I'm going to use this as an opportunity to explain why it makes little sense to fret about whether one's power meter is "accurate." I'm not saying you're doing this, Savechief; I think you're asking good questions. While I think the answers are interesting, they all point to the fact that all the strain-gage-based power meters currently on the market are just fine for training (even for pros). A meter that's more accurate than average is only really useful if you're running an exercise physiology lab or (more likely) using the Chung method to gauge aero gains without a wind tunnel.
Here's the tl;dr version:
- There's way more uncertainty in power measurements than manufacturers admit.
- Most people who know power well (e.g., athletes, coaches, marketers employed by power meter companies, even Ray Maker) misunderstand power meter accuracy and error.
- It turns out that they don't need to understand those things, because accuracy and error matter a whole lot less than consistency.
- Nearly all power meters return consistent results as long as their offset is set properly (i.e., they're zeroed regularly).
- The variation in meter-on-meter plots comes largely from algorithms in the power meter and in the bike computer that writes out the power file.
- Don't worry about accuracy. You can't validate manufacturers' claims, and besides, the meters are accurate enough. Buy based on features, price, service or whatever else matters to you.
- Don't forget to set your offset (zero your meter).
Manufacturers tend to quote a single vague accuracy number, and that doesn't tell you much. No one mentions this to consumers, but accuracy is usually quoted as a percentage of full scale. "Full scale" basically means the largest reading the meter is rated to read. For a power meter, that might be 2000 watts. If your meter claims ±1%, that's ±20 watts
no matter the current reading. That means if you're spinning along at a true 100 watts, your meter might read anywhere between 80 watts-120 watts. So accuracy that's ±1% of full scale can easily be ±20% of reading at low power outputs.
That sounds really bad, and it is. The assumption in the metrology field is that accuracies are quoted as %FS (percent of full scale) unless they're specifically claimed to be %R (percent of reading). So according to the manufacturers' claims, all power meters are pretty inaccurate at low power outputs.
But in reality, their strain gages are accurate to something like ±0.05%-±0.25% of full scale. If full scale is 2000 watts, that's more like ±5 watts in the worst case. But there's additional uncertainty added by thermal effects, hysteresis, material variations and other factors. That's why most instrument manufacturers quote accuracy as the sum of a small full-scale error plus a larger reading error. That lets you calculate the accuracy of your meter at any point in its operating range.
Reporting accuracy this way is mostly done when scientists design meters for other scientists. If SRM or Saris reported their accuracy this way, it would just confuse consumers. As a matter of fact, Stages reports their error as ±2% of reading. That's almost certainly a "dumbed-down" version of reality, but it shows that they know the difference between %FS and %R. Basically, power meter manufacturers fudge an accuracy number that's understandable for their target market. That's not necessarily a problem, either.
It can be taken too far, though. When Elite/Drivo asserts that their optical system has no use for thermal compensation, it's a triumph of marketing over science. They claim that they're not using strain gages, but of course they are. Theirs are optical rather than resistive: they're using optical occlusion to detect a phase shift* due to torsional strain in a shaft. It's true that this method is less sensitive to thermal effects than the more-common resistive strain gages. But temperature still matters a lot, and it's clear that whoever talks to the press for Elite doesn't have a great technical understanding of their device.*
The marketing BS spins out of control when Elite presents its fake "certificate" to Ray Maker. I don't doubt that they had their trainer tested by the lab they mention, and I don't doubt that it tested quite well. But if you read the "certificate," it's clear that it wasn't written by the German lab but rather by one of Elite's marketing staff, perhaps a 15-year-old intern. I mean, the tone is absurd. No credible lab anywhere is going to issue a report saying:
some adolescent Elite marketing person wrote:The current DRIVO [and all future DRIVO models] can be used as references for other powermeter systems in the market. ELITE DRIVOs are the new reference for precision power measurements.The question "who is right" is answered by ELITE today.
(Emphasis added). Seriously, that's what it says. See for yourself here:
https://media.dcrainmaker.com/images/20 ... mage12.pngWhoever wrote this "certificate" could just as well have added "Giulio Bertolo [Elite's CEO] is both handsome and extraordinarily well-endowed."
I've had several of my devices certified by third-party labs, and coincidentally, it's usually a German one: TÜV Rheinland. The certs we get back from labs just attest that our product is claimed to meet XXX standard and, after performing X Y and Z tests, the lab attests that our product meets XXX standard. They're really boring. They'd never make claims about future models or weigh in on what instrument is "the new reference" in my field.
In the comments section of Ray Maker's article, someone named Robert (comment #40) picks up on the fact that that Elite's accuracy claim is incoherent and that the "certificate" is meaningless. He's a lot more reserved about it than I am, though.
