Stages Or P2M Power Meters?

janesy

Ordered on the 23rd of September and arrived on the Friday 10th Oct

slowbike

janesy wrote:

I don't think my opinion is worthless. Its my opinion. Its not worthless to me. Aka my opinion.
Also I don't think the only reason people get a non stages because they have incompatible cranks.
Thats stupid.

I wanted the P2Max due to:
Good reviews
left/right balance
Accuracy.
Build quality etc
and of course fit. But then again I could have put a stages on my other bikes.

That's better - now your opinion has some value!

meanredspider

janesy wrote:

Ordered on the 23rd of September and arrived on the Friday 10th Oct

I got to the point where I ordered a P2M at but delivery, quoted at 2 week was confirmed at 4 weeks after I placed the order. Finally tipped me to Stages (I only had 3 months to train for my event) and it arrived the next day from Merlin. And the Stages crank only weighed 10g more than the crank it replaced.

janesy wrote:

I wanted the P2Max due to:
....
Accuracy.
....

"Accuracy" is an interesting question. I haven't yet seen a review that doesn't just compare meter readings (like the generally excellent DC Rainmaker reviews) and none that reviews versus some calibrated "Gold Standard". Is there a review that reviews the accuracy against a calibrated dynamometer? I know, from my years working for an in vitro diagnostic medical device company, that comparing results between products is no measure of accuracy - just of "agreement" (which might be useful if you use multiple meters). To be sure of real accuracy (not something, if I'm honest, that bothers me - I just need precision (repeatability) ) you have to test against a known, fully calibrated, gold standard dynamometer.

ETA - Based upon my medical device experience, I wouldn't be surprised if the manufacturers built in a small positive bias. Why? Because, if two meters disagree, it's human nature to believe the one that gives you the "better" result (I've seen thousands if not tens of thousands of data points to confirm this phenomena). Anyhow, unless you have a calibration certificate (which would be pretty much void if you then do a calibration yourself other than a zero reset), I'd not rely on your meter being accurate.

shockedsoshocked

@meanredspider.

In my correspondence with Power2Max they said they used a Torque Meter (this is the exact link they sent me http://www.hbm.com/de/menu/produkte/auf ... ent/t20wn/ ) so I do believe they are trying to calibrate their products to the highest standard. They couldn't go into further detail with me as they were entering propriety information territory, but the meter is DIN certified.

xavierdisley

Ergomos came with a calibration certificate too. It's a shame that people really don't understand the situations where the reported power from a Stages/Rotor/Garmin left only power meter becomes less accurate even after repeated discussion on here and elsewhere, and only look at price and weight. It's a power meter, not a stem!

meanredspider

ShockedSoShocked wrote:

@meanredspider.

In my correspondence with Power2Max they said they used a Torque Meter (this is the exact link they sent me http://www.hbm.com/de/menu/produkte/auf ... ent/t20wn/ ) so I do believe they are trying to calibrate their products to the highest standard. They couldn't go into further detail with me as they were entering propriety information territory, but the meter is DIN certified.

I don't doubt they all test/calibrate their products to some standard or other. My question was more about seeing an independent review where various meters are tested against a known gold standard (across a range of power, rpm, temperature etc). Just because two or three meters agree with each other, isn't a measure of accuracy although it's easy to mistake for accuracy (I've seen this in highly-regulated medical devices). I'm not having a dig at any particular meter either - just flagging that you need to be careful in what you read.

It's not a big deal as I don't much care about power meters being accurate compared with being precise.

meanredspider

xavierdisley wrote:

Ergomos came with a calibration certificate too. It's a shame that people really don't understand the situations where the reported power from a Stages/Rotor/Garmin left only power meter becomes less accurate even after repeated discussion on here and elsewhere, and only look at price and weight. It's a power meter, not a stem!

Why is accuracy important to most people? I don't believe it is. I'd be interested to see the calibration certificate too to see if it's worth the paper it's written on.

