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How do I know my power meter is working properly?

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How do I know my power meter is working properly?

Old 06-03-15, 02:50 PM
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Seattle Forrest
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How do I know my power meter is working properly?

I bought a Garmin Vector set recently. Had it installed at the store because I don't have a torque wrench or any desire to do mechanical ****. Unfortunately paid mechanics aren't immune to mistakes, and PM has been reporting nonsense. I think it's been installed properly now but at this point I'd like to verify the numbers before I trust them. How do I do that?
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Old 06-03-15, 03:00 PM
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have you dialed it up to 400 watts?
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Old 06-03-15, 03:02 PM
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Are you using a Garmin 500 or higher to display the power data? You can set the screen to display 1, 3, 5 second avg. Don't just use 1 second since you'll be chasing the numbers if you're trying to stay at a specific wattage during intervals. You will then need to download the data into a software program to analyze the numbers. There's several out there to chose from, Golden Cheetah, Training Peaks, Wko+, etc. Golden Cheetah is free and does the job for me, so I'd say give it a try. I does take some time to accumulate the data to look at trends and understand the curves.
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Old 06-03-15, 03:17 PM
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Yeah, what kind of numbers are you talking about? As noted above, don't set your readout to instantaneous, use 3" average. Much smoother. If you go instantaneous, it kind of looks crazy, seemingly all over the place. But if you are talking about obvious bad data, can you do a "reset" of the PM? I have a different model and after I did a battery change, the numbers were off the charts, like in the thousands. I had to do a reset of the power meter and it started working properly again. Don't know how the Garmin model works, but for mine I had to download an app, that used Bluetooth to reset it.
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Old 06-03-15, 03:22 PM
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Originally Posted by FLvector View Post
Are you using a Garmin 500 or higher to display the power data?
I'm using an Edge 800 and a Fenix 3 (watch), both are paired to the power meter. On the Edge, I have a screen with 3-second avg, L/R balance, cadence, and HR zone. On the Fenix, I have a graph showing power over the last 30 minutes.
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Old 06-03-15, 03:25 PM
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Originally Posted by cthenn View Post
Yeah, what kind of numbers are you talking about?
Example: after a ride last week Garmin Connect said I crossed a segment. It took me 28 watts to average 16.8 mph up a moderately steep hill.

Also, L/R balance has been all over the place, often pretty close to 100 % from one foot. Power would cut in and out, and for a while there was no cadence.

I had the pedals taken off and put back on, updated firmware, etc. The numbers are probably within "normal" for road cycling at this point, power and cadence are there whenever I'm pedaling, but I'm suspicious, I have no frame of reference at this point to know what 200 w "feels like." So I'd like to figure out how to determine whether these are accurate.
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Old 06-03-15, 03:38 PM
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28 watts is totally wrong. If you're pulling 17mph up a decent hill, that should be around 300 watts, not 28.

