I’m not capturing it.
I fitted a gas post to my fermenter lid, attach a jumper line from the fermenter to the gas in post on a no foam sanitizer filled keg, and place a jumper from the liquid out post into a bucket. As fermentation proceeds the sanitizer is pushed from the keg into the bucket via the CO2 produced by fermentation.
For example yesterday I brewed. Today the bucket is full of sanitizer which now acts like an airlock. I saved the bottle CO2 it would have taken to push the sanitizer from the keg.
To keg the beer, I remove the jumpers, turn the keg upside down towards the gas in post, attach a gas quick disconnect to get the little bit of remaining sanitizer out until it sputters, then reattach the gas jumper from the fermenter to keg. I attach the jumper back onto the liquid out post to allow CO2 to escape purging the liquid line, then attach it to the fermenter spigot. Place the keg below the fermenter, open the fermenter spigot, and the beer displaces the CO2 in the keg and the CO2 displaces the beer in the fermenter. I save bottle CO2 by not using it to push beer from the fermenter to the keg.