Question about yeast catcher on bottom of connical

[kellzey]

When using a PVC or other type yeast catcher on the bottom of the conical, how is it actually used?

Do you leave the valve open when the wort is placed in the conical and allow the yeast to build up in the yeast catcher?

Or do you allow fermentation to occur and then open the valve and allow the yeast to drain and fill the yeast catcher later? The issue I see with this is that the yeast catcher appears to be a sealed vessel and would contain air and oxygen. Opening it would allow the yeast to flow in and the oxygen to bubble up through the wort (and possibly increase oxidation or contamination).

Or... is it used another way?

 

[sloanfamilydsm]

I begin fermentation with the 1 1/2" valve open. After 3-4 days of active fermentation I will close the valve and remove my DIY pvc yeast/trub collection system (it's approx. 1.5 pints on 7.3 gal conical). After cleaning and sanitizing you can re-install, or leave it off and finish out the primary and/or secondary. The homemade system on my stainless conical allows me the option to secondary with the removal of the lees & trub. It also doubles as a yeast collection although I rarely reuse yeast as I'm frequently switching from wort to must in the fermentor as well as different styles of both wine and beer. I first tried to dump but ended up losing significant volumes (up to a gallon) and quickly ended that program and engineered my current "collection" system.