Is damping rate of a pendulum dependent on the number density of the gas?

Robert Boyle in 1660 conducted experiments where he pumped air out of a vessel with a swinging pendulum. He discovered that the damping rate of the pendulum is independent of the number density of the particles. At what pressure this stops being true?

• When the means free path of particles becomes comparable to the size of the pendulum body. Certainly when the mean free path becomes comparable to the diameter of the vessel. Oct 15, 2021 at 20:02
• I assume after that since collisions are very infrequent that the damping actually does not exist? Oct 19, 2021 at 9:52