Setting the Slope on a PC8

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leehollywoodturner
Posts: 4
Joined: Sat Jul 25, 2015 10:04 am

by leehollywoodturner

I am using a Stages Power Meter with a new SRM PC8 Head Unit, everything is is working brilliantly except my power is reading a lot higher. After looking into it in more detail and talking to friends I have come to conclusion that it is due to my slope.

I have the slope set (using the PC8 device agent) to auto but it still seems to read way too high. I've checked the slope via the Stages Power App on my iPhone and it gives me a slope value of 0.0555 and a temp slope of -0.38. When I go into the PC8 Device Agent to adjust it offers me a "Received Slope [N/m]" and a "User Slope [N/m]". It only lets me manually adjust the "User Slope" by 3 decimals, IE 00.0 which doesn't help (I think).

I'm not the smartest bloke so this has really confused me. I'm really lost on what to do.

Can anyone make sense of my rambling message and offer any advice.

Thanks heaps!!

:D

CAFERACER
Posts: 55
Joined: Sun May 09, 2010 3:42 am

by CAFERACER

Here's the problem, you are using a stages.
xo
Last edited by Frankie - B on Sun Jul 26, 2015 8:36 am, edited 1 time in total.
Reason: Cafe racer, please keep things civil around here. Thanks

by Weenie


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BdaGhisallo
Posts: 3250
Joined: Wed Mar 17, 2004 1:38 pm

by BdaGhisallo

You might try asking your question on the Google Wattage board. You could have more luck there:

https://groups.google.com/forum/?fromgr ... um/wattage

pantelones
Posts: 35
Joined: Mon Jan 31, 2011 2:24 am

by pantelones

leehollywoodturner wrote:I am using a Stages Power Meter with a new SRM PC8 Head Unit, everything is is working brilliantly except my power is reading a lot higher. After looking into it in more detail and talking to friends I have come to conclusion that it is due to my slope.

I have the slope set (using the PC8 device agent) to auto but it still seems to read way too high. I've checked the slope via the Stages Power App on my iPhone and it gives me a slope value of 0.0555 and a temp slope of -0.38. When I go into the PC8 Device Agent to adjust it offers me a "Received Slope [N/m]" and a "User Slope [N/m]". It only lets me manually adjust the "User Slope" by 3 decimals, IE 00.0 which doesn't help (I think).


Hi, I am 99.999% sure you can't set the slope on any 3rd party power meter using the PC8. PC8 can only adjust the slope for SRM powermeters. You will have to use the stages software, if possible within it, to set the slope. You can use the PC8 to perform zero offsets for all power meters though.

I have a quarq and I have to use the Qualvin software to adjust slope if I wanted to. Since the method of setting slope is specific to each powermeter (not part of the ANT+ communication protocol) it requires the special manufacture specific software.

leehollywoodturner
Posts: 4
Joined: Sat Jul 25, 2015 10:04 am

by leehollywoodturner

Thanks, so I should just set it to auto zero? That's what I thought and that's what I have done. I have it set to auto zero but it is still reading too high. This whole slope thing has really confused me.
Last edited by Frankie - B on Sun Jul 26, 2015 8:38 am, edited 1 time in total.
Reason: Deleted the quote. it is not needed when you reply.

sungod
Posts: 1702
Joined: Fri Dec 24, 2010 9:37 pm

by sungod

the pc8 can set zero offset aka 'calibration', it's a standard feature in the ant+ power protocol, for srm cranks you do it with the cranks horizontal, stages' website says do it with the stages crank pointing straight down

see page 13...

http://www.srm.de/fileadmin/user_upload ... e_V1_1.pdf

slope is a different matter...

on my srm the slope is fixed at factory calibration, but over time it'll drift a bit so when re-calibrating at home the head unit must be set to the new slope manually, pc7 allows this, from looking at the manual so does the pc8

the stages should tell the pc8 its slope when first paired, what does the pc8 say it is? you should be able to see it on the display (see page 12 of manual)

the units are Hz/Nm

if the stages app is saying 0.0555 then it is using different units, this is why you cannot put the 0.0555 in the pc8's xx.x slope units, you could ask stages what units their app is reporting so that you can do a conversion and check vs. the pc8 pairing value...

