Pressure-reducing valves for reducing the pressure of gases and maintaining that reduced pressure at a fairly constant level by means of a plug or stopper which is generally controlled by a pressure device (diaphragm, bellows, capsule, etc.) damped by an adjustable tension spring. These appliances directly regulate the pressure of gases passing through them; they are mounted, for example, on compressed gas cylinders, on pressure containers, or on feed pipe systems of the appliances which they serve.
The heading 8481 also includes pressure-reducing valves (sometimes called pressure regulators, pressure reducers or pressure regulator-reducers), also mounted at the outlets of pressure containers, of boilers, on connecting feed pipe systems or near the appliances which they serve, to perform the same function on compressed air, steam, water, hydrocarbons or other fluids.
If combined with a pressure gauge, pressure-reducing valves fall either in this heading (i.e. 84.81) or in heading 90.26 () depending on whether or not the combined apparatus retains the essential character of a tap, valve, etc. (see the fourth paragraph of this Explanatory Note).