  The uniaxial compression design of the diamond anvil high pressure cell (DAC)
necessitates the use of a soft pressure-transmitting medium (PTM) to minimize
non-hydrostatic effects at significantly high pressures, and many such media
are gaseous at ambient conditions. There now exist a number of
commercially-available instruments - high-pressure gas loading apparatus - for
the insertion of gases into the diamond anvil cell up to several kbar. These
machines allow the use of gases as a PTM, as a reagent for high-pressure
chemistry, or to be loaded as the sample material itself. We present the
development of two highly adaptable gearbox designs to allow the controlled
closure of the DAC within an atmosphere of gas at several kbar, contained
within the walls of a pressure vessel. The first applies a torque directly to
the pressure screws and is thus DAC-specific, while the second applies a load
to the body of the cell and may be used with a wide variety of DAC designs.
