The Surface Charge Drill: Understanding the Limits of Your Mouthfill

A Simple Way to Think About Mouthfill

In this document, I'm going to talk about my take on the most popular mouthfill drill - the surface charge.

To understand everything I am about to discuss, it helps to think about mouthfill like this.

At some point before reaching the residual volume of the lungs, we bring up as much air as we can store above the glottis - filling the throat and the oral cavity.

I like to think of this volume as the throat and oral cavities' vital capacity.

As you continue to descend, pressure is going to decrease that volume, at some point you're no longer able to pressurize that volume of air, I think of this as the residual volume of the airspaces in the head.

Defining EQVC and EQRV

Let's call these terms EQVC (the amount of air that we can charge into our mouthfill) and EQRV (the volume at which we are no longer able to equalise our ears).

What the Surface Charge Drill Is Really Measuring

The main purpose of the surface charge drill is to find out how much pressure we can move through before reaching our EQRV.

What Different Levels of Mastery Look Like

In my experience, somebody with an OK understanding of mouthfill can take a mouthfill at 0m and take that down to 20m

Somebody with quite a good handle on mouthfill can go from 0 - 25m

And somebody who has really mastered the mouthfill can go from 0 to 30m

Using Surface Results to Predict Depth Potential

Finding this number is great because we can then predict how deep we should be able to dive with our mouthfill.

0-20m in a surface drill means you can move through 3 atmospheres of pressure. We would then multiply the pressure (in bar) that we take our mouthfill by 3. The answer is the bar that we will reach EQRV.

So let’s say that we do our final mouthfill charge at 25m (3.5 bar).

3.5 x 3 = 10.5 bar or 95m

A Real-World Baseline: 610ml to 160ml

If you're performing mouthfill correctly, these numbers are entirely dictated by how large your EQVC is AND how small your EQRV is.

I have measured how much air I can charge into my mouthfill. It is 450ml. I can do a surface charge to 28m (3.8 bar) consistently. Therefore with a full mouthfill I am holding 610ml of air above my glottis. With this information, and a little help from ChatGPT I can calculate that my EQRV is 160ml

How Anatomy Shapes Your Limits

So to use me as a baseline, an EQVC of 450ml and an EQRV of 160ml means I can go from 0-28m.

Someone at an anatomical disadvantage to me may have an EQVC of 400ml and an EQRV of 180ml. This person will only be able to move through 3.2 atmospheres of pressure - this person's surface charge will go from 0 - 22m

Someone at an anatomical advantage may have an EQVC of 500ml and an EQRV of 140ml. Which would take them from 0 - 35m

The Relationship Between EQVC and EQRV

As you can see, if you have a relatively small EQVC and a relatively large EQRV, you're not gonna be able to reach the same numbers as me.

And vice versa, if you have an exceptionally large EQVC and a small EQRV, you're going to be able to go much deeper.

When the Drill Becomes Misleading

The problem with this drill is a lot of the time, people aren't truly using mouthfill. They may be using mouthfill for the first ¾ of the exercise. Then as the air starts to reduce in volume, and they start to get closer to EQRV - instinctively, these people reverse pack more air up without even realizing it. It only needs to happen once or twice and then, instead of your equalization running out at 25m, where it should have - now the person is going to 30+ meters.

Training Awareness Without Going Deep

One use for the surface charge drill would be to move through the entirety of the mouthfill without being deep. This means you should have more focus, more awareness, and be able to spot and improve things you might not notice on a deep dive.

Why Reverse Packing Invalidates the Drill

But if you're using this drill to try and understand how deep you can go on mouthfill - and you're reverse packing, then the drill becomes completely useless and it not going to have any real-world application.

Before You Use This Drill Properly

Before using this drill to find out how deep you should be able to take your mouthfill. You need to have already deadened your instinct to reverse pack, and built a high awareness of the latter stages of mouthfill.

Learning to Feel the End of Your Mouthfill

A simple but extremely useful drill for this is - instead of seeing how deep you can go. Start to experiment with what it feels like to completely run out of air. Do shallow dives, with little to no charge and full lungs. You're gonna run out of air for equalization very quickly - and that is the point.

Changing the mindset from seeing how deep you can take the charge to feeling what it is like to run out of mouthfill can make all of the difference.

When You’re Ready to Apply the Drill

If you're able to run out of air between 5 and 10 m, depths are becoming consistent, and less of a charge means less depth until EQRV. Then maybe you're ready to start the drill with a full surface charge and the information gathered will actually be useful.

Conclusion: Turning Awareness Into Reliable Depth Predictions

The surface charge drill is only as useful as the awareness behind it. When done correctly, it gives you a clear and measurable relationship between your EQVC, your EQRV, and the pressure you can move through. But the real value isn’t just in the numbers—it’s in understanding the sensations that define the limits of your mouthfill. If you can consistently recognize the point where the air is truly finished, without interference from reverse packing, then the predictions you make from this drill start to reflect reality. From there, depth is no longer a guess, but an extension of something you’ve already felt and understood in a controlled way.