  We report on a series of 42 measurements of the transition frequency of the
429~THz (5s$^2$)~$^1$S$_0$--(5s5p)~$^3$P$_0$ line in $^{87}$Sr taken over three
years from 2017 to 2019. They have been performed at the
Physikalisch-Technische Bundesanstalt (PTB) between the laboratory strontium
lattice clock and the primary caesium fountain clocks CSF1 and CSF2. The length
of each individual measurement run has been extended by use of a hydrogen maser
as flywheel to improve the statistical uncertainty given by the Cs clocks. We
determine an averaged transition frequency of
$429\:228\:004\:229\:873.00(0.07)$~Hz with $1.5\times10^{-16}$ fractional
uncertainty, at the limit of the current realization of the unit hertz.
Analysis of the data provides an improved limit on the coupling of the
gravitational potential of the Sun to the proton--electron mass ratio $\mu$,
and confirms the limits on its temporal drift.