Also, mine is a left only crank based model, so I don't know about the L/R balance stuff. You said you had the firmware updated, but can you do a "reset"? I don't pretend to know a lot about the "zero offset" and calibrating power meters, but I do know it is important, and if you don't do that, or do it incorrectly, you can get bogus numbers.
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Old 06-03-15, 03:47 PM
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This article should give you a general idea of what your power number should look like.
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Old 06-03-15, 03:48 PM
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Is this why the Garmin team use srm
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Old 06-03-15, 03:50 PM
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The manual should have instructions for a static zero torque reference procedure, which you could do if you are concerned enough. Personally, if it seemed to operating correctly, I'd trust the meter bit keep on eye on consistency of readings. And really, if it consistently says you're at 250w when you're really at 280w, that you train to the reported numbers is what matters, not that the numbers are unimpeachably accurate. I say git it some time.
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Old 06-03-15, 03:54 PM
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Originally Posted by chaadster View Post
The manual should have instructions for a static zero torque reference procedure, which you could do if you are concerned enough. Personally, if it seemed to operating correctly, I'd trust the meter bit keep on eye on consistency of readings. And really, if it consistently says you're at 250w when you're really at 280w, that you train to the reported numbers is what matters, not that the numbers are unimpeachably accurate. I say git it some time.
Yeah, except OP says the average at 17MPH was 28 watts going uphill. That's undeniably incorrect.
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Old 06-03-15, 03:55 PM
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Originally Posted by cthenn View Post
28 watts is totally wrong. If you're pulling 17mph up a decent hill, that should be around 300 watts, not 28.
Let's say I know the rise and run of that hill; I've also got my time on the segment or my average speed. I could do the math and figure out that I weigh X pounds if I did it with 28 watts, right? So if I ride the same segment, can I compare online power calculators to validate what my meter is saying?
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Old 06-03-15, 04:02 PM
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Originally Posted by Seattle Forrest View Post
Let's say I know the rise and run of that hill; I've also got my time on the segment or my average speed. I could do the math and figure out that I weigh X pounds if I did it with 28 watts, right? So if I ride the same segment, can I compare online power calculators to validate what my meter is saying?
I dunno about all that math stuff, but you should be able to calculate (roughly, at least) your power if you have the necessary numbers.

Is that segment on Strava? You could check that and get a very general idea of what kind of speed = what kind of power.

In any case, I can guarantee you 28 watts is way wrong, you almost have to be freewheeling to only produce 28 watts. What do you define as a "reasonably steep hill"? 17mph is pretty fast, so unless it's like 1 or 2%, you are probably in the 300-350 watt range, depending on the actual grade.
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Old 06-03-15, 04:05 PM
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Originally Posted by chaadster View Post
The manual should have instructions for a static zero torque reference procedure, which you could do if you are concerned enough. Personally, if it seemed to operating correctly, I'd trust the meter bit keep on eye on consistency of readings. And really, if it consistently says you're at 250w when you're really at 280w, that you train to the reported numbers is what matters, not that the numbers are unimpeachably accurate. I say git it some time.
Before every ride I set the pedals at 3 and 6 and tell my computer to zero the power meter. I think that's what you mean by static zero torque? Or do you mean hang a known weight from the pedals?

Accurate watts = accurate calories, in theory, and it plays into other stuff like VO2max. Anyway it's supposed to be within 2 % and there's no reason mine can't get there, so I'd like to make sure it is. I hear what you're saying, consistency is more important than precision for training. I pay for this stuff by making software, so I'm a numbers guy, and want to make sure the pretty charts mean something.
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Old 06-03-15, 04:11 PM
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Originally Posted by cthenn View Post
In any case, I can guarantee you 28 watts is way wrong, you almost have to be freewheeling to only produce 28 watts. What do you define as a "reasonably steep hill"? 17mph is pretty fast, so unless it's like 1 or 2%, you are probably in the 300-350 watt range, depending on the actual grade.
Yeah, 28 w is obviously WAY low, and it's probably working now but after seeing a lot of nonsense I don't trust it, and it's bugging me.

I didn't think of Strava, I've never had a membership with them, but you're right, I've heard people talk about a virtual power feature. I might be able to use that to see if what I'm getting is reasonable. Thanks!!

(The hill has an average grade of 4% but has a stretch of about 8% for a little while. The PM says 70 % of my power came from my left leg that day.)
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Old 06-03-15, 04:20 PM
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Originally Posted by cthenn View Post
Yeah, except OP says the average at 17MPH was 28 watts going uphill. That's undeniably incorrect.
OP already said that issue was fixed with firmware update, etc., so we're past that.
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Old 06-03-15, 04:23 PM
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OK, sorry I did not read the post thoroughly enough. If the low wattage issue is resolved, I missed that part. Yes, wattage numbers can fluctuate wildly. It's funny to shift gears and watch the numbers. If I go from the small to big chainrings, and apply more pressure to get my cadence up, my power will go way up for a brief period. I think part of it is that leg muscles are damn strong, and people don't realize just how easy it is to (at least momentarily) go from 150 watts to 400 watts, without feeling much difference. So unless you are practically a robot, you may see pretty big fluctuations.

