  Collisions between trapped atoms or trapped molecules with room temperature
particles in the surrounding vacuum induce loss of the trapped population at a
rate proportional to the density of the background gas particles. The total
velocity-averaged loss rate coefficient $\langle \sigma_\mathrm{tot} v \rangle$
for such collisions and the variation of the loss rate with trap depth has been
shown to depend only on the long range interaction potential between the
collision partners. This collision universality was previously used to realize
a self-calibrating, atom-based, primary pressure standard and was validated by
indirect comparison with an orifice flow standard. Here, we use collision
universality to measure $\langle \sigma_\mathrm{tot} v \rangle = 6.44(11)(5)
\times 10^{-15}~\rm{m^3/s}$ for Rb-Rb collisions and deduce the corresponding
$C_6 = 4688(198)(95)~E_ha_0^6$, in excellent agreement with predictions based
upon $\textit{ab initio}$ calculated and previously measured $C_6$ values.
