as a known weight? I can imagine that a weight lifting weight marked as 20kg is not precisely 20 kg.
How do you attach it to your crank arm? A rope or something?
The weight I use is an Ivanko 'calibrated' weight, which is supposed to be accurate to 10g. You can get scientific weights, too, but that is pretty close. You can hang it off an old spindle directly or with a rope, etc.
The reason that your calibration will differ from the factory calibration is that the factory uses a more accurate eddy current brake instead of a weight.
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