I've had some downtime while waiting on an electrician disconnect my old 60 amp fuse box and connect mains from the meter to my 200 amp panel which until now has been acting as a sub panel, long story, so I can finish up my beer area so I decided to see what Kind of dry hopping gizmo I could come up with. I based it on the hop cannon that
@leesmith posted a picture of a few pages back which I had seen a while back being used at 8:28in this video.
Here's a picture of what I came up with.
View attachment 618900
Excuse the cardboard pressure gauge. I made a template before I ordered a real one just to be sure there wouldn't be any clearance issues with the gas post fitting. The hopper is a 3"x1.5" offset reducer which I chose to avoid issues with clearance with the butterfly valve handle. Behind the fake pressure gauge is a purge valve with a barb fitting which I suppose could be used as a blow off, hooked up to a keg to purge it with co2 during fermentation, or just closed if I want to ferment under pressure. The Spike manifold is just sitting on a blank cap at the moment but is about where it will be once I get my cooling coil with the added TC back from the welder.
I figure when it comes time to dry hop you could put your hops in the hopper, crack the butterfly valve as @mongoose mentioned, open the purge valve and let the co2 in the fermenter purge the o2 from the hopper (as long as you maintained some pressure in the fermenter). Once this was done you close all the valves, pressurize the hopper, then open the butterfly valve and blow the hops into the fermenter. I expect some experimentation with pressures will be required to deal with the hop bridging problem mongoose mentioned but I don't expect it will require much. Someone also mentioned geyser possibilities but I'm hoping by adding pressure to the fermenter as the hops are going in this won't be a problem. We'll see.