Don't mistake what I'm writing for an argument that left-only meters being more accurate that left and right meters. I'm not saying that at all. Frankly I don't care about accuracy. I'm very happy to measure just my left leg because I believe I get most of what I need from that information. What I don't get I don't need or can't use.

What I am saying is that, until you've seen an independent study of accuracy against a gold standard across a range of conditions, you're just believing the manufacturer.

iPete

After following all this Stages vs P2M, left leg only, 4iii cometh, blah blah etc. I've purchased a bargain NOS Powertap.

Can't wait to see how useless I am

xavierdisley

Why is accuracy important to most people? I don't believe it is ... What I don't get I don't need or can't use.

If you're spending hundreds/thousands of pounds on a power meter, accuracy should be absolutely top of the list, as otherwise it just becomes as much use a glorified speedo! If you had a speed sensor that at times read an incorrect speed, heart rate monitor that spiked, cadence sensor that dropped out you'd buy a different one, especially if they were ten times the price.

What I am saying is that, until you've seen an independent study of accuracy against a gold standard across a range of conditions, you're just believing the manufacturer.

Absolutely - but you can check these things yourself, either dynamically or statically.

shockedsoshocked

meanredspider wrote:

ShockedSoShocked wrote:

@meanredspider.

In my correspondence with Power2Max they said they used a Torque Meter (this is the exact link they sent me http://www.hbm.com/de/menu/produkte/auf ... ent/t20wn/ ) so I do believe they are trying to calibrate their products to the highest standard. They couldn't go into further detail with me as they were entering propriety information territory, but the meter is DIN certified.

I don't doubt they all test/calibrate their products to some standard or other. My question was more about seeing an independent review where various meters are tested against a known gold standard (across a range of power, rpm, temperature etc). Just because two or three meters agree with each other, isn't a measure of accuracy although it's easy to mistake for accuracy (I've seen this in highly-regulated medical devices). I'm not having a dig at any particular meter either - just flagging that you need to be careful in what you read.

It's not a big deal as I don't much care about power meters being accurate compared with being precise.

I agree, there's very little literature comparing devices against the Gold Standard and I'm unsure as to why that is. You'd think with the burgeoning market in portable devices there'd be a greater interest. In my opinion there should be a uniform method of calibration for devices. In lieu of that i'd personally prefer a device that has been calibrated using a certified torque meter.

meanredspider

xavierdisley wrote:

Why is accuracy important to most people? I don't believe it is ... What I don't get I don't need or can't use.

If you're spending hundreds/thousands of pounds on a power meter, accuracy should be absolutely top of the list, as otherwise it just becomes as much use a glorified speedo! If you had a speed sensor that at times read an incorrect speed, heart rate monitor that spiked, cadence sensor that dropped out you'd buy a different one, especially if they were ten times the price.

I may be wrong but I think you're mistaking accuracy with precision. Accuracy is about the average result meeting the real result. Precision is repeatability - getting the same result for the same input over and over again. An accurate rifle spreads bullets around the bull. A precise rifle groups the shots in a tight cluster (maybe missing the bull completely).

Personally, I don't think accuracy is that important for a PM. Whether the meter tells you any particular number (whether that's Watts, horsepower, or whatever) I can't see makes much difference. What I believe most people want to know is whether than number is bigger or smaller or the same. You don't want spread - you want to know that, exactly the same effort today as yesterday, you'll get the same result - that's precision.

The only purpose of accuracy is to compare yourself with someone else or something else external. Apart from pub bragging rights or (and I doubt anybody much cares enough to this degree of accuracy) energy use for nutrition or whatever, I don't see what use an accurate meter reading is (to most people). A precise reading, on the other hand, can be used to compare your aero positions, FTP evolution etc etc I think that's more important.

slowbike

meanredspider wrote:

xavierdisley wrote:

If you're spending hundreds/thousands of pounds on a power meter, accuracy should be absolutely top of the list, as otherwise it just becomes as much use a glorified speedo!

