Equipment
Airlocks, bungs and headspace management
Do I need an airlock, and what does it actually do?
In short
An airlock is a small trap, usually of moulded plastic, filled with liquid. Gas pressure inside the vessel pushes the liquid aside and escapes as bubbles; when pressure equalises, the liquid falls back and seals the path.
What it prevents is the entry of oxygen, vinegar flies and airborne organisms. What it does not do is tell you reliably whether fermentation is happening — a leaking bung or a slow ferment both produce a silent airlock.
A bung or grommet is what makes the airlock work. A perfect airlock in a bung that does not seal is decorative.
The types and what to put in them
The S-bend or bubbler airlock is a bent tube with liquid in the bends. It is easy to read — the levels shift visibly with pressure — and easy to clean because it can be flushed through. Its weakness is that a sudden suck-back, caused by a drop in temperature, can pull the liquid into the cider.
The three-piece or cylinder airlock is a cup with an inverted cap floating over a central tube. It is harder to read but easier to dismantle and scrub, and it holds more liquid, which means it dries out less quickly.
What goes in the trap should be something that is neither a contamination risk nor a problem if it is drawn in. Cooled boiled water is the usual choice and is entirely adequate; a weak sulphite solution or a spirit is sometimes used where the vessel will stand a long time, on the argument that it will not itself grow anything. Plain tap water left for months grows a biofilm and evaporates, and an evaporated airlock is an open hole.
Vigorous ferments and blow-off
A cider fermenting hard produces a great deal of gas and a foam of yeast and pulp that climbs the vessel. If that foam reaches the airlock it blocks it, and a blocked airlock in a sealed vessel is a pressure vessel. The standard answer is either to leave enough headspace — a fifth of the vessel is a common allowance — or to fit a blow-off tube: a large-bore hose from the bung into a bucket of water, which passes solids without clogging and still acts as a one-way trap.
The blow-off arrangement is also what a maker uses for the first days of any large or warm batch, switching to an airlock once the vigorous phase is over.
Never seal a fermenting vessel with a solid bung. Glass has burst and plastic vessels have deformed for exactly this reason, and the failure is unpredictable in timing.
Reading and misreading the airlock
- A bubbling airlock means gas is leaving the vessel. That usually means fermentation, but it can also mean the vessel is warming up and expelling gas.
- A still airlock does not mean fermentation has stopped. Gas escaping past a loose bung, a cracked vessel or a poorly seated grommet leaves nothing to bubble through the trap.
- The only reliable test of fermentation progress is a gravity reading taken over time, not the airlock.
- Suck-back happens when a vessel cools. The liquid in the trap is drawn inwards; keeping the level modest and using a clean liquid limits the consequences.
After fermentation, the airlock is not enough
An airlock protects a vessel from bulk air exchange, but it does not remove the oxygen already dissolved in the cider, and it does nothing about the headspace above the liquid once fermentation has stopped producing gas. During fermentation the carbon dioxide blanket is continuously renewed and the headspace is protected; the day the ferment ends, that protection ends with it.
From that point the governing practice is topping up: keeping the vessel full so that there is almost no headspace, or displacing what remains with carbon dioxide. Cider that has been left under an airlock with fifteen centimetres of air above it for six months will show oxidation and often volatile acidity whatever the airlock is doing.
Also answered on this page
Questions this page covers, so you can tell at a glance whether it is the one you want.
- Is my airlock supposed to bubble?
- What liquid goes in an airlock?
- Why has my airlock stopped bubbling?
Related
What people ask next
Questions readers ask about the things this page mentions. Each one goes to the section that answers it rather than to a page written to receive the question.
- How do i know when my cider has finished fermenting — Take two hydrometer readings several days apart: if the gravity has not moved, the ferment is over. Airlock activity is not a reliable test, because a slow ferment can produce gas too slowly to see.
- How do i use a hydrometer to measure cider — A hydrometer floats at a depth determined by the density of the liquid it is in. Sugary juice is denser than water, so the hydrometer floats high; as fermentation converts sugar to alcohol the density falls and it sinks.
- How long does cider take to ferment — A warm ferment with cultured yeast can finish in one to two weeks; a cool wild ferment in a cellar may take three months or more. Slow is not the same as stuck — the test is whether gravity is still falling.
- Can you fix a cider that has gone vinegary — Not really. Volatile acidity cannot be removed at small scale, and blending it away only dilutes it. The realistic options are to accept it, to blend a lightly affected batch into a much larger sound one, or to let it finish becoming vinegar.
- Is a glass demijohn a good vessel for making cider
- What is oxidation in cider
Sources
What this page rests on. Where a source is marked as registered rather than read, CiderHQ is recording that the body is authoritative on the subject without claiming to have worked through the document itself. See our evidence policy for what each state means.
The New Cider Maker’s Handbook: A Comprehensive Guide for Craft Producers
Claude Jolicoeur, Chelsea Green Publishing, 2013. ISBN 9781603584739 · reference work · bibliographic record verified, not opened 2026-08-25
Bibliographic record verified on 2026-08-25 against the Open Library union catalogue: Chelsea Green Publishing, 2013, ISBN 9781603584739, one edition recorded. That establishes the citation points at a real book in a stated edition, which is what a citation needs and is all it establishes. No copy was opened and nothing is quoted from it. The book itself is in print and not digitised in any open collection; where CiderHQ needs a figure from this territory it uses an accessible research source instead and says so.
The Science of Cidermaking and associated technical writing
Andrew Lea · reference work · passage verified 2026-08-24
Written by a food chemist who worked at Long Ashton on apple phenolics. Unusual among specialist cider writing in that it is primary-research-adjacent: the author is describing work he did, and cites the literature. This is why it is registered at tier 1 for chemistry while a general cider book is not.
Where to go next
- Cider making equipment — The rest of the equipment material, grouped by what each piece is for.
- How cider is made — The methods this equipment exists to carry out.
- Troubleshooting — The faults that are traced back to equipment and to cleaning.