I decided to email support at SSBrewtech and this was their response.
This happens with all air tight fermenters, as the liquid cools it contracts, so it can draw in outside air or in this case sanitizer. I would recommend going to an airlock or bubbler during crash cooling, that way just air and not sanitizer get drawn into your Bucket.
No need to worry about oxidation, there is still the protective blanket of CO2 on top of the beer. Thanks!
So basically what he is suggesting is to use the typical three piece air-lock with no liquid inside. I will try this with my next batch and see how it goes. I typically use a blow-off tube for the first three days and then move to the airlock after the first big push.
YIKES, I would ask him for his DO reading to confirm that! You won't catch me doing that!
Yes, thinking that there will be a layer of CO2 protecting the beer is silly and unscientific.
There are YouTube videos showing how you can pour CO2 from a container, into another container and a candle will go out. Some think that this is due to no O2. One should not reach the conclusion that it is no O2, as combustion will not be sustained when the CO2 concentration is around 10%. I sat through many safety videos over the years that went over the dangers of gases pooling in confined spaces.
As an aside, if you ever see a video of a brewer going into a cellar with a candle, it may have been for light, but if the candle goes out, the brewer would know to turn around and get out! That clicked when watching the introductory video on the PU tour.