Abstract: An adapter for coupling a pressure transmitter to a vessel or tank carrying a liquid therein that establishes a hydrostatic pressure at a flanged opening has a planar surface that includes two spaced apart pressure ports that are aligned with and will mate with the isolator input ports of a differential pressure transmitter. The pressure ports are arranged so that the transmitter can be mounted very close to a side wall of the vessel or tank, with the hydrostatic pressure sensing port positioned closest to the vessel to eliminate errors caused by tilting of the adapter and transmitter from bulges that may occur in the vessel or tank side wall. Additionally, the adapter planar surface mounts the transmitter facing sideways, where the transmitter can be quite easily serviced, replaced, or examined.