How do people with conical (or other sealed) fermenters avoid sucking liquid from the airlock into the vessel when they take samples or remove trub through the bottom valve? Do they just remove a plug from the top and let air in to replace the liquid volume that's been lost? If so, doesn't that cause a contamination/oxidation risk?
There is a trick to pulling a small sample or drawing yeast.
Use a 1" blow off tube of ample length to reach the bottom of a bubbler jar on the floor.
The volume in the tube is enough to draw a small sample, as you draw the sample you will see the sanitizer suck up in the tube, don't let it crest the arch and you're ok.
For a large draw you can use a sanitary air filter in place of the airlock to prevent contamination, minimal air is ok, but you can also attach c02 to replace the lost volume if worried about o2 exposure.