How does Power Meter calibration work?

May 24, 18 11:31

Post #1 of 17 (3115 views)

So, I am a Power Meter noob and am thoroughly confused about a few things regarding calibration. So, I have a Garmin Edge 500 that is paired to the power meter. I also ride Zwift and have the same power meter paired to Zwift. Since Zwift doesn't give me a calibration option can I calibrate on the Edge 500 and have that calibration be valid on Zwift? Meaning does the calibration on the Edge actually save a calibration value on the power meter itself that then results in a valid calibration for Zwift? Or is the calibration value something that is only valid on the head unit that I used to calibrate the power meter with?

If my calibration is only valid for whatever head unit I use to calibrate with then how do you calibrate a power meter for use with Zwift?

Hopefully that is clear. Appreciate your responses to bring me up to speed on this stuff.

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Re: How does Power Meter calibration work? [Lock_N_Load] [ In reply to ]

May 24, 18 11:48

Post #2 of 17 (3091 views)

My understanding is the PM calibration like you are talking about is really a zero offset which sets the PM so that 0 watts = 0 watts, just like zeroing a scale before you get on it in the morning. Once that is done in any way it is good for all uses since whatever is being changed is being changed on the PM, not the head unit or whatever program you are using (e.g. calibrating on the Garmin means you are good to go on Zwift)

Re: How does Power Meter calibration work? [STP] [ In reply to ]

May 24, 18 12:04

Post #3 of 17 (3064 views)

If you imagine that this is simplified to something like a math formlula of y=mx+b, the "cal" on your bike computer is addressing the "+b" part, or just taring the zero point.

A true calibration would take several known applied forces to generate a calibration curve which is then re-applied to the device.

This would involve hanging weights or something like that.

Re: How does Power Meter calibration work? [burnthesheep] [ In reply to ]

May 24, 18 12:33

Post #4 of 17 (3035 views)

burnthesheep wrote:

If you imagine that this is simplified to something like a math formlula of y=mx+b, the "cal" on your bike computer is addressing the "+b" part, or just taring the zero point.

A true calibration would take several known applied forces to generate a calibration curve which is then re-applied to the device.

This would involve hanging weights or something like that.

Exactly - you need to know the frequency change in the strain gauges from a given load, and you should know how that frequency changes over a range of loads. In that way you can come up with a curve of outputs from the strain gauges for given loads and you can derive the slope from that, or the 'm' in your equation. That way your head unit can combine your cadence and the torque output readings from the strain gauges and deliver a power number and be able to do that accurately over the range of power outputs a cyclist is able to deliver.

Re: How does Power Meter calibration work? [Bdaghisallo] [ In reply to ]

May 24, 18 12:48

Post #5 of 17 (3008 views)

Got it. The initial calibration (with weights etc.) was only done once and would not have to be done unless I change the PM to another crank. The calibration the had unit is doing is really a zero offset of the PM. This helps me. At least I am not wasting my time when I calibrate on the Garmin prior to riding on Zwift. Thanks all!

------------------
http://dontletitdefeatyou.blogspot.com

Re: How does Power Meter calibration work? [Lock_N_Load] [ In reply to ]

May 24, 18 12:51

Post #6 of 17 (3002 views)

Your garmin and zwift do a zero offset not a calibration. Garmin just calls it a calibration even though its not.

The garmin zero offset (calibration) is done in the head unit. Not the pm. Your pm broadcasts pm slope (hz/Netwons*Meters), instantaneous frequency (hz), and cadence. Your garmin does the calculation in the head unit to calculate power (watts). Zwift does the same exact thing on its own, independent of your garmin. Doing a calibration/zero offset on your garmin, has zero affect on what zwift reads. Currently, there is no way to do a calibration/zero offset on zwift.

Re: How does Power Meter calibration work? [stevej] [ In reply to ]

May 24, 18 13:36

Post #7 of 17 (2944 views)

stevej wrote:

Your garmin and zwift do a zero offset not a calibration. Garmin just calls it a calibration even though its not.

The garmin zero offset (calibration) is done in the head unit. Not the pm. Your pm broadcasts pm slope (hz/Netwons*Meters), instantaneous frequency (hz), and cadence. Your garmin does the calculation in the head unit to calculate power (watts). Zwift does the same exact thing on its own, independent of your garmin. Doing a calibration/zero offset on your garmin, has zero affect on what zwift reads. Currently, there is no way to do a calibration/zero offset on zwift.

This is true only for the SRM. All other power meters do their zero offsets internally, and therefore, when you do a zero offset (calibration) using your head unit, it will affect what happens on Zwift, because the calibration is a command that gets sent to the power meter and it internally corrects the zero setting. If you do not think that is the case, do a calibration while you lean on one of the pedals and then ride Zwift.

Less is more.

Re: How does Power Meter calibration work? [Big Endian] [ In reply to ]

May 24, 18 13:39

Post #8 of 17 (2933 views)

Thanks. I did not know that was specific to srm. (I own 2 srm’s).

Learned something new today.

Re: How does Power Meter calibration work? [stevej] [ In reply to ]

May 24, 18 14:14

Post #9 of 17 (2900 views)

stevej wrote:

Thanks. I did not know that was specific to srm. (I own 2 srm’s).

