Don Young wrote:
"Pat Coghlan" wrote in message

When the little tab - which is part of the plate that moves the contacts -
below the brass cylinder (connected to tank pressure) rises, the contacts
open. The needle in the cylinder just sits there, with the tab rising to
meet it. If the needle were to get pushed down (from air pressure), this
would force the contacts to close and activate the motor. Why would the
designer want the motor to operate when pressure rises?
He wouldn't. The needle doesn't push the tab down, the tab pushes the needle
up when the contacts open to stop the compressor. This releases the pressure
in the compressor while a check valve holds pressure in the tank. When the
tank pressure drops, a diaphragm in the switch closes the contacts to start
the compressor. At the same time the needle is released so that the
compressor air is no longer vented and can build up pressure to open the
check valve and repressurize the tank.

Ah, okay, that makes sense. I figured there must be something under the
switch (diaphragm) moving the plate, but started to think that maybe the
needle might be moving the contact plate, rather than as you described.

Does the diaphragm suddenly "pop up", opening the contacts and releasing
pressure from the compressor? It would be hard to make the two happen
gradually.