Last edited by cthenn; 06-03-15 at 04:29 PM.
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Old 06-03-15, 04:27 PM
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Originally Posted by Seattle Forrest View Post
Before every ride I set the pedals at 3 and 6 and tell my computer to zero the power meter. I think that's what you mean by static zero torque? Or do you mean hang a known weight from the pedals?

Accurate watts = accurate calories, in theory, and it plays into other stuff like VO2max. Anyway it's supposed to be within 2 % and there's no reason mine can't get there, so I'd like to make sure it is. I hear what you're saying, consistency is more important than precision for training. I pay for this stuff by making software, so I'm a numbers guy, and want to make sure the pretty charts mean something.
I meant the "hang weights and take three measurements" thing. I've never done it (for my Powertap hubs), so I can't speak to the procedure in detail.

I certainly understand your lack of trust and concern for accuracy, and as I said, if it matters enough, do the static zero calibration (in addition to normal pre-ride torque zero through head unit).

I imagine you may be able to use a second meter, hub or crank, to compare data over same ride. Dunno that's any easier to pull off than static zero, but at least you'll have an external reference.
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Old 06-03-15, 04:31 PM
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Originally Posted by Seattle Forrest View Post
Yeah, 28 w is obviously WAY low, and it's probably working now but after seeing a lot of nonsense I don't trust it, and it's bugging me.

I didn't think of Strava, I've never had a membership with them, but you're right, I've heard people talk about a virtual power feature. I might be able to use that to see if what I'm getting is reasonable. Thanks!!

(The hill has an average grade of 4% but has a stretch of about 8% for a little while. The PM says 70 % of my power came from my left leg that day.)
No, the virtual power is useless as a reference. Don't even think about it. I can't even comprehend how you'd consider trusting a wild-assed guess over a powermeter?? Sh*t, Strava doesn't even accurately know how fast you were moving (unless you're feeding in wheel speed sensor data).
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Old 06-03-15, 07:19 PM
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Borrow a power tap wheel
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Old 06-03-15, 07:32 PM
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Strava compares pretty well with my pm on steady grades and no wind if I've entered weight accurately.

You can also ride steady on your trainer and compare to its published power curve.
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Old 06-03-15, 07:39 PM
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Originally Posted by chaadster View Post
I meant the "hang weights and take three measurements" thing. I've never done it (for my Powertap hubs), so I can't speak to the procedure in detail.
.
Hanging weights is painful. I prefer to torque my nuts. Take three measurements and Powertap away.
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Old 06-03-15, 08:50 PM
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Originally Posted by f4rrest View Post
Strava compares pretty well with my pm on steady grades and no wind if I've entered weight accurately.

You can also ride steady on your trainer and compare to its published power curve.
My point is that there is far less cause to trust Strava estimates than a powermeter's readings.
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Old 06-03-15, 10:16 PM
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Strava will provide reasonable estimates if the hill is steep enough and the data you enter for you weight is accurate. If you can find a 3-5 min hill then you can calculate the average power based on your time up the hill and the elevation gained. The steeper the hill (and slower the ascent) the more accurate the results will be.

Best would be to borrow someone's powertap. I always thought Vector pedals were better for someone who already had a powermeter.
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Old 06-03-15, 10:19 PM
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Can you do a static calibration check with Vector pedals? I didn't know that.

People often state that you don't need accuracy, all you need is consistency. You're seeing right now why accuracy can be useful -- you can tell when the power meter is off.

If you can't do a static calibration check your other two options are to borrow a known good PM, or else to do a dynamic check. Both are kind of hassles.

The most important characteristic of a power meter is the quality of the data it produces. Everything else is an optional feature. Being able to check the accuracy of your power meter is an important step in determining data quality. If you can't easily check it, you're unlikely to do it very often (or at all).
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