A valve manifold (18) between a main flow line (10) and a differential pressure transmitter (24) having a body (40) with a bore (48) in which three spherical ball valve members (72, 74, 76) are mounted for rotating among run, zero and calibration positions. A low pressure calibrating port (28) and a high pressure calibrating port (46) are controlled by calibration valve member (76). Low pressure calibrating port (28) is connected to a source of a predetermined low pressure calibrating fluid and high pressure calibrating fluid port (46) is connected to a source of a predetermined high pressure calibrating fluid to calibrate transmitter (24) between any desired low pressure and high pressure ranges. Low pressure calibrating port (28) and high pressure calibrating port (46) may be selectively connected to atmospheric pressure if desired and the calibrating fluids may be controlled from a remote control site (26).