  Although flow in a circular pipe is stable to infinitesimal perturbations, it
can be excited to turbulence by finite perturbations whose minimal amplitude
shrinks as R goes to infinity (R = Reynolds number). Laboratory experiments
have appeared to disagree with one another and with theoretical predictions
about the dependence of this minimal amplitude on $R$, with published results
ranging approximately from $R^{-1/4}$ to $R^{-3/2}$. Here it is shown that
these discrepancies can be explained by the use of different definitions of
amplitude by different authors. An attempt is made to convert the existing
results to a uniform definition of amplitude, the nondimensionalized $L^2$
definition common in the theoretical literature. Although subtleties in the
physics raise some questions, agreement appears to be reached on a minimal
amplitude that scales as $R^{-3/2 \pm 0.3}$.
