What is the restricting element in a gas regulator?

The valve disk is the restricting element in a gas regulator because it controls the flow of gas through the regulator. Its primary function is to open and close in response to the changing pressure within the system, effectively regulating the amount of gas that can flow through to the downstream side. The position of the valve disk, which is influenced by the diaphragm's movement in response to pressure changes, determines how much gas is allowed to pass, thereby ensuring a consistent and safe outlet pressure.

The other components, while essential for the operational integrity of the regulator, do not serve the primary function of flow restriction. The diaphragm, for example, helps sense pressure changes and transmits that information to adjust the valve disk but does not itself restrict flow. The spring provides the necessary force to close the valve disk against the gas pressure but is not the part that directly restricts flow. Inlet pressure refers to the pressure of the gas entering the regulator, which can influence how the regulator performs but is not, on its own, a component that restricts gas flow. Thus, the valve disk's role as the direct flow controller makes it the correct answer.