The 3328 is a poppet-type normally open adjustable relay valve. The diaphragm spring forces the valve stem down, which keeps the poppet open. This allows air to flow from the source of supply directly to
the device actuated.
These valves come in a normally open setting, in a variety of light to heavy duty configurations. They have a pipe port size of 1/4″ NPT and have a maximum supply pressure of 200 PSI (13,8 bar).
The 3325 is the the non-differential version of this valve.
Photo shown is of a 3325.