The 3325 is a poppet-type normally open adjustable relay valve. The spring keeps this valve in an open position and lets the air pass from the source of supply 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 3328 is the the differential version of this valve.
Photo shown is of a 3325.