I may be wrong but I think you're mistaking accuracy with precision. Accuracy is about the average result meeting the real result. Precision is repeatability - getting the same result for the same input over and over again. An accurate rifle spreads bullets around the bull. A precise rifle groups the shots in a tight cluster (maybe missing the bull completely).

Personally, I don't think accuracy is that important for a PM. Whether the meter tells you any particular number (whether that's Watts, horsepower, or whatever) I can't see makes much difference. What I believe most people want to know is whether than number is bigger or smaller or the same. You don't want spread - you want to know that, exactly the same effort today as yesterday, you'll get the same result - that's precision.

The only purpose of accuracy is to compare yourself with someone else or something else external. Apart from pub bragging rights or (and I doubt anybody much cares enough to this degree of accuracy) energy use for nutrition or whatever, I don't see what use an accurate meter reading is (to most people). A precise reading, on the other hand, can be used to compare your aero positions, FTP evolution etc etc I think that's more important.

Exactly ...

All this fuss over one legged readings and varying balance over time/fatigue - unless you're looking to change that then it's immaterial - what's important is your overall power output and how that compared to previous readings.

I got a power meter this summer and in conversation with a friend it turns out that my power reading for a 10 is the same as his on a turbo - he is faster than me (by a few minutes) - But, what does that mean? Is his turbo underreading or my crank based PM over reading? Or is it that we're reading input/output power and there is signficant loss in the drive train? Perhaps it's because our CdA is different?

We will (at some point) do a direct comparison by putting my bike on his turbo and comparing the outputs - but it doesn't matter as both our methods of recording are repeatable and we can use our own numbers to evaluate our own training plans.

xavierdisley

meanredspider wrote:

Personally, I don't think accuracy is that important for a PM. Whether the meter tells you any particular number (whether that's Watts, horsepower, or whatever) I can't see makes much difference. What I believe most people want to know is whether than number is bigger or smaller or the same. You don't want spread - you want to know that, exactly the same effort today as yesterday, you'll get the same result - that's precision.

They're not precise either. The left leg contribution to total power output is not a constant, no matter how much people want or expect it to be. So a Stages/Rotor/Garmin left only power meter will not report the same value from day to day, or even within a session because of L/R imbalances due to fatigue, position, cadence, effort etc.

My argument is that if you're spending >£500 on a power meter, then you should be paying for something that will give you repeatable, accurate results. If a left only power meter was £200 then I would be more in favour for some people, but they're typically >3x that price. I have coached riders with Stages (or just heart rate/RPE) but you can see inconsistencies in the data.

xavierdisley

Slowbike wrote:

All this fuss over one legged readings and varying balance over time/fatigue - unless you're looking to change that then it's immaterial - what's important is your overall power output and how that compared to previous readings.

And a left only power meter is less comparable day to day.

I got a power meter this summer and in conversation with a friend it turns out that my power reading for a 10 is the same as his on a turbo - he is faster than me (by a few minutes) - But, what does that mean? Is his turbo underreading or my crank based PM over reading? Or is it that we're reading input/output power and there is signficant loss in the drive train? Perhaps it's because our CdA is different?

It could be due to a number of factors (CdA is the most likely).

meanredspider

xavierdisley wrote:

They're not precise either. The left leg contribution to total power output is not a constant, no matter how much people want or expect it to be. So a Stages/Rotor/Garmin left only power meter will not report the same value from day to day, or even within a session because of L/R imbalances due to fatigue, position, cadence, effort etc.

My argument is that if you're spending >£500 on a power meter, then you should be paying for something that will give you repeatable, accurate results. If a left only power meter was £200 then I would be more in favour for some people, but they're typically >3x that price. I have coached riders with Stages (or just heart rate/RPE) but you can see inconsistencies in the data.

I think you're now confusing variability of the input with precision of the device. Output in general will vary with fatigue, position, cadence, effort etc whether it's one leg or not. For what it's worth (not a lot as N=1, I've found the Stages very consistent (NL with its long straight flat rides would show up variability - especially on calm days. In fact, on windy days, direction is the only variable that shows up in the results).