http://support.stagescycling.com/support/tickets/new

...if the stages reported slope to the pc8 is wrong wrt to actual measurement value then it implies the pm has a significant error and probably needs to be serviced

you say "my power is reading a lot higher", higher than what? are you sure this other measurement is accurate? also, if your other measurement is based on both legs and you actually have a significant l-r imbalance then the one-sided stages measurement would be thrown off by that

edit:

zero offset - as the name implies, it cancels out any simple offset error in power reading

slope - this is the conversion factor between the value measured by the circuitry of the power meter, a frequency, hence the Hz unit (hertz), and the mechanical force measured in Nm (newton meters)

example:

assume that with cranks in the calibration position if the frequency is, say 500 Hz, the zero offset calibration remembers this 500 and will subsequently substract it from the value used to calculate power, also i'll assume pm slope is 20 Hz/Nm

now when you are pedaling, if the frequency reported by the pm is, say, 1500Hz, then the pc8 takes this value, subtracts the zero offset to get 1000Hz, then divides this by the slope 20Hz/Nm, to get the value of the force which is 50Nm, then it takes this and the cadence to calculate the final power figure based on 1 watt == 1 newton meter per second

pantelones
Posts: 35
Joined: Mon Jan 31, 2011 2:24 am

by pantelones

Sorry, but you have some basic misunderstandings. **edited due to some of MY misunderstandings with SRM**

sungod wrote:the stages should tell the pc8 its slope when first paired, what does the pc8 say it is? you should be able to see it on the display (see page 12 of manual)

the units are Hz/Nm

if the stages app is saying 0.0555 then it is using different units, this is why you cannot put the 0.0555 in the pc8's xx.x slope units, you could ask stages what units their app is reporting so that you can do a conversion and check vs. the pc8 pairing value...

...if the stages reported slope to the pc8 is wrong wrt to actual measurement value then it implies the pm has a significant error and probably needs to be serviced


This is not true. The headunit doesn't need to know and won't display the slope for non-srm power meters. Each power meter transmits a power value over the ant+ protocol which is displayed and recorded by the head unit. The head unit doesn't do any calculations to determine a power value. [edit: SRM head units and SRM powermeters don't transmit just a power number, they store the zero offset in the head unit.]

I don't think the slope of stages is able to be changed regardless of head unit.

You can't use the PC8 to change or adjust the slope for a non-srm power meter.

sungod wrote:assume that with cranks in the calibration position if the frequency is, say 500 Hz, the zero offset calibration remembers this 500 and will subsequently substract it from the value used to calculate power, also i'll assume pm slope is 20 Hz/Nm

now when you are pedaling, if the frequency reported by the pm is, say, 1500Hz, then the pc8 takes this value, subtracts the zero offset to get 1000Hz, then divides this by the slope 20Hz/Nm, to get the value of the force which is 50Nm, then it takes this and the cadence to calculate the final power figure based on 1 watt == 1 newton meter per second


Again, the head unit (all head units) do not use the slope or zero offset to calculate the power. They display the power value transmitted from the powermeter. When to do a zero offset from the head unit you as asking the PM to perform a zero offset and it reports it back to the head unit just so a user can see it (and see if anything wonky is happening). [edit: only true for non srm meters ]

Zero offset is not "calibration". Calibrating the powermeter would be setting and verifying the slope. That is, ensuring that the force deflection line for the powermeter is correct. Unfortunately people use the terms interchangeably when they are distinct.
Last edited by pantelones on Mon Jul 27, 2015 1:19 am, edited 1 time in total.

sungod
Posts: 1702
Joined: Fri Dec 24, 2010 9:37 pm

by sungod

er, no. you can't use a srm head unit to change the slope of an srm power meter either, it's purely local to the head unit

ant+ actually has multiple options for transmitting the data that will be presented as power, but you can't really fit that into a simple explanation, so i didn't bother and went for something simple to explain zero-offset and slope to the op

in ant+ calibation IS zero offset correction, it's not the best choice of terminology, but it is the one they used

if you examine the pc8 manual, you'll find the same use of the term

you may not like it, but that's what they call it

leehollywoodturner
Posts: 4
Joined: Sat Jul 25, 2015 10:04 am

by leehollywoodturner

Thanks so much for all the responses. I feel like I need a double university degree to be able to work this stuff out.

So basically I can't do anything re the slope. I just set it at "zero offset automatic" and "slope automatic" and thats it?

I have 3 powermeters. 2 X Stages and 1 X Quarq. My avg 20 min power has jumped by approx 60-80 watts which isn't right. My normal power is reading around what I believe is about 60 watts higher.

When would one use the manual feature for both and input it themselves as opposed to using the "automatic" tick box?

Thanks again!!

han1337
Posts: 91
Joined: Tue Oct 07, 2014 5:55 pm

by han1337

I have the feeling that my power is a little bit off too, when using stages and my SRM PC8.
But to take the Questionmarks out of this conversiation, I want to check exactly what is transmitted over ANT+.

This should be possible to read from my PC with an ANT+ USB Dongle and some kind of Debugging software.
Has anyone tried this and got some kind of Software for me to read the transmission data?

avispa
Posts: 262
Joined: Tue Mar 11, 2008 2:36 am

by avispa

Question: does the Stages Power Meter tell you the watts if you use your hand to pedal the bike while setting it up?

I was able to pair the head unit, but I do not see power displayed when I pedal with my hand.

On the Stages App, also doesn’t display anything.... it says “must ride the bike to display power”.

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