How do the HVAC pressure switches operate? A pressure switch for sensing fluid pressures(gas or liquid) contains a capsule, bellows, Bourdon tube, diaphragm, or piston element that deforms(changes shape), or displaces fluid proportionally to the applied pressure. The resulting motion is applied, either directly or thru amplifying levers (cantilevers), to a set of switch contacts.

Since pressure may be changing slowly and contacts should operate quickly, some kind of over-center mechanism, such as a miniature snap action switch (Micro Switch), is used to assure quick operation of the contacts.

One sensitive type of pressure switch uses mercury switch contacts mounted on a Bourdon tube, the shifting weight of the Mercury provides a useful over center characteristic, (sort of like an off center fulcrum teeter-totter).