Frenzel: Should you transfer air to your mouth to dive to 30m?

In this article we will focus on the technique needed to dive in « classic » Frenzel (there is no question of « mouthfill », « charge », « reverse pack » and other advanced techniques…) to a depth of, say, 30/35m (but it can work much deeper).


Whilst talking with some instructor friends I recently realized that even on this type of « shallow » dives we do not always use the same equalization technique:

On a personal basis (I’m not the only one, I assure you) I just repeat in the Frenzel a sequence of « equalization, release, equalization, release… ». (I will come back later on what that means exactly). I do not think about the air in my mouth, it comes naturally, all by itself. Others add a step to this sequence, «reload» the air in their mouth regularly (so that they can make a few equalizations or just one), and start again once this air is gone.

In discussing this difference with equalization specialists I was told that « my » version was theoretically impossible and that I simply should not be aware of bringing air back into my mouth. The two reasons I was given for this (by different people) were as follows:

1st reason: « Air has a natural tendency to come to the surface, so upside down it’s impossible for it to go down into the mouth. »

2nd reason: « The air naturally goes where there is the least pressure (diffusion principle) and the mouth being deeper than the lungs (upside down), it naturally goes in the direction mouth => lungs » (with the idea that the pressure is created first at the lowest point).

Proponents of one (or both) of these arguments therefore suggest that « my » version is theoretically only possible by diving feet first, otherwise there must be an « action » to transfer air.

Let’s take a closer look at these two reasons…

1st reason: As we saw in a previous article, the natural behaviour of air is to occupy all the space available. It is not because we are under water that the air in our body « floats », there is no fluid to make this air ‘float up’ in our lungs. On the other hand, air has mass, and in the presence of gravity, even when it is floating up in the water column, an air bubble continues to exert pressure (an extremely small one) downward ; It is only rising to the surface because Archimedes’ thrust is greater than this force. In other words, when you are « losing » in arm wrestling, your force is always present in the opposite direction of the movement you are observing: it is the same for everything you can see floating, including the air. This argument is therefore not at all admissible. I understand that it is counter-intuitive, but please take the time to think about it : air is not « pulling » you up, it’s water that is PUSHING you up. This video explains it better than i ever could

2nd reason: Now it gets interesting … This time the logic is good and I agree 100% with the idea that the pressure difference between 2 spaces is the only thing to look at to understand the natural direction of the air! However, although good, this logic is unfortunately misused here and forgets a crucial element …

A small (easy) trick question: if I dive with my closed metal flask full of air to a depth of 5m, what will be the air pressure inside? 0.5bar? 1bar ? 1.5bar ? 2bar ?

If you answered 1.5bar, you have fallen into the trap.

It is the decrease in volume that generates the pressure…

A metal flask is a very difficult object to compress and 0.5bar extra (down to 5m) of external pressure will not be enough to deform it: and without compression (without volume decrease) there is no pressure increase! The pressure inside our flask has not changed; it is still at 1bar, as it would be on the surface!

If you are a maths addict (category to which I do not belong):
P (pressure) = K (quantity) / V (volume)

Our metal flask is closed so the quantity of air did not move, and because of its elasticity (very very low) the Volume did not move either… Therefore, the pressure remains the same!

« But the human body is flexible » you may say!

Well, it depends… Not all areas have the same flexibility, sinuses for example are not at all flexible where the lungs are flexible as long as the diaphragm can move up (bloodshift also plays its role in the propensity to compress the air). Anyway, to understand in which direction the air goes it is not so much a question of knowing if an area is flexible or not, but more importantly if it is MORE flexible than the area to which it is connected or not… In other words, imagine our metal flask at the end of which we attach a plastic bottle: whether I dive with my assembly in one direction or the other, the air inside this space will always go from the plastic bottle to the flask. Indeed, the air in the plastic bottle will increase in pressure due to the compression of the plastic bottle and will therefore move towards the flask whose air would otherwise have remained at surface pressure (the pressure naturally balances in the same volume).

Here the example is extreme on the differences in rigidity (metal Vs plastic bottle) but the principle will be the same regardless of the materials used: what matters is to know which will first transmit the external pressure to compress the air, and then the air will travel naturally in the direction « high pressure => low pressure » until equilibrium. To be a little more precise, we can easily add as a parameter the additional external pressure of the deepest part: if we consider a gap of 50cm (the deepest part would be 50cm « lower » than the shallowest part), this means an additional pressure of 0.05 Bars (0.05kg per cm2). In summary, if you have a zone A connected to a zone B that would be 50cm deeper (the whole forming the same volume), to understand in which direction the air will « naturally » go as our volume goes down in depth, we must solve the following equation:

RIGIDITY A – (RIGIDITY B – 0.05kg/cm2)

-If you get a positive result, this means that by combining its properties with the excess external pressure, zone B transmit the pressure first and therefore the air goes in the direction B => A (which would be the case for example if A and B had exactly the same properties).

-If you get a negative result, it means that despite the excess external pressure, zone B will transmit the pressure AFTER zone A and therefore the air goes in the direction A => B.

-Finally, if you get 0, it means that the 2 zones compress at the same rate and that the air does not need to move according to the principle of diffusion between the 2 zones.

