[Richard]

  I saw a review of a dry hopper on Brulosophy that attaches to the top of a stainless conical and has a butterfly valve and a chamber that can be purged with CO2 before dropping the dry hops into the beer. Nice looking hardware, but it costs over $200 !!! I began to brainstorm how to something similar on my setup for a lot less money. I ferment in a Fermonster, a PET fermenter with a 4" plastic screw-on lid with an O-ring seal. I have been dry hopping with mesh bags or stainless mesh tubes that I hang in the beer with dental floss. I know that the lid will seal tightly even with dental floss going under the O-ring, and I can then pull out the hops when I am finished. Up to now I have been removing the lid to add or remove the hops, letting some oxygen in.
  The next time I dry hop I am going to put the hops in a mesh bag (with some stainless plumbing fittings for weight) and use dental floss to hold the bag up just under the lid, out of the wort, at the start of fermentation. When the time comes to add the dry hops I will loosen the lid ever so slightly and let the weight pull the hops down into the beer, then tighten the lid again. After a few days I can loosen the lid again and pull the hops up out of the beer but leave them hanging until after the beer is transferred out. This should allow for dry hopping with no oxygen exposure.
  I haven't tried this with real beer and hops yet but I have made test runs with an empty fermenter and it seems to work just fine. And it's free!

[denny]

People do something similar using magnets in metal fermenters.  I just don't worry about it.  It takes me less than 5 seconds to put in dry hops on my setup amd there will be minimal gas mixing in that time.

[hmbrw4life]

  I saw a review of a dry hopper on Brulosophy that attaches to the top of a stainless conical and has a butterfly valve and a chamber that can be purged with CO2 before dropping the dry hops into the beer. Nice looking hardware, but it costs over $200 !!! I began to brainstorm how to something similar on my setup for a lot less money. I ferment in a Fermonster, a PET fermenter with a 4" plastic screw-on lid with an O-ring seal. I have been dry hopping with mesh bags or stainless mesh tubes that I hang in the beer with dental floss. I know that the lid will seal tightly even with dental floss going under the O-ring, and I can then pull out the hops when I am finished. Up to now I have been removing the lid to add or remove the hops, letting some oxygen in.
  The next time I dry hop I am going to put the hops in a mesh bag (with some stainless plumbing fittings for weight) and use dental floss to hold the bag up just under the lid, out of the wort, at the start of fermentation. When the time comes to add the dry hops I will loosen the lid ever so slightly and let the weight pull the hops down into the beer, then tighten the lid again. After a few days I can loosen the lid again and pull the hops up out of the beer but leave them hanging until after the beer is transferred out. This should allow for dry hopping with no oxygen exposure.
  I haven't tried this with real beer and hops yet but I have made test runs with an empty fermenter and it seems to work just fine. And it's free!

I mean I wouldn't call it oxygen free(nothing really is), you still have the oxygen molecules that are adsorbed (yes adsorbed) to the hops. That would likely have to be removed via multiple vacuum cycles. But I would call it a very good way to do it (maybe the best all things considered?). The dropper with a chamber as mentioned would probably the second best (most expensive for sure!) way because you would have adsorbed, and Co2 gas impurities (I mean you could slightly crack the butterfly valve and allow fermentation Co2 to purge it for a more elegant solution, but its still ludicrously priced).

[majorvices]

I think it is an interesting idea. You can also add dry hops to a corny keg, purge the keg and then rack the beer onto the hops.

[tommymorris]

Here is $13 alternative that at least claims to be purgeable.

https://www.kegland.com.au/pco1881-x-pco1881-female-ball-balve-dry-hop-device-for-fermzilla.html

[hopfenundmalz]

I think it is an interesting idea. You can also add dry hops to a corny keg, purge the keg and then rack the beer onto the hops.

I've done this. Purged by low pressure CO2 applied to the liquid out post.

[majorvices]

Here is $13 alternative that at least claims to be purgeable.

https://www.kegland.com.au/pco1881-x-pco1881-female-ball-balve-dry-hop-device-for-fermzilla.html

That's seems pretty simple to configure. If the hops are resiny it's going to be a clog problem.

For large batches (30-60bbl) I used to purge a yeast brink after the hops were added then recirculate the wort through the brink. It worked well about 50% of the time.The other 50% it clogged.