I bought the Stages because it was the most convenient. I'd actually ordered the Vector (L&R) and the P2M until, with both, I found issues with my application. Vector wouldn't actually fit on my SL-K cranks and P2M (apart from the 4 week delivery) would have meant swapping chain rings between bikes too. The Stages has done exactly the job I wanted from it. The price is the price. Given how popular they are, you can't argue that they're too expensive. If anything, they've driven down the prices of all other meters.

Alex_Simmons/RST

meanredspider wrote:

xavierdisley wrote:

They're not precise either. The left leg contribution to total power output is not a constant, no matter how much people want or expect it to be. So a Stages/Rotor/Garmin left only power meter will not report the same value from day to day, or even within a session because of L/R imbalances due to fatigue, position, cadence, effort etc.

My argument is that if you're spending >£500 on a power meter, then you should be paying for something that will give you repeatable, accurate results. If a left only power meter was £200 then I would be more in favour for some people, but they're typically >3x that price. I have coached riders with Stages (or just heart rate/RPE) but you can see inconsistencies in the data.

I think you're now confusing variability of the input with precision of the device.

Since the Stages uses an algorithm that makes an assumption about the input, i.e. that power = 2 x left crank power, then it is very much a matter of precision of the device.

It may well be quite precise at measuring left crank power, but that's not what it's reporting. The algorithm used is a key factor in the device's precision.

Alex_Simmons/RST

Slowbike wrote:

We will (at some point) do a direct comparison by putting my bike on his turbo and comparing the outputs - but it doesn't matter as both our methods of recording are repeatable and we can use our own numbers to evaluate our own training plans.

Keep in mind that comparisons on a turbo are probably the lowest hurdle for testing relative performance of two power measuring devices.

slowbike

xavierdisley wrote:

meanredspider wrote:

Personally, I don't think accuracy is that important for a PM. Whether the meter tells you any particular number (whether that's Watts, horsepower, or whatever) I can't see makes much difference. What I believe most people want to know is whether than number is bigger or smaller or the same. You don't want spread - you want to know that, exactly the same effort today as yesterday, you'll get the same result - that's precision.

They're not precise either. The left leg contribution to total power output is not a constant, no matter how much people want or expect it to be. So a Stages/Rotor/Garmin left only power meter will not report the same value from day to day, or even within a session because of L/R imbalances due to fatigue, position, cadence, effort etc.

It is precise - in that it is always measuring the left leg and giving a result based on that.
I won't argue that it's easy to fool the reading - just stop pedalling so hard on the right leg and start pulling up on the left and the power reading goes right up, but the absolute number is immaterial, it's measuring the power output on the left leg - it has to assume you're doing equal work on the right.
I'd argue that for the average MAMIL this is more than adequate - however if you're an elite athlete then you should be looking at these things in far more detail..

xavierdisley wrote:

My argument is that if you're spending >£500 on a power meter, then you should be paying for something that will give you repeatable, accurate results. If a left only power meter was £200 then I would be more in favour for some people, but they're typically >3x that price. I have coached riders with Stages (or just heart rate/RPE) but you can see inconsistencies in the data.

But, as I understand it there are inconsistencies in most power meters - some need recalibrating during a ride ... and they're not cheap!
I'd love it to be cheaper and it's hard to argue that P2M isn't a better tool - however, like MRS I went for the Stages for the portability - yes, you can swap the P2M over with compatible bottom brackets, but the issue comes in having different chainrings on different bikes - I have a compact on the road bike and 39/53 on the TT bike - swapping over the Stages is quick and easy, swapping a P2M on that would be more time consuming.
Alternatively, go for a wheel based PM - Yes, you can swap it between bikes, but you're restricted to the one wheel - a fine solution for some, but not all.

meanredspider

Alex_Simmons/RST wrote:

meanredspider wrote:

xavierdisley wrote:

They're not precise either. The left leg contribution to total power output is not a constant, no matter how much people want or expect it to be. So a Stages/Rotor/Garmin left only power meter will not report the same value from day to day, or even within a session because of L/R imbalances due to fatigue, position, cadence, effort etc.

