Certainly more accurate than most other ergs (that's how we rowers call them). However, it's not perfect. Nothing is ever perfect when it comes to measuring real world biomechanics. I did competitive rowing in my youth + I've spent a couple years building rowing technology, sensors + telemetry, so I can maybe offer a few interesting insights about their issues. Don't get me wrong, C2 ergs are great craftsmanship, built like tanks and I've spent a few hundreds hours on these machines (blaming the weather gods and the winter season).
1) The PM2 units (the old LCD ones, stock on Model C ergs) were particularly bad, suffering from some well known "issues". The sensors in them had not a high enough sample rate + high hysteresis in the sensors. That means that "pulling hard" at the catch (for non-on-the-water rowers: the beginning of a stroke cycle, when your hands are closest to the flywheel) and then backing off the handle force rewards you with higher measured power than actual.
2) This became a lot better with the PM4 generation (stock on Model D and up).
However they still have another issue. The force exerted by the suspension cord that pulls the chain back into the housing is not subtracted from the measurement. The sensor only measures effective force/rpm on the flywheel. This is the reason why virtual regattas (serious competitions anyway) are always staged on brand new ergs where the suspension cords have not different levels of fatigue. The ergs are typically sold off after the events (which is of course a marketing/logistics trick just as well).
"Proper" testing on a C2 thus involves measuring handle force directly. This can be done by linking a strain gauge between handle and chain.
3) Speaking of racing events, it's pretty interesting how C2 has enabled virtual/digital racing long before "digitalising" things was a big trend. The interface that the PM4s use for regattas is based on RS485 over RJ45 (you can also use USB but that's not the recommended way for large scale > 16 seats races). The PM3 were wired using telephone/RJ11 and I think it was RS232 IIRC.
4) The single most important "calibration" on a Concept rower is the Drag Factor (the displays can show it in an advanced menu). The drag factor is essentially how fast the flywheel slows down, i.e. how much drag it has. Different ergs have different amounts of dust in them and wear on the bearings, which is why the "flap setting" really shouldn't be used as an absolute/comparable measure between machines.
Racing with a higher/lower drag factor is purely a matter of preference of the athlete and where their personal optimum for performing mechanical work lies. You can think of it like a gearing to increase resistance. In "on the water" rowing we can change gearing using inner/outer handle length of the oar on the gate to achieve a similar effect. This is very important since speed vs. the water varies a lot between boat classes from about ~4m/s in a single (1x) to ~6m/s in a 8+/4x. Our testing protocols recommended 125-140 (female-male).
5) While everyone obsesses over watts, that's not the whole picture. To move a boat (virtual or real) you need work, not power. Since ergs don't float, their mechanics are a lot simpler than a real rowing boat. Work on an erg is handle force x distance, i.e. you integrate the force-distance curve of the handle. That means long strokes are better - the most common beginner mistake is to waste stroke length.
6) Related to 1) and 5) there's a special "erg technique" of pulling the chord up the chest as you move angle your back backwards at the end. This gains you extra stroke length and "stealing" a bit more mechanical work due to sensor hysteresis. This works on a PM4 and up too. You can only get away with this on an erg. On the water pulling off this sort of trick at race speeds will most likely eject you from the boat. It may look like a funny catapult but ejecting from the boat can 100% kill you if your head hits a rigger or blade. See [0] for a close encounter.
7a) "Ergs don't float" is a saying for another important reason. They fail to simulate the impulse exchange between rower and boat mass. This is very important for real world rowing performance because minimizing boat shell velocity fluctuations means less mechanical work required to travel the same distance [1]. This is also the reason why it's preferable to have a "front-loaded" force curve profile. Coincidentally the sensor hysteresis of the concept ergs rewards that... maybe it's engineered intentionally? :-)
7b) Anyways, Avoiding vertical shell movement is of similar importance as wetted surface (and thus drag) increases massively if you push the boat down into the water. Athletes thus have a to maintain very delicate balance between stretcher and handle forces not to offset the systems center of mass during the stroke cycle. None of that matters on an erg.
7c) And most important: Ergs don't simulate the interaction of impulse exchanges between a whole crew of rowers that can never be perfectly in sync. Concept sells "Sliders" for the C2 ergs that you can use for a single erg but also to link together multiple ergs to a crew. If you haven't tried this, I can highly recommend this as it makes it much more realistic and also a lot more challenging. From my coaching experience I can also recommend it especially to beginners. It may sound counterintuitive, but it's much less forgiving to errors in your motion sequence/force application and will thus make you a better (and more healthier!) rower much quicker.
8) Rowers are an incredible bunch of creative and resourceful people. There's so many tinkerers in the community... a shout to a couple of things in the erg space:
- Waterrower hit big thanks to House of Cards. This plus cross-fit certainly made rowing much more popular as a sport. Waterrowers are not used in serious capacity in competitive training though as far as I'm aware.
- Augletics are a bunch of former competitive rowers building an erg using a electromagnetic brake (that can support more realistic force profiles/dynamics). And you don't get that "wooooosh" sound ringing in your ears that won't go away for hours after a 90min C2 erg session...
- Biorower simulates proper sculling technique, boat rotation and uses mechanical gears instead of a flywheel
Anyhow - great to see more innovation like Aviron in the space even if it's "just" targeted at recreational/fitness use case. It's a fabulous sport and I applaud you for making it more accessible. Hope it gets people to dip their toes so they may one day decide to get their feet wet and hit their local rowing club.
[0] https://www.youtube.com/watch?v=pH2k_026KrY
[1] https://www.rowinginmotion.com/drag-efficiency-rowing/