We wish to use elastic (this experiment) and inelastic neutron scattering (experiments on MARI and MERLIN submitted in this round) as an approach to quantify the change in phonon entropy at a metamagnetic phase transition relevant to magnetocaloric materials research. We have recently used bulk probes and high-resolution neutron diffraction to study CoMnSi, a helical antiferromagnet that undergoes a magnetic field-driven metamagnetic phase transition with a large magnetocaloric effect. In the process we have discovered giant magneto-elastic interactions within the antiferromagnetic state and also that the simple Debye model often used by magnetocaloric materials researchers underestimates the change in phonon entropy. This PEARL experiment will gather data on compressibility and lattice response required to form a fuller picture of the role of phonons in such an anisotropic solid.