My argument is that if you're spending >£500 on a power meter, then you should be paying for something that will give you repeatable, accurate results. If a left only power meter was £200 then I would be more in favour for some people, but they're typically >3x that price. I have coached riders with Stages (or just heart rate/RPE) but you can see inconsistencies in the data.

I think you're now confusing variability of the input with precision of the device.

Since the Stages uses an algorithm that makes an assumption about the input, i.e. that power = 2 x left crank power, then it is very much a matter of precision of the device.

It may well be quite precise at measuring left crank power, but that's not what it's reporting. The algorithm used is a key factor in the device's precision.

Except that I know (like most/all users) that it's just measuring my left leg. I'm not sure that it actually affects the precision of the device. In fact one could possibly argue that devices like the two pedal Vector are likely to be less precise partly due to the inherent additional variables from two different measurements and partly because the torque value seems critical (2 torque values - one for each pedal). Temperature is probably one of the greatest threats to precision - I believe this was something Stages did pretty well. I think the others have since improved too but it's a minefield - one I'm not sure people understand fully. I did my final year engineering degree project on cutting tool failure prediction. That was a strain gauge array on a tool that's very similar to a crank (it was a very long time ago) but, more recently (the last 10 years in fact), I've been heavily involved in medical devices (design and manufacture) from a quality engineering perspective. Precision, accuracy, calibration, bias, offset, validation etc etc were what my days were all about.

BeaconRuth

meanredspider wrote:

I may be wrong but I think you're mistaking accuracy with precision. Accuracy is about the average result meeting the real result. Precision is repeatability - getting the same result for the same input over and over again...

Personally, I don't think accuracy is that important for a PM. Whether the meter tells you any particular number (whether that's Watts, horsepower, or whatever) I can't see makes much difference. What I believe most people want to know is whether than number is bigger or smaller or the same. You don't want spread - you want to know that, exactly the same effort today as yesterday, you'll get the same result - that's precision.

Sorry, meanredspider, but you are wrong. Precision has nothing to do with coming up with the same result time and time again. Precision is to do with the resolution of the readings, not their reliability.

A very precise reading would be 321.157W
A very inprecise reading of the same would be 320W

The terms I think you're searching for are 'validity' and 'reliability'. The Stages is very valid if you accept it's only measuring left leg. It's not very valid if you take it as a measure of your total power output. Xavier is suggesting the Stages isn't very reliable either.

Ruth

meanredspider

BeaconRuth wrote:

meanredspider wrote:

I may be wrong but I think you're mistaking accuracy with precision. Accuracy is about the average result meeting the real result. Precision is repeatability - getting the same result for the same input over and over again...

Personally, I don't think accuracy is that important for a PM. Whether the meter tells you any particular number (whether that's Watts, horsepower, or whatever) I can't see makes much difference. What I believe most people want to know is whether than number is bigger or smaller or the same. You don't want spread - you want to know that, exactly the same effort today as yesterday, you'll get the same result - that's precision.

Sorry, meanredspider, but you are wrong. Precision has nothing to do with coming up with the same result time and time again. Precision is to do with the resolution of the readings, not their reliability.

A very precise reading would be 321.157W
A very inprecise reading of the same would be 320W

The terms I think you're searching for are 'validity' and 'reliability'. The Stages is very valid if you accept it's only measuring left leg. It's not very valid if you take it as a measure of your total power output. Xavier is suggesting the Stages isn't very reliable either.

Ruth

Hi Ruth - I'm absolutely sure I'm right in the terms that I'm using. Maybe you work in a completely different field in which they have very different definitions. I grabbed this off Wiki (for convenience) but Google Accuracy vs Precision to see loads of example

In the fields of science, engineering, industry, and statistics, the accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantity's actual (true) value.[1] The precision of a measurement system, related to reproducibility and repeatability, is the degree to which repeated measurements under unchanged conditions show the same results.