I’m sure there are much more elegant ways of presenting things and if someone has the background to do so, he is more than welcomed. However, I think the idea behind it is quite simple and I leave it to you to make it your own.

Coming back to the human body, I do not have any precise and « official » scientific data on the subject but I will bet (which seems quite reasonable to me) that the belly (and the propensity of the diaphragm to rise) has the capacity to be more flexible than the oral cavity (protected by our jaw and larynx). I think that this flexibility gap is potentially large enough to absorb the 0.05Kg/cm2 of extra pressure on the oral cavity. Moreover, this demonstration does not take into account the bloodshift which contributes to decrease the volume of the lungs and thus to increase the pressure (there is no equivalent mechanism in the oral cavity) which again favours the diffusion of air towards the mouth.

So as long as the lungs first transmit the external pressure the natural direction of the air is to travel in the direction « Lungs => Mouth », even head down.

The 2 stages of the Frenzel: equalization and general release.

Since you started reading this article, how have you been breathing? It’s a good bet that you are breathing through your nose. This means that by default when you are relaxed, your soft palate and glottis are open, it’s good news you’ll see! Moreover, the equalization consisting in a contraction of the tongue/larynx to decrease the air volume of the mouth, allows us to affirm without too much of a problem that when our mouth is « relaxed », it has a greater volume of air (still speaking « basic » equalization without mouthfill !).

After each equalization, there is an increase in volume when you release the pressure (the tongue is relaxed again) but with a smaller amount of air than before (this air now being in your ears).

An increase in volume combined with decrease in quantity => decrease in pressure in the oral cavity.

Because of this decrease in pressure the air will travel by itself into the mouth if the glottis relaxed and you will be able to repeat the operation.

In addition to the « natural » direction of the air going towards the mouth the post-equalization release creates a reduction in pressure accentuating the phenomenon!

If you are an equalization nerd, it is the exact same mechanism of a reverse pack (decreasing pressure in the oral cavity). Just done in a « natural » way.

Equalization issues…

« Well, that’s all well and good, but why am I out of air in my mouth at 5m/10m/15m/20m/25m/30m while I know I can equalize in Frenzel? »

Well I don’t know how to answer this question directly, there are several possibilities, let’s explore them together:

1. You have two separate volumes instead of one. If your glottis is tense and therefore closed and tight it means that you create a strong separation between the volume of your mouth and the volume of your lungs. The diffusion cannot therefore take place and it is normal that the air does not recharge itself. The glottis only needs to be closed during the equalization action, keeping it relaxed the rest of the time you will allow the air to flow naturally to your mouth. Solution: dive more relaxed! Don’t be « afraid » to breathe out underwater, if you have your mouth closed it won’t happen, especially at depth. To persuade you, observe how you need very little energy to hold your breath: you can thus have fun doing short (20 seconds will be enough) static breath holds in neutral lungs (FRC) while using the absolute minimum of tension to « hold » the air. You will see that it still works with very little amount of tension !

2. You « refuse » the pressure: remember, the key point is the difference in elasticity between two spaces. The trick is that this difference is not « constant and guaranteed » as it evolves as you go down (full lungs compress much more easily than empty lungs) and with your general level of relaxation; if you have tension in your abdominal area, this will decrease the elasticity of the space that is supposed to generate the pressure. Less elasticity -> less pressure generated -> less pressure difference with the mouth -> less diffusion -> less air naturally in the mouth! Solution: Dive more relaxed! Forget your stiffness: only your legs have to work (in constant weight), the rest of your body (especially your belly) has no reason to be mobilized and can behave in a neutral way as if you were on a « walk ». You can also probably work on your freefall position and make sure you are « slouched » when you do it; imagine the walking position of an old man with his knees bent and his upper body curled forward.

3. You don’t relax your mouth/tongue enough after an equalization: for X reason, once your Frenzel equalization is done you don’t (or not enough) relax your tongue. So even with a relaxed glottis and belly, the space of your mouth remains considerably reduced (potentially reduced to its minimum even) so the air has nowhere to go and it’s normal that you can’t continue to equalize. Solution: dive more relaxed! What position is your tongue in right now? It is possible that « by default » it is already contracted and stuck to your palate. If this is the case, take the time several times a day to relax your entire oral cavity (jaw, tongue, larynx, and glottis): then become aware of the sensation. During your next sessions, pay attention to the reproduction of this release of tensions after each equalization.

If we want to extend the question to deeper dives: up to 45m let’s say but there is absolutely no « rule » behind this number:

4. Lack of flexibility: yes, another term for elasticity! Your diaphragm exercises and intercostal muscle stretches will have a direct impact on the propensity of the air in your lungs to compress! Being more flexible will directly improve your equalization as the air pressure in your lungs will be able to continue to increase more easily and for longer periods of time.

5. Depth adaptation / bloodshift. If we assume that a person who is trained has a « better » bloodshift than someone who is less experienced, then in addition to the elements already mentioned, physiological depth adaptation also has a role to play in equalization. The bloodshift also contributes to compressing the air in the lungs and thus increasing the pressure in the lungs: be patient and give your body time to create the necessary adaptations to what you are asking of it.

For those who would like to read some pertinent discussions around this article, you can check the post on the freediving science Facebook page