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Deep Equalization Technique: The Mouth-fill

By: Emil Lars

Updated: Feb 2022

Mouth-fill is an equalization method that is performed when freediving to depths where the standard Frenzel equalization method will fail. What happens is that there will be a point in your dive when the volume of air in your mouth will be so compressed that you will no longer be able to use it to equalize. At the same time, the volume of your lungs will be just as compressed that it’s nearly impossible to move air from the lungs to the mouth for equalizing. To remedy the problem, you need to move a big bubble of air from the lungs to the mouth while you still can.  This is usually around 20-25 meters, just before your lungs reach residual volume.

While it takes time to master, mouth-fill is simple in theory and the process itself is just as simple if you break it down in steps. Mouth-fill is a more advanced version of Frenzel so we need to first understand the idea behind Frenzel. Some people do Frenzel without even thinking about it so it’s a good idea to discuss it so everyone understands what’s happening.  And a little anatomy always helps!

The Frenzel Technique

In Frenzel, the movement of the tongue or the larynx compresses the trapped air inside the mouth and the nasal cavity.  The pressure forces the air to enter the nasal cavity and tries to escape out the nose, but the nostrils are pinched shut.  Air cannot escape back into the lungs because the glottis is closed.  Air cannot escape out the mouth because the tongue applies an airtight seal either against the upper teeth or in the back of the mouth.  Nowhere else to go, the air enters the eustachian tubes and into the middle ear equalizing the pressure.

For a more detailed explanation of the Frenzel maneuver along with learning tips, exercises, and troubleshooting, refer to our Frenzel article.

Working out the math

Remember Boyle’s law from your first freediving course?  If you dive from the surface to 10 meters, the pressure will double in a matter of 10 meters. Makes sense? 1 Bar to 2 Bar. But… if you begin your dive from 30 meters, you will reach 70 meters before the pressure doubles. 4 Bar at 30 meters, 5 at 40m, 6 at 50m, 7 at 60m, 8 at 70 m. It took 40 meters before the pressure doubled!

This goes to show that the deeper you dive, the slower the relative effect of pressure will be. This is why mouth-fill is so effective on deeper dives. You do a mouth-fill at roughly 20-25 meters and it allows you to equalize to great depths. Simply because the relative effect of pressure is much slower at depth than from the surface.

You know by now that it becomes very difficult, if not impossible, to equalize beyond 30 meters because the air in the mouth gets too compressed and the volume significantly reduced. At the same time, because the air in the lungs is also too compressed, it’s very difficult, if not impossible, to move air from the lungs to the mouth. So why not fill your mouth with air before 30 meters? Say, 20 meters? This way you will have plenty of air to equalize with. If you think about it, you’ll get to 50 meters before the volume of air in your mouth gets reduced to half its size when the pressure will have doubled (3 Bar at 20m, 4 Bar at 30m, 5 Bar at 40m, 6 Bar at 50m). You know you can equalize without difficulty from the surface to 10 meters, the point where the volume of air in the mouth is reduced to 50%, right?

Breaking down mouth-fill step-by-step

1. Before you reach residual volume, say 20 meters, shift as much air into your mouth either by contracting your abdominal muscle or by pulling air using your jaw and cheeks. By now, your cheeks should be fat. Some may find that extending the chin forward, as it opens the glottis, makes it easier to shift air to the mouth.

2. Immediately close your glottis so the air doesn’t escape back into the lungs. You need to be mindful to keep the glottis closed throughout the entire mouth-fill process.

3. Tilt your chin back towards your chest if you extended it. This should help make it easier to keep your glottis closed.

4. Keep your soft palate in a neutral position.

5. As the air in the mouth slowly get compressed, start compressing the air in your mouth using your jaw and cheeks to equalize. With a mouthful of air, some may find it easier to keep a constant pressure instead of doing sequential pumps.

6. When you can no longer equalize with your jaw and cheeks because of the reduced air volume, start doing standard Frenzel. Reach new depths!

Simple right? A good way to practice mouth-fill dry is to let a little bit of air escape out your mouth to emulate the increasing pressure at depth. By doing this, you’ll get to experience equalizing with your jaw and cheeks and then the back of the tongue (creating movement in the larynx) eventually.

Exhale / FRC dives

An exhale dive or Functional Residual Capacity (FRC) dive is when the air in your lungs is neutral. It’s a passive exhale meaning you have the same volume of air as when you do a normal exhale.

So why are we talking about FRC? Exhale dives are a very effective way to train mouth-fill. When you do an exhale dive, you can simulate depth without going deep. Exhale or FRC dives are generally used as an effective warm-up technique before a big dive. However, it is also an effective way to train mouth-fill! On exhale dives, you could theoretically dive to 10 meters and be able to simulate a dive to 30 meters. That means we can train mouth-fill without having to go past 30 meters each time! How cool is that?! What an easy, convenient and safe way to practice the mouth-fill technique!

Exhale dives expose you more to the possibility of a lung squeeze. On an exhale dive, not only are great depths simulated but are reached at a greater speed which may not give you enough time for blood-shift to happen in full degree. Be very conservative with depth and have a regular thoracic stretching program in place! Remember that the more flexible your chest is, the less likely you are to experience a lung squeeze. Also, use a line you can pull down on (free immersion) so you have total control of your descent.

To train mouth-fill using FRC, perform a mouth-fill on/near the surface and descend head-first very slowly concentrating on not losing the air in the mouth as you equalize. This will take some time to master.

Common problems with Mouth-fill and how to fix them

1. Losing the mouth-fill by swallowing. This happens when the glottis is opened. Practice closing the glottis on dry land. This is perhaps the most common problem when starting to learn mouth-fill. I had this problem myself. Most everyone I’ve trained with had this problem too. But if I can learn it, so can you! So don’t be discouraged in the beginning. The only difference between you and me is that I have spent more time practicing mouth-fill. Practice makes perfect!

2. Air escapes through the lips. We don’t want to waste air. On land, practice not shifting more air in the mouth than it can contain and always have an airtight seal with the lips.

3. An important matter that is often neglected in understanding equalization is relaxation. When we tense up, equalization becomes difficult and even impossible. We tend to tense up when we aren’t comfortable with a dive. When you’re not ready or prepared for the dive, unable to relax through the descent, having early contractions, maybe feeling tired, anxious, or stressed, equalization suffers greatly. Tension causes us to raise the soft palate. The blockage makes it impossible to pump air into the eustachian tubes. It takes a certain level of awareness to realize this on a dive, but it is important to understand that a lot of equalization issues are due to tension. Eliminate the potential causes of stress and tension on a dive.

One key thing to remember when doing a mouth-fill is to go big and don’t swallow (if you will forgive the pun) – inflate your cheeks fully and keep your glottis closed. Concentrate on the technique rather than depth. It will come.  As I always say to students, with proper technique comes depth. The success and effectiveness of mouth-fill depend on many factors, but with proper technique, relaxation, and practice, mouth-fill will take you to new depths.

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This article is geared towards providing intermediate to advanced freedivers an easy, user-friendly and practical way to learn and understand the Frenzel and Mouthfill equalization techniques. The information herein is offered solely as informational and not intended as a substitute for instruction from equalization experts or from attending a formal freediving course. Freediving and breath-holding can be dangerous if practiced without proper knowledge and education. They should be practiced with a trained buddy or a qualified instructor. Attend a formal freediving course.