Grill

It isn't reliable. Look at all the water ingress and battery issues. There are multiple threads on pretty much all forums that show the issues aren't isolated.

Buy cheap, buy twice. Considering you can get Quarq and P2M for under 1k, there isn't really any reason that someone serious about their power would use Stages. The way I see it, Stages is a niche product made for the sportive crowd.

meanredspider

Grill wrote:

It isn't reliable. Look at all the water ingress and battery issues. There are multiple threads on pretty much all forums that show the issues aren't isolated.

Old news, I think - I've not seen any issues of this type on the updated versions. In the same way that some other meters had no temperature correction on earlier models.

slowbike

Grill wrote:

Buy cheap, buy twice.

One for each bike ... good idea ...

RChung

meanredspider wrote:

What I am saying is that, until you've seen an independent study of accuracy against a gold standard across a range of conditions, you're just believing the manufacturer.

I've seen independent studies of accuracy against a gold standard. I've also done independent studies of accuracy against a gold standard. It's not that hard if you know what you're doing.

As I said above, whether accuracy matters depends on what your needs are, and what you intend to do with the data. Training for long endurance rides doesn't put particularly high demands on the data so if that's all you're doing (and many people are like you) then accuracy may not matter much. However, it would be wrong to think that if accuracy doesn't matter for your needs then it must not matter at all for anyone else's.

meanredspider

Interesting, RC - what gold standard of dynamometer did you use? Is there a published study?

BTW, I've been very careful not to claim what suits me suits everybody (I see that far too often on here). I'm interested to learn where accuracy (absolute power in Watts) is really useful.

Grill

meanredspider wrote:

Interesting, RC - what gold standard of dynamometer did you use? Is there a published study?

BTW, I've been very careful not to claim what suits me suits everybody (I see that far too often on here). I'm interested to learn where accuracy (absolute power in Watts) is really useful.

Keep in mind that there's a lot of data out there unpublished because why release your numbers and watch others reap the benefits when you're the one who put the time and money into it. It's rare to see power data from top amateurs as well as pros and rarer still to get a glimpse at their training plan. This holds truer still with aero data.

Alex_Simmons/RST

meanredspider wrote:

BTW, I've been very careful not to claim what suits me suits everybody (I see that far too often on here). I'm interested to learn where accuracy (absolute power in Watts) is really useful.

It's a reasonable question. Keep in mind that if your data is predictably consistent, then accuracy is only a calibration setting away (which BTW is not possible on either a Stages or a P2M - you are reliant on factory calibration being correct after it's installed and used in your specific situation).

The trouble with discussing the accuracy of power data is that it isn't a simple issue, and the nuances are lost on many. It will vary depending on the application and circumstances of use. Heck even the choice of head unit and software can affect the quality of the data, not to mention user error. For some uses many meters will appear to be equally accurate (or show a consistent predictable bias), but under other usage conditions they won't.

As for the comment on meters with separate left and right measurement gauges, yep that's right, you'd need to know both sides were accurate, although even if there was some inherit bias between sides, that data still tells you more than a blanket "left x 2" equation would (but it requires some mental skill and knowledge to recognise that).

Maximal power-duration profiles and any modelling on it and metrics that flow from that are dependent on consistent quality power data across the duration spectrum (GIGO principle). This is fundamental to the next wave of understanding power data that is beginning to emerge and how it can help make better informed choices about training interventions and how one is responding to training.

You can forget about any sort of peak power / peak pedal force & speed assessment or analysis with single side only measurement. But that's not just a singled sided meter issue. Very few meters and online or magazine testing reports tell us how meters perform under such circumstances. Most meters just don't have the temporal resolution necessary.

