Some questions for those of you who are doing closed transfers and who also dry hop in the keg.
I understand the keg purging process of filling the keg with sanitizer and pushing it out with CO2.
Do you then open the keg to put the dry hops in?
Do you “purge” the hops somehow? I’ve read/heard that the hops bring a substantial amount of O2 to the finished beer.
Finally, with the keg closed while transferring beer from primary, do you fill the keg by weight? Or, how do you know when the keg is full?
I haven’t done any keg hopping since I’ve started using kegs as primary, so I can’t really comment on that.
As far as telling when the keg is full, I cold crash before jumping to my serving keg. Since the beer going in is ice cold, I can just watch the condensation line on the outside of the keg and feather the gas once it gets near the top. You can also tell by sound if you use your spunding valve as a vent when filling the keg. Or you can just wait until it starts spraying out of the spunding valve. Been there, done that, mopped it up.
I use a scale. Beersmith has a calculator that factors in finishing gravity, but a 41-42lbs is safe.
I push out the sanitizer and add my hops. I then purge and burp a bunch. I use pellets in a stainless filter, so I’m not too worried overall.
I am VERY worried. Will do my first closed transfer with my next brew and I’m NOT dryhopping. I’ll do that when I’m all grown up. 
Same here.
Thanks for the responses.
I try to fill to just below the in dip tube to avoid beer in the gas line and, when the keg is over-filled, it seems to take a lot longer to carbonate.
I dry hop the keg all the time it’s not a problem. I’ve even released pressure and opened kegs to throw in more hops not going to hurt anything. We’re talking IPA which your not going to age and should be drunk fresh so O2 should not be a problem.
I open the keg to add more hops to APA or AIPA on occasion, but I like to open the gas valve back up to let CO2 blow over the surface of the beer as I do it, though.
You could but I’m not sure you have to. I’ve heard the co2 is heavier than O2 so if you’re quick it’s probably negligible
Same here too, word of caution. Go slow when transferring beer to serving keg, if you start out to fast, depending on the amount of hops, you will clog that screen.
The only issue I have observed with this, is that the condensation line is usually below the actual line of the beer as it takes a bit of time for the condensation to actually form as the beer is filling the keg.
That makes sense to me.
Is it a good idea to transfer from an auto syphon to keg outlet for a closed transfer? I have BMB and I’m not sure I would be able to add CO2 to transfer under pressure