We have constructed a low-temperature helium pressure device that incorporates optical-reflectivity detection and is useful for the study of thermochromic materials. We have tested the device up to 1600 bar (0.16 GPa) and down to 10 K. The performance of the device is illustrated by the results of constant-pressure and constant-temperature runs for the spin-transition solid [Fe(btr)₂(NCS)₂]·H₂O. The data establish a consistent pressure-temperature phase diagram, which is discussed.