Full Face Snorkel Mask Test and Dry Top Test: Comprehensive Analysis

Full face snorkel masks have grown popular for their natural breathing, better visibility, and anti-fog properties, but safety concerns exist. NIH research papers have documented issues with CO2 rebreathing and water intrusion resistance. In this video, Dr. David Zhang provides a detailed analysis and demonstrates how to test your mask for potential problems.

Understanding Mask Design

Every full face mask has two pockets: the eye-pocket and the oronasal pocket. Understanding the separate inhale and exhale paths, and the location of unidirectional valves, is critical for safety. The video traces these paths for two different mask models using colored shoelaces for visualization.

The 3-Step Mask Test

Step 1: Test unidirectional valve function. Step 2: Test the face-to-mask seal. Step 3: Verify that the breathing paths work correctly under conditions simulating actual use.

The 3-Step Dry Top Test

The dry top mechanism is designed to seal the snorkel when submerged. The three-step test verifies that the dry top seals properly, opens fully when above water, and drains correctly when water enters.

Safety Awareness

Full face masks should not be used for deep diving because equalizing ear pressure is not possible. They are designed for surface snorkeling only. The video references two NIH papers that document the specific risks.

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Why Full Face Snorkel Masks Deserve Scrutiny

The full face snorkel mask market has witnessed significant growth in recent years because of obvious advantages: natural breathing, masks conceal better than goggles, better visibility, and anti-fog properties. They are also far easier for beginners who do not intend to dive deep. However, you may hear some people advise against using them. Today we will have a detailed analysis and show you how to test your mask to spot potential problems.

Even though manufacturers are continuously focusing on product innovation to enhance performance and safety features, there are two potential problems to watch for. The first concern involves CO2 rebreathing. A 2023 NIH paper concluded that full face masks “may result in rebreathing due to non-unidirectional flow leading to hypercapnia.” A 2022 NIH paper noted that “mask models showed patterns of increasing breathing resistance with water intrusion, and this increased resistance could potentially create elevated levels of respiratory distress.”

Understanding the Mask’s Internal Air Paths

Three masks are compared here: two full face masks and one traditional snorkel. Of the two full face masks, one has a double tube and one has a single breathing tube. Both full face masks can be divided into two compartments: the oronasal pocket and the eye pocket.

For the single-tube mask, inhaled air is drawn down the snorkel and travels through the eye pocket first, then into the sealed oronasal pocket through a set of one-way valves. Those one-way valves are designed to isolate the oronasal compartment from the eye pocket during exhalation. A white shoelace is inserted to trace this inhale path visually.

When exhaling, CO2 passes through another one-way valve at the bottom. From there it can take two routes: out through a port directly into the water, or up through a separate exhalation channel in the snorkel tube. The tube is divided so that one half handles inhaled air and the other handles exhaled air. A red shoelace traces the exhalation path.

When the mask functions correctly, unidirectional airflow means the eye pocket contains normal air and only the oronasal pocket accumulates CO2. If the seals or valves fail, the larger eye pocket may also build up elevated CO2 — which is the scenario the NIH papers warn about.

The double-tube mask works differently. Air enters both the eye pocket and oronasal pocket simultaneously, and there are no unidirectional valves separating the two. CO2 is expelled through three paths: a large unidirectional valve at the front into the water, back through the tubes along the inhale path, and partially into the eye pocket where some CO2 may remain. How much CO2 is trapped in the eye pocket depends on the pressures present under water and in the tube. This design is still concerning, and you should not use this type of full face mask for extended time underwater.

As the NIH paper states: “At least some full face snorkel masks enhance the risk of hypercapnia and possibly hypoxia due to rebreathing. Manufacturers and future snorkelers should be made aware of this new information to prevent unsafe situations.”

The 3-Step Mask Test

The following three steps verify that the unidirectional valves and face seal are working correctly.

Step 1 — Tape the bottom exhaust port. Cover the bottom port with tape so that when you exhale, air cannot escape through it. This forces all exhaled air to take the internal path.

Step 2 — Test the face-to-mask seal. Inhale while covering the entire top of the snorkel tube with your palm. Because the tube top is the only remaining inlet, you should feel suction pressure on your face. If the seal leaks, you will not feel significant pressure.

Step 3 — Test the unidirectional valves. Exhale while covering the half of the tube marked by the red exhalation shoelace, blocking the exhale path. The mask should fog up, and you should feel high pressure building inside the oronasal pocket. If there is a leak in the valves, pressure will be absent. Do not use the mask if the unidirectional valves leak.

The 3-Step Dry Top Test

Full face masks and some traditional snorkels include a dry top design. A floating ball inside the breathing tube rises to close the valve automatically when submerged or when a wave washes over, preventing water from being inhaled. When the snorkel top is above water, the ball drops and opens the tube. A washer inside the tube provides the seating surface for the ball to seal against when it floats up.

On the exhalation side, a unidirectional valve allows air out but remains closed otherwise, preventing outside water or air from entering through the exhale channel.

The three steps to verify dry top function are:

Step 1 — Point the tube downward. This simulates the ball floating up as it would when submerged. Suck on the tube; you should feel elevated pressure confirming that the ball has sealed the inhale path.

Step 2 — Point the tube upward. This simulates being above the water surface, where the ball drops and opens the path. Suck on the tube; you should feel no resistance and air should move freely.

Step 3 — Verify the exhale path is not clogged. Blow into the tube; you should feel no resistance regardless of which direction the tube is pointing. You can blow directly into the tube or through the mask.

If the dry top fails on a full face mask, or if the face seal leaks, water can enter and drain through the bottom of the mask. Note that the dry top design can occasionally become clogged at the top of the snorkel, making breathing difficult, and may itself contribute to increased breathing resistance.

Limitations of Full Face Snorkel Masks

Full face snorkel masks are designed for surface snorkeling and come with two significant limitations for deeper use.

The first is ear pressure equalization. Pressure at the ocean surface is 14.7 psi, increasing by 0.445 psi per foot of depth. At 10 feet, the pressure difference across your ears is 4.45 psi. To equalize, most snorkelers pinch their nostrils shut and gently blow through the nose. Some full face mask designs include a flexible silicone nose section specifically to allow nose pinching. If pinching is not possible with your mask, you can try equalizing by swallowing or rotating your jaw instead.

The second limitation is buoyancy. The larger airspace inside a full face mask contributes to your positive buoyancy, making it harder to dive beneath the surface compared to traditional goggles with a separate snorkel. If you enjoy free-diving even a short distance below the surface, a full face mask is not the right choice.