Manostats (Automatic Pressure-Regulating Instruments)

A manostat automatically regulates pressure by controlling an electrical circuit or a modulating valve to maintain a preset pressure level — it is a sensing-and-switching instrument classified in Chapter 90. A pressure-relief or safety valve (heading 8481) is a purely mechanical device that opens to release excess pressure without any electronic sensing or switching function. If a device combines both mechanical relief and electronic pressure-sensing control, classification depends on which function constitutes the essential character.

Generally no. Chapter 90 note provisions treat instruments and apparatus of heading 9032 as classifiable in that heading even when they are dedicated to a specific machine or system, unless they are integral parts of that machine and lose their identity as separate instruments. A manostat supplied separately — even if destined for installation in a pneumatic conveying line — retains its classification under 9032.20. If it is pre-installed and shipped as an integral assembly with the conveying equipment, the composite-goods rules would apply.