Learned something new today.

ANT+ has two different data transmission protocols for power meters.

ANT+ call them PWR and CTF devices. See here:
https://www.thisisant.com/developer/ant-plus/device-profiles

SRM is a CTF device (Crank Torque Frequency)
Most others are PWR devices.

http://www.cyclecoach.com
http://www.aerocoach.com.au

Re: How does Power Meter calibration work? [Big Endian] [ In reply to ]

May 24, 18 16:28

Post #10 of 17 (2837 views)

I was under the impression you could do a calibration with a quarq as well. I remember they had to include the ability to calibrate the Cinqo because if you changed the chain rings the PM had to be recalibrated.

Have they kept that function with the Riken, Elsa and Dzero and Dfour?

Re: How does Power Meter calibration work? [AlexS] [ In reply to ]

May 24, 18 16:31

Post #11 of 17 (2837 views)

To piggy back onto this conversation, I bought a used powetap Elite+ wheelset and use a Garmin 310XT to record power. How do I do a zero offset? The power readings I'm getting are higher than I would expect.

"They're made of latex, not nitroglycerin"

Re: How does Power Meter calibration work? [gary p] [ In reply to ]

May 24, 18 16:59

Post #12 of 17 (2819 views)

gary p wrote:

To piggy back onto this conversation, I bought a used powetap Elite+ wheelset and use a Garmin 310XT to record power. How do I do a zero offset? The power readings I'm getting are higher than I would expect.

page 42.

https://static.garmincdn.com/...unner310XT_OM_EN.pdf

Note that garmin calls zero offset a calibration.

Re: How does Power Meter calibration work? [AlexS] [ In reply to ]

May 24, 18 17:07

Post #13 of 17 (2812 views)

AlexS wrote:

stevej wrote:

Thanks. I did not know that was specific to srm. (I own 2 srm’s).

Learned something new today.

ANT+ has two different data transmission protocols for power meters.

ANT+ call them PWR and CTF devices. See here:
https://www.thisisant.com/developer/ant-plus/device-profiles

SRM is a CTF device (Crank Torque Frequency)
Most others are PWR devices.

Thank you for the link. I have always been curious of how ant\ant+ communicates. Guess I never searched hard enough for info. Lots of reading planned for this rainy weekend.

I have never seen a sticker with CTF or PWR showing. Am I not looking hard enough? Is there a list of what mfg's use somewhere?

Re: How does Power Meter calibration work? [stevej] [ In reply to ]

May 24, 18 17:18

Post #14 of 17 (2801 views)

stevej wrote:

gary p wrote:

To piggy back onto this conversation, I bought a used powetap Elite+ wheelset and use a Garmin 310XT to record power. How do I do a zero offset? The power readings I'm getting are higher than I would expect.

page 42.

https://static.garmincdn.com/...unner310XT_OM_EN.pdf

Note that garmin calls zero offset a calibration.

I've seen that, it's more than a little vague. And the manual for the Elite+ only tells you how to zero offset with the Powertap Computer. ¯\_(ツ)_/¯

"They're made of latex, not nitroglycerin"

Re: How does Power Meter calibration work? [jaretj] [ In reply to ]

May 24, 18 19:10

Post #15 of 17 (2746 views)

jaretj wrote:

I was under the impression you could do a calibration with a quarq as well. I remember they had to include the ability to calibrate the Cinqo because if you changed the chain rings the PM had to be recalibrated.

Have they kept that function with the Riken, Elsa and Dzero and Dfour?

You can use their Qalvin App to test/check slope and update the power meter with a new slope.

The app differs depending on the model, either Qalvin Legacy for older models or Qalvin BLE for newer models using Bluetooth.

https://www.quarq.com/product/qalvin-legacy/
https://www.quarq.com/product/qalvin-ble/

http://www.cyclecoach.com
http://www.aerocoach.com.au

Last edited by: AlexS: May 24, 18 19:10

Re: How does Power Meter calibration work? [gary p] [ In reply to ]

May 25, 18 4:50

Post #16 of 17 (2621 views)

The PowerTap app will quickly / easily do the zero offset thing :-)

WD :-)

Re: How does Power Meter calibration work? [Bdaghisallo] [ In reply to ]

May 25, 18 6:24

Post #17 of 17 (2574 views)

Bdaghisallo wrote:

Exactly - you need to know the frequency change in the strain gauges from a given load, and you should know how that frequency changes over a range of loads. In that way you can come up with a curve of outputs from the strain gauges for given loads and you can derive the slope from that, or the 'm' in your equation. That way your head unit can combine your cadence and the torque output readings from the strain gauges and deliver a power number and be able to do that accurately over the range of power outputs a cyclist is able to deliver.

I thought about making my own with some Arduino/Adafruit or whatever stuff.

The hardest part is getting it to play nice on ant+/bluetooth and having some kind of starting point for that.

The hardware, sensor, wiring, cal isn't much to it.

In college there was an engineering lab where we had a bread-board and we glued the strain gauges in various configurations then calibrated and used them. Much like you would a power meter.

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