Fermentation is measured with a hydrometer and weighed as carbon dioxide going out of the vessel, and the two have to agree, because every gram of sugar that goes produces a known amount of each. In tank 6 the gravity falls and the gas does not come out.
I am a brewer at a regional brewery and I have done the night shift on fermentation for nine years.
What we measure and why it cannot lie.
Specific gravity is the density of the liquid compared with water. Wort full of sugar is heavy, about 1.045 for our bitter. As the yeast eats the sugar it makes alcohol, which is lighter than water, and carbon dioxide, which leaves. So the gravity falls, and where it stops tells you the beer is done.
You take it with a hydrometer in a trial jar, corrected for temperature, every two hours through the night, and you write it on the chart on the tank.
The other measurement is the gas. Each tank vents through a blow off into a header, and we have a flow meter on each one, because the brewery recovers carbon dioxide and sells it.
The chemistry fixes the ratio. Fermenting the sugar out of a hundred barrel brew gives you a known weight of gas, near enough four hundred kilos, and in nine years of charts that figure comes out within a few percent every time on every vessel.
Tank 6 ferments from 1.045 to 1.010 in about thirty hours, which is fast but not impossible.
The gas meter on tank 6 records about a third of what it should.
Not a third less. A third of it.
Two hundred and sixty kilos missing on every brew in that vessel, and a hundred and forty out of the blow off.
What has been checked, and the engineering manager has been decent about all of it.
The meter. Swapped with tank 5's, which is the same age and the same model. Tank 5 reads right in tank 6's position and tank 6 reads a third in tank 5's.
Which means it is not the meter, it is the tank, and that sentence took two years to be allowed to say out loud.
The blow off line. Pressure tested, rodded, and the arm swapped. A blocked arm would pressurise the vessel and lift the relief, which is alarmed and logged, and it has never lifted.
The vessel. A leaking fermenter would smell, and you would hear it, and you would find beer on the floor. It holds a head of pressure on the conditioning cycle and the gauge does not move overnight.
And the part that matters most to me, because I am the one who signs the chart: the hydrometer readings.
Three hydrometers, calibrated against a standard solution. Two different brewers on two shifts. A digital density meter borrowed from the laboratory at a larger brewery, which reads to four places off a small sample.
The gravity falls the way I have been writing down for nine years.
The other check, which is the one that settles what kind of problem this is.
Alcohol.
If the gravity is really falling, the alcohol is really rising, and you can measure alcohol directly rather than inferring it.
Every brew goes to the laboratory for duty purposes, because the tax is on the alcohol and the figure is a legal declaration.
Tank 6's beer comes back at the alcohol the gravity drop says it should be, to a tenth of a percent.
So the sugar went. The alcohol is there. It is in the beer, it is on the duty return, and it has been drunk in about forty pubs for nine years.
The gas that has to come off at the same time did not come off.
What that means, as carefully as I can put it.
Making alcohol out of sugar makes carbon dioxide in a fixed ratio. There is no route that makes one without the other, and if there were, somebody would have built an industry on it.
Two hundred and sixty kilos of gas a brew is about a hundred and forty cubic metres. That is not a smell you would miss and it is not a volume that hides in a cellar, and the cellar has a carbon dioxide alarm because that gas kills people in confined spaces and we test the alarm weekly.
It has gone off twice in nine years, both times on tank 2, both times for a real reason.
What is done about it.
Tank 6's yield is recorded as a gas recovery shortfall, and the recovery plant's annual figures carry it, and the accountant has asked about it once in nine years and was satisfied with the answer that vessel 6 has a known venting inefficiency.
I wrote that phrase. I am not proud of it. It is the words somebody needs in a spreadsheet and it describes nothing.
The beer out of tank 6 is good. That is the thing nobody believes when I tell this. It wins its category at the regional awards about every other year and the head brewer puts tank 6 on for the festival beer on purpose.
What I do on my shift.
The readings, the chart, the temperature control, and the walk round every two hours, which is eleven tanks in a hall with the lights on a timer.
The one thing I do differently is that I take tank 6's sample from the top hatch rather than the sample point.
The sample point is at the bottom of the cone, which is where you are taught to take it, and the hatch means climbing the gantry with a jar on a string, which is slower and which two different managers have told me is unnecessary.
I started doing it in 2019 after a night when I was crouched at the sample point with the jar under the tap, which puts your head about a foot from the steel, and a fermenting tank is a loud thing, a sort of heavy fizzing you stop hearing after a week on the job.
Tank 6 was silent. It had been silent at every reading all night, and the gravity had dropped four points since midnight, and when I put my hand flat on the cone it was warm the way a fermenting tank is warm, and under my palm it was absolutely still.