  We describe a new implementation of the one-fluid method in the SPH code
Phantom to simulate the dynamics of dust grains in gas protoplanetary discs. We
revise and extend previously developed algorithms by computing the evolution of
a new fluid quantity that produces a more accurate and numerically controlled
evolution of the dust dynamics. Moreover, by limiting the stopping time of
uncoupled grains that violate the assumptions of the terminal velocity
approximation, we avoid fatal numerical errors in mass conservation. We test
and validate our new algorithm by running 3D SPH simulations of a large range
of disc models with tightly- and marginally-coupled grains.
