  Smoothed Particle Hydrodynamics (SPH) schemes need to be enhanced by
dissipation mechanisms to handle shocks. Most SPH formulations rely on
artificial viscosity and while this is working well in pure shocks, attention
has to be payed to avoid dissipation where it is not wanted. Commonly used
approaches include limiters and time-dependent dissipation parameters. The
former try to distinguish shocks from other types of flows that do not require
dissipation while in the latter approach the dissipation parameters are steered
by some source term ("trigger") and, if not triggered, they decay to a
pre-described floor value. The commonly used source terms trigger on either
compression, $-\nabla\cdot\vec{v}$, or its time derive. Here we explore a novel
way to trigger SPH-dissipation: based on the entropy growth rate between two
time steps we identify "troubled particles" that need to have dissipation added
because they are either passing through a shock wave or become noisy. Our new
scheme is implemented into the Lagrangian hydrodynamics code MAGMA2 and
scrutinized in a number of shock and fluid instability tests. We find excellent
results in shocks and only a moderate (and desired) switch-on in instability
tests, despite our conservatively chosen trigger parameters. The new scheme is
robust, trivial to implement into existing SPH codes and does not add any
computational overhead.
