# SRM & Garmin



## Ghost234 (Jun 1, 2010)

I'm seriously considering a SRM to go with my Garmin. However, I've been reading things about the SRM slope and off set values, etc. and I have no idea what they are talking about. 

Can someone explain this too me? How would I go about setting the right slope/offset? Would I need a PC7 to do it?


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## JustTooBig (Aug 11, 2005)

they are not difficult to determine or calibrate.

No, you do not need a PC7.

If you google something like "SRM slope and offset" I'll wager you will find the SRM Users Manual in the results, that will give you all the info you want -- and more.


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## the mayor (Jul 8, 2004)

I have a SRM with both the PC7 and a Garmin 705.....I usually use the 705 on the road and the pc7 on the trainer


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## multirider (Nov 5, 2007)

SRM sets the slope. You calibrate your PC7 and/or Garmin at the beginning of each ride. The PC7 does it automatically if you coast early in the ride. I don't know if the Garmin does that. But it is pretty trivial to calibrate at the beginning of a ride. And not necessary if the temp is the same as your last ride. But very easy to do.


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## Alex_Simmons/RST (Jan 12, 2008)

multirider said:


> SRM sets the slope. You calibrate your PC7 and/or Garmin at the beginning of each ride. The PC7 does it automatically if you coast early in the ride. I don't know if the Garmin does that. But it is pretty trivial to calibrate at the beginning of a ride. And not necessary if the temp is the same as your last ride. But very easy to do.


What you are describing is pairing a handlebar computer with the power meter and making sure the computer is using correct values for both the slope and the zero offset. Garmin rather unfortunately refers to setting the zero offset as "calibration", when it is nothing of the sort.

Calibration involves a test procedure to validate the torque (or power) readings from the power meter align with accurately known torque (or power) values applied to the power meter. IOW, calibration determines the slope of a power meter (which may or may not be the same as the slope that happens to be stored in the SRM or in the powercontrol).


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## multirider (Nov 5, 2007)

I was not describing pairing. Both my PC7 and my Garmin are paired with my SRM. I can ride off without pushing any buttons on either unit and, because they are already paired, they report power, speed, etc. Garmin calls it "calibration". SRM calls it "setting zero offset". The process is intended to ensure accurate power.


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## Alex_Simmons/RST (Jan 12, 2008)

multirider said:


> I was not describing pairing. Both my PC7 and my Garmin are paired with my SRM. I can ride off without pushing any buttons on either unit and, because they are already paired, they report power, speed, etc. Garmin calls it "calibration". SRM calls it "setting zero offset". The process is intended to ensure accurate power.


1. Yes, setting off and the units will begin to auto read/record speed, power etc. You need to tap one button on the Powercontrol to wake it up from sleep mode though. However this does not require one to set the zero offset. Setting the ZO is a separate function.

2. They will read/report data whether or not one's zero offset has been correctly set. The data may or may not be accurate as a result and setting the ZO is only one element to ensuring accurate data. Manually checking/setting the ZO is recommended before starting a ride and occasionally during a ride. For an SRM, I recommend disabling the auto torque zero function.

3. The SRM uses the slope and zero offset stored _in the Powercontrol_ to calculate power. The Powercontrol may take its slope setting from that which is stored in the power meter, however that's only the case if the auto slope detection function is enabled in the Powercontrol. Otherwise the Powercontrol uses the slope setting that was manually entered by the user.

4. Just because the Powercontrol is set to auto detect/use the slope stored in the power meter does not mean that the slope is correct (or that a slope that's been manually entered is correct).

5. Calibration is the process of checking the actual slope of a power meter (versus what might currently be being used/stored in the meter). I recommend all power meter users validate the slope of their power meter, and use the validated number. It's not difficult to do, but it is not an automatic function and is something you might check every 6-12 months.


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## Got Time (Jan 23, 2009)

Why do you "recommend disabling the auto torque zero function" for the SRM (even when used with a PC7)?

BTW: whether you need to press a button on the PC7 at the start of the ride seems to depend on its mode (turned off by pressing the "Mode" button for 10s? otherwise the motion sensor kicks in).


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## multirider (Nov 5, 2007)

Got Time said:


> BTW: whether you need to press a button on the PC7 at the start of the ride seems to depend on its mode (turned off by pressing the "Mode" button for 10s? otherwise the motion sensor kicks in).


That is correct. 

In the interest of absolute precision, I have to push the on/off button on my Garmin 310 to turn it on. I am in the habit of, but am not actually required to, hit the "Start" button on my Garmin.

My PC7 almost always turns on automatically because I don't typically put it to sleep manually. It wakes up on its own and works immediately.


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## Alex_Simmons/RST (Jan 12, 2008)

Got Time said:


> Why do you "recommend disabling the auto torque zero function" for the SRM (even when used with a PC7)?


Because setting a torque zero on a crank based power meter while still clipped into the pedals makes no sense and can introduce a significance source of uncontrollable error.

Put it this way, I have seen the auto-zero function alter a torque zero setting by over 300Hz.

There's no auto-zero on a Quarq. There are two manual zero options, and one is preferable to the other for accuracy reasons.

Powertap is different as it's at the freewheel hub and less prone to clip in errors, but freewheel drag can be a problem.

P2M have some firmware to detect if a "clipped in" zero is valid. I can't say how well that works in practice.

The secondary thing I dislike about auto zero features is not knowing how and when they are applied (and hence what impact they've had on the data). It would be preferable if torque zero change data was also stored/time stamped.


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## Alex_Simmons/RST (Jan 12, 2008)

Ghost234 said:


> I'm seriously considering a SRM to go with my Garmin. However, I've been reading things about the SRM slope and off set values, etc. and I have no idea what they are talking about.
> 
> Can someone explain this too me? How would I go about setting the right slope/offset? Would I need a PC7 to do it?


While you'd need to consult each meter's specific information to understand how these thing apply to it, here's an item to explain the general principles of zero-offset and slope - using a bathroom scales analogy:

Alex's Cycle Blog: The bathroom scale analogy


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