Equipment

The floating lid and the variable-capacity tank

How do I stop air sitting on top of a half-empty tank?

In short

A variable-capacity tank is an open-topped cylinder with a lid that sits on the liquid rather than on the rim. The lid is lowered onto the surface and its seal is expanded against the wall, so the volume of the vessel becomes whatever volume of cider is in it.

It exists because vessel sizes and batch sizes never match. Racking loses volume, blending changes it, and a tank correctly sized for a ferment is oversized for what comes out of it — and the gap is filled with air.

Air above a finished cider is the mechanism behind most of the oxidation and most of the volatile acidity that spoils otherwise sound batches, so a device that removes the gap entirely is worth more than its appearance suggests.

The headspace problem, stated properly

During fermentation a vessel protects itself. Carbon dioxide is produced continuously and, being heavier than air, forms a blanket over the surface that is renewed faster than it can be displaced. Nothing gets in. The day the ferment stops, that protection stops with it, and from then on the headspace is whatever the vessel admits.

Two organisms take immediate advantage. Acetic acid bacteria are obligate aerobes — they cannot work without oxygen — and a cider under air at the surface gives them exactly what they need, which is why a half-empty vessel goes to vinegar and a full one does not. Film yeasts colonise a liquid surface and grow as a visible veil, and the area available to them is the area of the headspace.

Chemical oxidation is the slower half of the same story. Oxygen dissolves at the surface and is consumed by phenolic compounds, producing quinones and eventually the browning, flattening and sherry-like character of an oxidised cider. A vessel also breathes: as it warms and cools it draws air in and pushes it out through whatever opening it has, so a headspace under an airlock is not a sealed volume of air but a continuously refreshed one.

A carbon dioxide blanket helps and does not settle the matter. Gas laid over a surface is diluted by diffusion and lost through every joint, and over months it needs renewing. It is a good answer for a week and a poor one for a year.

How a floating lid seals

The commonest design is an inflatable gasket: a hollow tube of food-grade elastomer running around the rim of the lid, inflated through a valve with a hand pump or from a low-pressure gas line until it presses firmly against the tank wall. The lid is set on the cider, the gasket is inflated, and a small amount of cider is usually left standing on top of the lid as a visible tell-tale that the seal is holding — if the level on top drops, the seal is passing.

The other design is a liquid seal. The lid carries an upstand and a channel around it, or the lid sits within a channel formed at its edge, and the channel is filled with water, a neutral oil, a glycerine solution or a food-grade sealing liquid. Gas cannot cross the liquid in either direction. It is simple and has nothing to perish, and its weakness is evaporation: a channel that has dried out looks exactly like a channel that is working.

Either way, the lid has to match the tank closely. Tank walls are not perfectly round, particularly on a vessel that has been knocked, and a lid that seals at one point on the circumference and not at another is not sealing. The lid also has to be lowered evenly; a lid that jams askew on the way down is the standard frustration of the design.

Where they go wrong

A perished, cracked or hardened inflatable gasket, which looks intact and does not seal. Gaskets are consumables with a service life measured in years, not decades.
A gasket that has slowly leaked down. Checking and re-inflating is a routine task and is exactly the routine task that gets forgotten between racking and bottling.
A liquid seal channel that has evaporated over a summer, leaving an open path to the cider.
Cider left standing on top of the lid for months, which goes to vinegar in the open air and then drips back in through a seal that has begun to pass — a slow contamination that is easy to miss.
The underside of the lid and the inside of the channel, both of which hold residue and neither of which is reached by a spray ball. They are cleaned by hand every time.
Overfilling. A lid pressed down onto a tank that is too full has nowhere to go and the seal is forced.
Assuming the lid does the job of a lid. It seals against air; it does not restrain pressure, and a vessel with a floating lid is not a pressure vessel under any circumstances.

The alternatives, and where each is right

Read together, the table says something that a tank catalogue will not. A floating lid is a convenience for a producer who already owns tanks of the wrong sizes; a set of vessels in a range of sizes solves the same problem more cheaply and more reliably, and is what most small operations should buy first. Where the floating lid genuinely earns its place is at a scale where a spare tank is a serious capital item and racking down through three vessels is a day’s work.

Ways of dealing with a part-full vessel
Approach	Cost	How well it holds over months	Where it fits
Floating lid, inflatable gasket	High, and part of the tank	Very good if the gasket is checked	A producer whose batch sizes and tank sizes do not match
Floating lid, liquid seal	High, and part of the tank	Very good if the channel is topped up	The same, where a simpler seal is preferred
Topping up with a similar cider	The cider used	Complete, while the topping cider lasts	Anyone with more than one lot; the traditional answer
Carbon dioxide or argon blanket, renewed	Low per application	Good for weeks, poor for months without renewal	Bridging a short period, or protecting during an operation
Racking into a smaller vessel, or several	The cost of owning odd sizes	Complete	Small producers; the most reliable answer at any scale
Bag-in-box or a collapsing container for the remainder	Low	Good, within the format’s own shelf life	The last few tens of litres that fit nothing
Leaving the headspace and hoping	None	Poor; this is how sound cider becomes vinegar	Nowhere

A lid is heavy and a tank is a confined space

A floating lid for a tank of any size is a substantial weight suspended over an open vessel, usually on a hoist or a screw. Nobody puts a hand, an arm or a head under a raised lid, and the lifting arrangement is inspected like any other lifting equipment. The tank beneath it is a confined space: entering it is a permit operation with atmosphere testing, forced ventilation and a trained attendant outside, for the reasons set out under inert gas — carbon dioxide from a ferment sits in the bottom of a vessel and gives no warning at all.

Also answered on this page

Questions this page covers, so you can tell at a glance whether it is the one you want.

What is a variable capacity tank?
How do I store cider in a half-full tank?
Why does my cider go vinegary in the tank?

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.

What are carbon dioxide, nitrogen and argon used for in cider making, and how dangerous are they — Air is what damages finished cider, and the cheapest way to remove air is to displace it with a gas that will not react with the cider. That is the whole purpose of inert gas in a cider house.
What should i know about stainless steel fermentation tanks — A stainless tank is a welded vessel, usually cylindrical, in an austenitic stainless grade chosen for corrosion resistance in acidic, chloride-containing conditions. It contributes nothing to the cider and lets no oxygen in.
What vessel should i ferment cider in, and does the shape matter — The material question comes down to three properties: how much oxygen crosses the wall, whether the vessel adds or absorbs flavour, and whether the surface can genuinely be cleaned and sanitised.
Why does my cider taste like vinegar — Acetic acid bacteria have reached the cider and, given air, are converting its alcohol into acetic acid. The cause is almost always oxygen — an unfilled vessel, a leaking bung, or a slow transfer.
What is oxidation in cider
What is racking in cider making — Racking is siphoning cider off the sediment it has thrown into a clean vessel, leaving the lees behind. It clarifies the cider and, in traditional practice, is also used to slow a ferment by removing yeast with the lees.

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.

Cider and perry production technical training material
Peter Mitchell / Cider and Perry Academy · reference work · registered as competent for this subject
Searched again on 2026-08-25 and it remains the one printed source here that a bibliographic check cannot fix, because it is not a published work with an edition. It is course material issued to participants of a training programme, so there is no catalogue record to verify, no ISBN, and no lawful public copy. Its state stays `registered` for that reason rather than through inattention.
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.