By a dealer or sent in to a srm service center?

In any case, I've had the slope change before..

In any case, you can do a recalibration on the bike and it should be perfect
You'll need a weight... And you need to know what it actually weighs (not what it says it weighs... Most people suggest at least 20k... I've done it with 10, but heavier is better.

Heres a tool that has instructions
http://www.cyclingpowerlab.com/...eterCalibration.aspx

You can update the slope with the garmin and then should be all set.

Thanks for the reply - it was a SRM service centre.

In order to perform the calibration as instructed do I not need some weights to hang off the cranks? I don't have any that I can use. Shouldn't this have been done already when they replaced the battery? I paid for the calibration too, so I am assuming this was done already?

Likely was, but something could have happened in transit or installing it?

Or, it was out of calibration before, so watts were too high?

As a side note is you can find access to a kettle bell, your can hang it off the pedal spindle.

Maybe just drag your bike to the local gym? I always did my calibration at my pt place, plus an accurate scale to measure the weight

Another thread illustrating how the advantages of training with a power meter are often negated and far outweighed by the time wasted wanking about with the power meter and useless misleading data, instead of actually doing some proper training.

Do you buy it brand new or second hand? Maybe intiaitially it was reading high? I've never had any issues with my SRM coming back from a service center unless I changed the chainrings or crank arms and did not torque them to SRM specifications.

As I am sure you know make sure you zero offset the SRM to each Garmin prior to use.

In my model (PC VI), I had to set the correct altitude to get relevant wattage numbers

Yeah brought it brand new. I have never zero'd it - when I first got it just put it on and away we go. I have mailed the service centre, so I assume they'll get back to me tomorrow now, but should I try a zero? It is along the lines that I am thinking - the zero has changed with the service, and I have the old value in my Garmin.

I don't think there was an issue with the watts before the service. I ran the old battery into the ground - it lasted for four years! It gave me a very reliable reading for those 4 years, and although I never tested it against a second power meter it read consistently, so high or low, doesn't really matter as I based all my training around reliable numbers.

Did slope change post service?

Because it is telling me a flat out max effort sprint is 75 watts and my 55km ride today I did at an average watts of 21 - i.e. the numbers are not a few watts high or low, they are so out so as to be useless. I have 4 years of data that I'm not willing to chuck away because my SRM has gone wonky.

I have no idea - how would I know?

Do you zero offset prior to each ride now? Did they ( SRM ) change the slope?

I've never once in 4 years of using the SRM had to do a zero offset. It has just worked. This is what I am not sure about - would anything they have done in replacing the battery and calibrating the power meter have changed things so drastically that it is now as good as not working?

I always do a zero offset when using Garmins with my SRMs, when I use them with the PC8 not so much, when I set the offset on the Garmin it matches the PC8, when I don't it tends to be off and drift. Won't hurt to try, especially if the SRM is indoors and you go outdoors and the temp change is significant.

Ok, so I've just gone into the Edge 1000 and looked into the previously stored zero offset - it was set to 0. This would explain a lot hey! Went into the menu and did a calibrate, and now have a number of 532 stored. A quick ride up and down the road and everything is looking good - my watts are back up to where they should be. I wonder what happened!? Does taking the battery out and re-calibrating make the Edge 1000 lose its values? The power meter ID is still the same as before the service, but for some reason my Garmin unit set everything to 0. Anyway, thanks for the ideas.

Could have been a SRM firmware update maybe. Just check the offset prior, it's a quick process that'll keep you sane.

Possibly. I'll wait and see what they reply with tomorrow as to the reasons. Anyway, thanks for the input.

If you have not been zero offsetting pre ride some of your 4 years of data could be junk anyway .never rely on the garmin auto zero, in fact turn it off

based from those two posts, I think you aren't out of the woods yet, as the correction may be only 500W (so unless you expect to see a peak sprint of ~600W, then something is still wrong). What number do you expect for your max sprint, and what is the slope of your PM?

The other poster suggested getting kettle balls. This is a good idea. Buy two of them, one at 10 lb and one at 20lb, and have their masses determined at your local post office (whose scales are certified once a month). Having this info allows you to determine what your slope should be and if there's a significant difference btwn current slope and the actual slope.

FWIW, I also have a Rotor SRM that's four years old. The slope on that unit is ~45 Hz/Nm. Which means that the error introduced by having an offset that's off by 530 would result in only ~110W at 90 rpm and ~150W at 120 rpm.

Also, I was under the impression that the offset is subtracted from the raw value reported by your PM. Which means that an offset of 20 Hz should yield a higher wattage than an offset of 500 Hz.

Ah so that would explain the sticker on the inside of the SRM that was attached - it says 30.5hz/nm. I wondered what that was all about. Thanks.

As for what I'm expecting, I can push somewhere around 1000w for a few seconds in a sprint. Is the 532 that is now stored a watts value? I've done some reading around but can't say I'm overly confident in my understanding. The power meter has for the past 4 years been very consistent with it's values with no zeroing. I do CP20's on it etc etc and what it has given me season on season has been very steady.

If you have not been calibrating the power meter you have no idea if the numbers are accurate or consistent and all your data is worthless. Garbage in garbage out.

It's time to send it back. Key thing here is to be aware of units. The PM reports what it measures in units of Hz. That is converted to NewtonMeter (which is a measurement of torque). Torque x angular velocity (in units of radians/second) gives power.

The 532 is an offset value, and is in units of Hz. It represents a torque value of 532Hz / (30.5 Hz/Nm) or 17.4 Nm. Multiply that by your cadence (in terms of radian/second), and we get power, which is 180W assuming a cadence of 100 rpm. Assuming that only zero offset is off and nothing else is amiss, that would correspond to 255 Watt in your situation. It is safe to say that your slope is most likely off.

THis offset value is stored until you do another determination of offset.

Do the following when you get a chance. Have one pedal forward (at 3 o'clock position and the other at 9 o'clock); go to the zero offset screen on your Garmin. First, determine offset with no additional force applied to the pedal. Now apply some force onto the pedal and you'll see that zero offset number will increase. The rate of increase (less the offset) is the slope of your PM, and the slope is essentially what is obtained when doing a linear regression on the data points. If you have weight plates you can hang off of the pedal, you can create regression charts like below.



This is an independent way of ensuring that the slope on the PM is correct. For Edge 1000, there a way to check the stored slope value; that would be the thing i check next.

Not sure I understand - I can follow the calculation bit, but how have you come to 255 watts in my situation? My situation of what? There was no pedalling involved, the bike was stationary.