It's telling that Ray Maker published that "certificate" on his web site, though he immediately explains that he's more interested in evaluating the Drivo himself. Maker is clearly a very smart guy, but he doesn't have a hard-science background at all. That's not a criticism, either. He does what he claims to (write about and evaluate power meters) very, very well. But quantifying error and accuracy for instruments like this is really hard...undergrad engineering curricula often don't address it, and I only got a little exposure to it in grad school. It wasn't until I started working in the metrology field that I wrapped my brain around it to any degree. It's not Ray Maker's fault that he doesn't know this stuff; it's esoteric. And I understand that, to his credit, Maker consults with Tom Anhalt and Robert Chung when getting into the details of data analysis. I'm not sure what more anyone could ask of him.
The point is that unless you're designing power meters, it doesn't matter whether you understand accuracy and error. There are two reasons why:
- While powermeter accuracy numbers are fudged and somewhat made-up, most makers quote numbers that represent realistic error levels for a broad range of power inputs. The specs are technically wrong, but in a broader sense, they're basically right.
- There are so many factors that affect power readings (altitude, fitness, muscle fatigue, injury, time discretization, meter algorithms, head-unit algorithms, etc.) that the raw error/accuracy of the meter just fades into the noise. Meters don't need great accuracy to be fully valuable training tools; they just need to be consistent. And all of the strain-gage-based meters I've seen (including left-side-only ones) meet the consistency criterion.
I'm defining "consistency" as "pretty precise" and "somewhat accurate." A "pretty precise" meter will give essentially the same output under the same conditions every time. It may be off by a bit (like a left-side-only meter for someone with a large L/R imbalance) but it's off by the same amount every time. A "somewhat accurate" meter will be off from the "true" value, but maybe only 7-10 watts at your FTP.
And you can get that consistency from nearly any power meter on the market. Stages meters do just fine. People have complained that "they only capture half the data," but that's not true; they capture most of the data. Most people's dominant limbs are a little stronger than their non-dominant ones, but so what? If, at FTP, your left leg puts out 150 watts and your right leg puts out 160 watts, you'll think your FTP is 300 watts instead of the true 310 watts. So what? As athletes, we're interested in relative improvement--how our speed now compares to our speed before--and not our absolute power output. For all intents and purposes, a one-sided Stages will capture a 5% improvement in FTP just as accurately a Quarq or a Powertap will. Your one-sided meter will report that your FTP went from 300 to 315 watts, while a Powertap hub, capturing both sides, would report an increase from 310 watts to 325.5 watts.
But wait! A Powertap hub measures power after the drivetrain, so it reads low compared to a crank-based meter. Reasonably well-maintained bike drivetrains are around 96% efficient, so if your "true" FTP is 310 watts, your Powertap will suggest that your FTP is 96% of that, or just under 298 watts. That's lower than the supposedly-awful left-side-only Stages at 300 watts. If you reject one-sided power meters, you should also reject hub-based power meters, because in most cases, the discrepancy in reported power is about the same. One might counter that "at least with the Powertap you know what you're not measuring." Well, sort of, but drivetrain losses vary with chain tension, gear selection and lubrication condition. So you don't really know; you've just got a rough idea. My point is that no matter how you measure, as long as your rough idea is consistent, you're producing perfectly useful data.
When you're talking about high-quality power meters, there's no point to claiming that one is useless while the other is not. Comparing data traces, while good fun, often comes down to the truism, "A man with a watch knows what time it is. A man with two watches is never sure."
With rare exceptions, SRMs, Powertaps and Stages power meters are equally useful as training tools. If you're in the market for a power meter, choose the one that fits your bike, budget and sensibilities best. You'll get the most useful results if you frequently set the offset/zero, and a $500 power meter that's properly zeroed will give much more useful, repeatable results than a $2000 meter that gets zeroed once a month.
* And since they're detecting phase shift, they'd better not be doing so in the time domain. Detecting phase differences is considerably more accurate in the frequency domain, though shifting from the time domain to the frequency domain requires some fancy math in the form of a fast Fourier transform (FFT). That's a computationally intensive operation best done with an FPGA or ASIC, which are specialized computer chips that can be dedicated to a single set of operations. They're not cheap, though.
** If we assume a 50°C operating range for the Drivo meter's internals, then its accuracy varies over an extra 2% range simply because for human-friendly temperatures, every 50°C rise in temperature makes chrome-moly steel 2% more flexible. So Elite could considerably improve the accuracy of their meter just by implementing temperature compensation. They seem to make a pretty accurate power meter, and it's fair to say that it's considerably less temperature-sensitive than resistive-strain-gage-based meters. But the statement Elite makes in Ray Maker's article--that thermal compensation doesn't apply to their technology--is just plain wrong. Again, I expect their marketing department's braggadocio is no reflection on the accuracy of the Drivo power meter.