A spring by-pass valve is used to regulate water bypass around filter and eliminate high filter pressure on systems with 2 speed pumps. They are also used in ozone injector manifolds.
Valve opens at 5 lbs and is fully open at 17 lbs.
 

DelCheck valve can be used as a bypass or as a regular check valve.
1-1/2" Slip or 2" Spigot, 5 lb spring, ( used on ozone injector manifolds ), PVC

Flo Control, 2" Slip X Slip

Use for air blower installations, spring loaded, 1/4 lb. spring for a minimum restriction of air flow
which results in less burden on the blower motor.
 

Spring air check valve, 1-1/2" Slip or 2" Spigot,
1/4 lb spring, white body, PVC

Smart 1/4" barbed spring loaded, red, break apart body

Del 1/4" barbed ozone check valve, one piece, spring loaded

Rainbow spring check valve, 1/4" tube compression,
corrosion resistant for chemical feed

Rainbow spring check valve, 1/2" tube compression,
corrosion resistant for chemical feed

Rainbow  1-1/2" Slip or 2" Spigot, 2 lb spring,
white body, used with some Rainbow chemical feeders, corrosion resistant