A strange fermentation thing

[guypettigrew]

My beers are fermented in a SS Brewtech 7 gallon chronical with a blow off tube dropping down into a bucket of water.

When the top tap on the chronical is opened to run off a small amount of the beer to test the SG after a few days there is an inrush of air and a massive explosion of bubbles in the blow off bucket.

This has happened with the last three brews. Before then the blow off tube was taken out of the bucket before opening the ball valve so I wouldn't have seen it.

What on earth is happening?

Guy

[Kev888]

Without a picture of the setup, my best guess would be a column of liquid in the blow off tube is being held up by suction (low pressure in the fermenter). As you open the valve and allow pressure inside the FV to equalise (by sucking in air) the column of liquid would be allowed to drain out of the blow-off tube, pushing out air ahead of it.

I'd guess that you previously dumped some sediment (and/or the FV has cooled) with the blow-off tube still in the bucket - creating negative pressure in the FV and sucking up liquid from the bucket into the blow-off tube. (BTW, if this is happening you may want to check that liquid from your bucket isn't being sucked right up into the FV). Personally I remove the blow-off tube before letting anything out of the fermenter or cooling it, some people go as far as to put CO2 in instead of letting it suck in air.

[McMullan]

Your top tap hasn't taken the oath of least resistance

[guypettigrew]

Hi kev888

Good thoughts, but I clearly hadn't given enough info.

The beer ferments nicely for (say) four days. Regular bubbles popping out of the tube into the blow off bucket. No yeast had been harvested. Nothing done to the FV except kept the temperature steady.

At the time of opening the tap to test the beer the bubbles were still coming through the blow off tube at about one every four or five seconds. You'd kind of assume positive pressure in the FV wouldn't you?

Even if there was negative pressure in the FV, causing the inrush of air through the valve, I can't make sense of the huge rush of bubbles from the blow off tube. The air going in through the valve was tiny compared to the amount of gas vented through the tube.

Guy

[Kev888]

Thats strange indeed, Guy. It seems to me that the blow-off tube must for some reason be sucking more strongly/rapidly than the natural tendency for wort to flow out of the racking valve. I may have guessed wrongly at the cause, but I still can't think of any way that could happen unless there was liquid in the blow-off tube being affected by gravity (i.e. similar to the beginnings of a syphon effect). Unless your 'bucket' was something more special, anyway.

[Rad]

Here's a thought, how smooth does the valve work? Could the jolt of the tap opening shake the fermenter enough to release more CO2 rapidly?