As the physics of the erg were explained to me by Kellerman (or was it
Nielsen?), it is quite elegant how the performance meter calculates
how hard you are working. There are a few basic assumptions:
1) to accelerate the flywheel on the drive, the rower works
against a) the flywheel's moment of intertia, and b) fan forces
2) flywheel losses on the recovery are due to fan forces;
bearing friction is "negligable"
3) all erg flywheels have the same moment of inertia
(Kellerman said that the Dreissigacker (sp?) brothers are
particularly fanatical in this regard), which is some known
quantity
4) the fan laws that apply are fairly well-known

By virtue of 2), 3) and 4), the performance monitor is able to
calculate coefficients for the fan laws from slowdown of the flywheel
on the recovery. With the fan law coefficients known, it is now
possible to calculate the total work for the preceeding drive. This
pair of calculations is repeated for each stroke cycle.

So, the determination of the rower's work (pick a unit: joules,
calories, etc.) or power (e.g. watts, calories/hr), is very precise
physics. The conversion of this to a number of meters is less exact:
Nielsen & Kellerman were just given a conversion factor, but I can't
remember if it was a conversion based upon power or work; the
distinction may be very important in determining the relation of a
power curve to the "score."

It is because of how the performance meter calculates rower output
that a score is a score, no matter how the erg is set up (vents
open/closed, or partially open; big or little sprocket). As long as
the vents aren't changed in the middle of a stroke, the physics used
by the monitor will apply. Even so, one stroke will be inaccurate, but
those before and after will be fine since the fan resistance is
calculated for each stroke.

As to obtaining the moment of intertia, your best source is probably
Nielsen-Kellerman. Give them a call. Kellerman was happy to explain it
to me when I met him at the Head of the Charles last year.

You may also want to check out the latest issue of _CII Ergo
Update_. They published formulae for converting erg scores at sea
level to those at altitutde. I did not study the formulae, but I seem
to remember that the conversion procedure involved converting a time
score into work, then re-calculating time based upon what appeared to
be an empirically-derived formula.

Matthew Fremont
Undine Barge Club