RPM bug fix created a issue when rescaling hertz

[seanbrant]

The bug fix in commit 393d37e introduced a bug when rescaling rpm's to hertz's.

Example:

import quantities

print(quantities.Quantity(60, 'rpm').rescale('Hz'))

# used to equal 1.0 Hz before the bug fix
# now equals 6.283185307179586 Hz

[apdavison]

hi, do you have any suggestions on how this should be fixed? What is the relationship between a cycle and a revolution?

[benhamill]

According to https://www.convertunits.com/from/hertz/to/RPM, 1 hertz = 60 RPM.

This seems to have occurred because of the more complex modeling of rpm that includes pi (notice that it's saying that 60 rpm = 2pi Hertz). I'm not sure the best way to account for this without simplifying the definition of revolution again or writing special case logic somehow.

[benhamill]

I think it might help if you could explain the cases that rpm being defined in terms of revolution instead of just 1/min was important. The commit that made that change didn't have tests or anything and the Issue that inspired it didn't have a lot of discussion, IIRC.

[carl-youngblood]

If I'm reading this correctly https://en.wikipedia.org/wiki/Revolutions_per_minute then RPM should be 1/60.

The value that rpm was changed to in 393d37e of 2pi / 60 is the value you would expect when asking for the angular velocity of something completing 1 RPM. Specifically, the value of 2pi / 60 radians per second.

The key distinction is that revolutions per minute do not have a unit associated with them, but angular velocity does explicitly include radians.