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part #4-1303 Pressure Relief Valve $3.99
Brand new Norgren 25-50psi.

Open up and remove SS spring.  Take a poppet out of a corny valve body
and grind off the "rivet" on the bottom sacrificing the poppet and
releasing the spring.  Install the poppet spring in the Norgran
valve.  It is now a 15-40 psi relief valve.  Connect it to a corny QD
fitting and you have controlled pressure relief during fermentation.
I also have a pressure guage so I can calibrate what setting the
pressure is set at.  I pump a corny full of air (the QD has a brass
cross with the relief valve, the guage and bicycle type air valve) from
my air compressor beyond the point where I want the release to occur
and adjust the valve to the correct level.  It is then ready to
install on the secondary fermenter.