Pressure controllers or regulators, also called manostats. These consist essentially of a pressure sensitive device, a controlling device which compares (e.g., by means of an adjustable spring) the pressure to be controlled with the prescribed pressure, and an electric contact or a small valve operating a servo-circuit.
This apparatus may be used, for instance, to control a motor pump or compressor which supplies a pressure tank, or to operate pneumatic valve positioners, or with a valve to regulate the flow, pressure, etc., of liquids or gases
These pressure regulators differ from the pressure reducing valves of heading 84.81 ( sometimes also called "pressure regulators")