Primary standards (often also referred to as mechanical dead-weight testers or pressure balances) are the most accurate reference instruments for pressure. Their functional principle is based on the physical principle of pressure = force/area. Mass pieces placed on the top of a piston-cylinder system are the source of a precisely defined force. By producing a certain (counter) pressure inside the primary standard an equilibrium is achieved: the mass pieces, including the free-running piston of the piston-cylinder system, are floating, which will lead to a very accurate pressure at the test port.
