Temperature Change Effect on Airlock

As the weather is cooling down, I've noticed the airlocks on my sours are reversing direction. There is still a water barrier, but the change in temperature is creating a vacuum that might be pulling in some of the airlock water until the airlock is reversed, and then possibly bringing in air.
The carboy is about a year old, so there is little or no CO2 production to correct the pressure.
I have several questions:
1. Is the vacuum primarily due to headspace volume change, or the liquid volume change due to temperature drop.
2. Should I top off the lambic with water? Will the topping off process introduce more oxygen and dilute the flavor making it worse off.
3. How bad is it for my airlock water to touch the beer? How much does it add to my beer cost to use cheap vodka for the airlock?
4. If I introduce fermentables or dextrins for Brett today, how long will the positive pressure last? Should I make it a policy to add (eg. 1/8 cup of sugar) every 3 weeks during the Sept-Nov months to keep positive CO2 pressure?
5. Sometimes I put a drop of iodophor to the airlock water. If this gets sucked into the fermenter will it kill the Brett in the beer, or affect the beer's taste?
6. I've been removing the airlock to remove the vacuum and allow the water to redistribute. Is the amount of air I'm introducing bad (or good)? My pellicle is long gone, and was very thin to start with.

Have you thought of replacing the airlocks with a blowoff tube setup? Even with pressure changes in my basement, the starsan water would never be able to travel all the way up the tube into the carboy. I just use a carboy cap with some regular siphon tubing--doesn't have to be large diameter b/c there is no blowoff activity actually occuring...

1. as the temp goes down the gasses in the head space will contract creating a vacuum

2. do not top off the lambic. your correct about the consequences.

3. it shouldn't be that big of a deal. the water has been sanitized by the iodophor.

4. the positive pressure will last till the fermentation stops again and the temp drops.

5. no at that concentration the iodophor is harmless.

6. no there is still a thin layer of CO2 on the surface of the labmic.


at this point i would switch to a carboy cap with both plugs in. the only reason an airlock is used is because the yeast are creating CO2 but they are no longer doing that so there is no need for an airlock.