Aero testing beyond the obvious very low hanging fruit won't really be reliable. When you are teasing out small changes, well you need a high level of consistency with data and need to eliminate or accurately measure as many uncontrolled variables that matter as you can. It might be that with large quantities of data you can still assess such things as there are ways to make sense of less than perfect data if you have enough of it, but I suspect the effort involved in both collecting enough data and analysing it makes it impractical for the purpose. Aero testing takes long enough with high quality data (whether it's in a tunnel or field testing).

When fitness is at or approaching peak levels, then we are looking at assessing small changes, both within season and season to season, as well as parse out the physiological nature of those changes. There is too much variance with single sided measurement to rely on it for that purpose.

Eventually you'll quite possibly change meter. You'd like to think the old data was comparable with new data.

But it may well be that such things are not important to some, while they are far more important for others.

I make no judgement, and think the Stages is a suitable choice for some riders, while it's not for others. I just suggest that people understand what they would like to do with the data, both now and into the future when making their choice.

Of course there are a host of other factors as well which I won't go into here, I cover those in my blog item Which Power Meter?

RChung

meanredspider wrote:

Interesting, RC - what gold standard of dynamometer did you use? Is there a published study?

If you know what you're doing you actually don't need a dynamometer. A dynamometer is much handier and faster but the real gold standard is just a known and verified mass. I'm lucky enough to work at a university where guys in the physics lab can check regular barbell weights against NIST standards to well within a gram or so. For these purposes a gram is plenty close enough. With known masses you can do either static or dynamic checks, as Xav mentioned above.

You pointed to DC Rainmaker's reviews. He doesn't often mention it but he does do periodic static torque checks on his PT and Quarq. Most guys who care about the quality of their data will want to do periodic torque checks. The old Polar chain sensor power meter, the old Ergomo optical BB power meter, and the iBike can't be checked that way, so they have to be checked in more complicated fashion. The left-side torque of the Stages can be checked but (obviously) not the right side.

If you check pubmed you can find published studies comparing the accuracy of power meters. These are old studies because, frankly, they're not publishable nowadays. People who care just check the accuracy themselves.

BTW, I've been very careful not to claim what suits me suits everybody (I see that far too often on here). I'm interested to learn where accuracy (absolute power in Watts) is really useful.

I mentioned some examples above. Alex mentioned some examples above. There are some others that I can't discuss but which rely on absolute not just relative accuracy.

meanredspider

@Alex - thanks for this thorough response. I think you've misunderstood my question though. To be very clear, I FULLY understand the limitations of the Stages (and other one-sided meters) - my contention is that my needs are simple and the Stages meets those needs better than any other meter on the market. To use an analogy: I want a can opener not a food processor. Several people insist than a can opener is useless and you can't whisk eggs with a can opener but if all you want to do is open cans, a food processor is pointless, expensive and/or cumbersome. It won't fit in my cutlery drawer. I know you've understood that. There are some people who haven't.

So let's please park Stages (and one sided meters in general).

At the risk of appearing stupid, I read what you wrote and understood that most of it could be met with a meter with high precision - nearly everything you talked of was looking for small changes (aero, for instance). What I haven't understood (and I'll read your post and Robert's posts again) is when accuracy is important: when it's important to know that the power measured in absolute terms is 310W (rather than xxxW+2 ie 2W better than the last measurement). Anyhow, I'll have another read later.

@Robert - thanks, too. Yup, for sure you can do various static checks on the strain gauges. Maybe I'm making it more difficult than it needs to be for a PM but the type of test you talk of simple wouldn't cut it the medical device world even for the simplest things. All you're really doing is a static calibration of the strain gauge array. What I'd like to see, as I said above, is how the various systems (including comms and head units as Alex mentions) perform in a range of conditions of load, cadence, changing temperature (humidity and air pressure would be good), L vs R loading etc As Xav, I think, said - if you're paying a lot of money for an expensive system, you need to know that it's coping with all of this. Ray found in one of his tests that a couple of the meters drifted horribly when the conditions changed. I'd like to know that for my day in the Alps when the temperature started at 0C peaked at 33C and dropped back to 10C my PM was still telling me the truth, for instance.