I just crashed 5 gallons of German Pilsner for lagering and I noticed that my airlock starsan level was significantly lower after cooling. I presume this is due to the liquid (beer) shrinking after a 20F drop in temp, but I am not sure how to prevent this or protect against it. I am not terribly concerned with starsan going into the carboy, but more that it can drop below the airlock level which essentially is opening up the carboy to the air in the chest freezer.
How do you handle shrinkage when crash cooling in a closed system?
[edit: clarified that i was talking about the beer volume shrinking not the starsan]