How do counter pressure bottle fill PET bottles with a stainless carbonation cap

I'm looking at purchasing a stainless steel carbonation cap.

E.g. https://www.homebrewfinds.com/2018/12/hands-on-review-ferroday-stainless-steel-carbonation-cap-for-pet-bottles-draft-line-cleaning-builds.html

If you do a Google search for stainless carbonation cap you can see them listed at lots of online brewing stores.

Most of these online stores list the following uses for the cap:

  1. Carbonate
  2. Cleaning your beer lines
  3. Use it to counter pressure bottle fill PET bottles

Numbers 1 and 2 make sense to me. However, number 3 is confusing me. How can you counter pressure fill a bottle if there is no way to vent the CO2 as you fill the bottle with liquid. I know that the cap I'm looking at has a barb so you cap put a dip tube to fill from the bottom up but I would have thought that no beer could flow into the bottle if it is sealed and full of CO2. Are all these homebrew stores just putting the wrong description on these caps or am I missing something?

I suppose maybe you could undo the cap a little bit to let the CO2 escape as you fill the bottle. Has anyone done counter pressure filling with a carbonation cap?

Topic keg co2 carbonation homebrew

Category Mac

1
answered at

Probably something like this: https://www.youtube.com/watch?v=CCrfp8ha9Dw

You unscrew it a little to let gas go.

1
answered at

Counterpressure filling involves purging the container, pressurizing it it CO2, and then bleeding off the pressure gradually to make beer flow into the container from another pressurized vessel (typically a keg). I don't see how that would be possible with a carbonation cap.

I made my own carbonation cap ages ago (before you could buy them) using some industrial gas fittings an a coke bottle cap. But I only used that to make my own sodawater (i.e. by carbonating water) and to carbonate some beer for a quick sample.

So yeah, I think the description of being suitable for counterpressure filling is dubious at best.

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