The Models 764P and 764PD are pneumatic pressure controllers.
They measure the controlled or differential pressure and develop an output signal that varies linearly with changes in the controlled pressure.
The 764P controls pressure between 30" Hg vacuum and 2500 psig (760 mm Hg Vac. and 172.4 Barg) using six ranges.
The 764PD variation controls differential pressures between 1 and 150 psid (.07 and 10.3 Bard) using three ranges.