The disclosure describes an adapter assembly for a conventional fluid system having a filter unit mounted to a filter pad. The adapter assembly allows the pressure differential across the filter element to be conveniently measured by a differential pressure detector. The adapter assembly, which is mounted between the filter pad and the filter unit, generally comprises an adapter plate, a mechanism for attaching and sealing the adapter plate to the filter pad, a mechanism for attaching and sealing the filter unit to the adapter plate, and a fitting for connecting the differential pressure detector to the adapter plate. The adapter plate includes at least one aperture which allows fluid to flow between the inlet chamber of the filter pad and the inlet of the filter unit and another aperture which allows fluid to flow between the outlet of the filter unit and the outlet chamber of the filter pad. The adapter assembly further comprises structure definig a fluid channel to the detector fitting from the fluid flowing between the inlet chamber of the filter pad and the inlet of the filter unit and structure defining another fluid channel to the detector fitting from the fluid flowing between the outlet of the filter unit and the outlet chamber of the filter pad.