The Mouthfill Technique: How to Equalise Past 30 Metres

The mouthfill technique is a way of equalising at depth by taking one charge of air into your mouth before your lungs get too compressed, sealing it in with your glottis, and then using only that mouthful to equalise for the rest of the descent. It exists because a normal Frenzel depends on refilling the mouth from the lungs, and at some point on a deeper dive the lungs are squeezed so small that you can’t pull air up any more. If you keep stalling somewhere around 25 to 35 metres with ears that won’t clear, this is a likely reason.

It is also the fiddliest equalisation skill in freediving. Most divers spend months on it. That isn’t a sign you are doing it wrong.

Why Frenzel runs out of air at depth

A Frenzel uses a small pocket of air in the mouth and nose. Every few equalisations that pocket runs low, and you top it up by opening the glottis briefly and letting a little air rise from the lungs.

Now follow the lungs down. Take a diver with about 6 litres of air at full inhale. Pressure doubles by 10 metres, so that air takes up roughly 3 litres. At 30 metres, four times surface pressure, it is down to around 1.5 litres, which is close to the residual volume of many adults: the air that stays in your lungs even after you have breathed out as hard as you can.

Long before that point, pulling air up from the lungs gets harder. The diaphragm is drawn up into the chest, the ribs are compressed, and the small push that worked at 10 metres now moves nothing. That moment is often called your Frenzel limit. Where it sits depends on your lung volume, how flexible your chest and diaphragm are, and how good you are at moving air, which is why lung capacity is only part of the story.

How a mouthfill works

Think of it in three stages.

Charge

A few metres before your Frenzel limit, you open the glottis and move air up from your lungs into your mouth. Your cheeks fill slightly and your jaw drops a little to make room. This is the last time on the descent that the lungs contribute anything.

Lock

You close the glottis and keep it closed. Your mouth, the back of your nose and your Eustachian tubes now form a sealed system with a fixed amount of air in it.

Spend

As you go deeper, you equalise by squeezing that reserve in small doses: lifting the tongue, gradually closing the jaw and letting the cheeks flatten. Each equalisation uses a little of the volume. By the bottom your jaw is closed and your cheeks are hollow.

The mouthful obeys Boyle’s law too. Air you charge at 20 metres, three times surface pressure, is squeezed to half its volume by 50 metres, at six times surface pressure. Charge deeper and the air goes further, but the deeper you are, the harder it is to charge at all. That trade-off is what you spend months tuning.

When to take your mouthfill

There is no single right depth. A common starting point is to charge a few metres shallower than the depth where your Frenzel gets difficult, and your instructor adjusts from there based on how you dive.

• Too early and the fill compresses before you reach the bottom, so you run out.
• Too late and your lungs are already too compressed to give you a decent charge.
• Too big and you clench your jaw and cheeks to hold it, which wastes oxygen and makes the lock leak.

A medium fill you can hold with soft cheeks is usually more useful than a heroic one. For many divers, the charge falls around the start of the freefall phase, when the body is still and there is attention to spare.

Holding the lock: glottis, jaw and cheeks

The glottis lock

The glottis has to stay shut for the rest of the descent, through contractions and the turn. If it slips open, the mouthfill rushes back to the lungs and it is gone. Land drills that hold the glottis closed for longer than feels necessary pay off here.

Soft palate control

You want to let air into the nose a little at a time, not all at once. Some divers keep the back of the tongue against the soft palate between equalisations, sometimes called a K-lock, to separate the mouth reservoir from the nose and release only small amounts.

Jaw and cheeks

The jaw becomes your second piston. Closing it a millimetre at a time compresses the reserve gently. Cheeks help without straining. If your face looks like a trumpet player’s, the fill is too big.

Dry drills for the mouthfill technique

These build the control you will need, without any depth.

1. Cheek hold with nose breathing. Puff your cheeks and breathe slowly in and out through your nose for 30 to 60 seconds without losing the cheek air. This trains the tongue and soft palate to separate your mouth from your airway.
2. One-fill equalisations. Take a small mouthful, close the glottis, pinch your nose and see how many gentle equalisations you can get from it by slowly closing your jaw. Aim for smooth, small steps rather than a high count.
3. Charge after a relaxed exhale. Breathe out passively, as you would at the end of a normal breath, then draw a little air up into your mouth and lock it. This mimics charging with smaller lungs.
4. The glottis hold. With a mouthfill locked, keep the glottis shut for 20 to 30 seconds while your shoulders and jaw stay relaxed.

Do these sitting or lying down, a few minutes at a time. Stop if you feel any chest discomfort, and don’t turn the exhale drill into a strained, empty-lung breath-hold.

Learn it with an instructor, not from a video

Mouthfill is where self-teaching stops being sensible. The charging depth depends on your body, the mistakes happen at 30 metres and below, and some of the methods coaches use, such as dives on a partial exhale, raise the risk of lung squeeze if done badly. You want someone who can see your jaw and cheeks on video and set a careful depth progression.

Mouthfill usually appears on intermediate or advanced courses rather than beginner ones, but check the syllabus before you book. Our guide to choosing a freediving instructor includes the questions worth asking about depth training.

Safety: Mouthfill training happens at depth, and depth means a trained buddy on every dive, a lanyard on the line and a conservative progression. Never practise exhale dives or mouthfill dives alone, and never use them to chase a number. If your ears won’t clear, turn.

What to work on before your next depth trip

A mouthfill is not a fix for a weak Frenzel. If your Frenzel isn’t reliable head-down at 15 to 20 metres, that is the job first. Once it is:

• Do the cheek hold and one-fill drills three or four times a week for a month before the trip.
• Add chest and diaphragm flexibility work, which helps you charge with compressed lungs. Our post on stretching for freediving has a routine.
• Film yourself doing the land drills and look for tension in your jaw, neck and shoulders.

An online community for Indian freedivers is on its way here at freediving.in, so stay tuned.