  We calculate the nonzero-temperature correction to the beta equilibrium
condition in nuclear matter under neutron star merger conditions, in the
temperature range $1\,$MeV$ < T \lesssim 5\,$MeV. We improve on previous work
by using a consistent description of nuclear matter based on the IUF and SFHo
relativistic mean field models. This includes using relativistic dispersion
relations for the nucleons, which we show is essential in these models. We find
that the nonzero-temperature correction can be of order $10$ to $20\,$MeV, and
plays an important role in the correct calculation of Urca rates, which can be
wrong by factors of $10$ or more if it is neglected.
