fermenting Why do I keep getting negative pressure in my ferments?

[Johnaton]

I have 4 ferments that have been going for a little under 20 days right now.
All of them took about 3 days to really get going, were then very active for about 6 days, after which things slowed down.
 
About 2 days after the ferment seemed to have slowed down I found all of them with negative pressure, as seen in the airlock.
Some of them had even swallowed a bit of water. I was kind of bothered by the fact they were getting possibly contaminated water injected, so I decided to gently unscrew the lids to equalize the pressure, after which I tightened them again, thinking that was that.
However, I've had the same thing happen 3 times already now! It just seems to happen every 3 days or so.
 
I know temperature can be a big influence on this, but these ferments are in a controlled environment where temperature fluctuations are 4 degrees at most over a 24 hour period. I've also checked the barometric pressure in my area, and at first I thought it might be the culprit as the first instance of it happening seemed to line up with a big change in pressure. This time the barometric pressure was quite stable and it still happened though.
 
I'm at a loss as to why this is happening, and would really hate to see these batches ruined. Would appreciate any advice!

I've added some pictures for reference.
 
 

 

[The_NorthEast_ChileMan]

I have 4 ferments that have been going for a little under 20 days right now.
All of them took about 3 days to really get going, were then very active for about 6 days, after which things slowed down.
 
About 2 days after the ferment seemed to have slowed down I found all of them with negative pressure, as seen in the airlock.
Some of them had even swallowed a bit of water. I was kind of bothered by the fact they were getting possibly contaminated water injected, so I decided to gently unscrew the lids to equalize the pressure, after which I tightened them again, thinking that was that.
However, I've had the same thing happen 3 times already now! It just seems to happen every 3 days or so.
 
I know temperature can be a big influence on this, but these ferments are in a controlled environment where temperature fluctuations are 4 degrees at most over a 24 hour period. I've also checked the barometric pressure in my area, and at first I thought it might be the culprit as the first instance of it happening seemed to line up with a big change in pressure. This time the barometric pressure was quite stable and it still happened though.
 
I'm at a loss as to why this is happening, and would really hate to see these batches ruined. Would appreciate any advice!

 
No experience with this issue but a search turned up,Airlock funny business

 

[Johnaton]

 
No experience with this issue but a search turned up,Airlock funny business

 
Yes, I've seen that topic. Like lots of other reference it discusses temperature and barometric pressure changes.
I'm certain temperatures have been quite stable in my fermenting location.
 
I've added a chart showing the barometric changes in my area. I've no idea if those fluctuations are dramatic enough to see the changes I'm seeing, especially considering I don't have a lot of headroom in my ferments (not a lot of air to be compressed).

 

[Wiriwiri]

Hi Johnaton,
I sent an email with some info.,  that may be helpful r/t to  pressure and air locks.

 

[Chorizo857_62J]

I saw this with one of mine.  I removed the airlock, checked the mash, and then re-installed the air lock after stirring the ingredients.  Did not have any adverse results.  I've also had them burp out, but managed.

 

[Bou]

Alwways safer to use Star San (homebrewing acid base no-rinse sanitizer) inside an airlock, just in case...