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Density Discords: Understanding and Applying Gas Density Research

Do you know the density of your breathing gas at your planned working depth? New research conducted by Gavin Anthony and Simon J. Mitchell suggests that you better! A gas density of 6 grams/liter (g/l)—the equivalent of diving nitrox 32 at 110 ft/34 m, or trimix 18/35 at 200 ft/61 m—significantly increased the risk of dangerous CO2 retention, resulting in test subjects experiencing problems at three times the rate of divers using gas even 1 g/l less dense. Divers Alert Network risk mitigation leader Reilly Fogarty explains.

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By Reilly Fogarty

Header photo by Derek Remmers

As divers we’ve learned to adapt to the effects of depth without fully understanding the mechanisms behind them. Gas density is one of those misunderstood dynamics lurking in the background of many of our dives. We minimize our work at depth, improve our fitness, and add helium to reduce narcosis and work of breathing (WOB) at depth, but these are reactionary responses. While we typically can’t discern the difference during a dive, gas density and the accompanying increase in WOB causes decreased respiratory capacity, increased CO2 production, and decreased ability to eliminate CO2 in the blood. Some agencies (e.g., GUE) have been proactive in modifying standard gases and tailoring training to adapt to these concerns, but the forefront of hyperbarics research is constantly pushing us toward increasingly conservative gas choices.

Just to clear the air, we’re working with two terms here — WOB and gas density. WOB is an integral of pressure as a function of volume that’s used to measure the effort required to breathe. A high WOB means it takes more effort (measured in energy, typically joules) to draw breath (a measure of volume, typically in liters). High WOB results in increased CO2 production, and that CO2 increase can result in hypercapnia, narcosis, and loss of consciousness among other symptoms.

Gas density is a measure of mass per unit volume, measured in grams per liter (g/l). A high gas density means a given volume of gas weighs more and takes more effort to move, resulting in increased WOB. Increased gas density also skews the pressure gradient between inspired and arterial CO2, resulting in further decreased CO2 off gassing efficiency and a recurring system that results in further complications.

In the past, these factors were taken for granted, but recent research by Gavin Anthony and Simon J. Mitchell from the University of Auckland Department of Anaesthesiology (see link below) has cast gas density in a new light. Working with both open-circuit and rebreather divers, Anthony and Mitchell found that gas density near the 6 g/l mark significantly increased the risk of dangerous CO2 retention during dives, resulting in their test subjects failing more than half their attempted dives and experiencing issues at more than three times the rate of divers using gas even 1 g/l less dense. Their takeaway from this research was an ideal maximum gas density of 5.2 g/l (equivalent to air at 102 fsw/31 msw), and a hard maximum of 6.2 g/l (equivalent to air at 128 fsw/39 msw).

The implications of these results are both complex and far-reaching. Recreational and technical divers alike face issues with gas density under these new guidelines. The use of EANX 32 (32% oxygen, 68% nitrogen) at 110 fsw/34 msw exceeds recommendations with a gas density of 5.66 g/l and more than 6.54 g/l at 132 fsw/40 msw. Technical divers using trimix 18/35 (18% oxygen, 32% helium, balance nitrogen) will overshoot recommendations, reaching 6.93 g/l at 200 fsw/61 msw and a PO2 of just 1.26, and trimix 10/70 (10% oxygen, 70% helium, balance nitrogen) reaches an impressive 6.73 g/l at 396 fsw/121 msw and 10.29 g/l at 495 fsw/151 msw.

Checking supplies. Photo courtesy of GUE archives.

The reality is that gas density is another in a series of dynamic risk factors that divers of all levels must contend with. Treating gas density like DCS by acknowledging and mitigating the hazards via personal fitness, decreased work at depth, and appropriate dive planning has worked in the past and will continue to work. What this research shows us is why we face the issue we do at depth, a possible understanding of corollary hazards like DCS and IPE, and how we might be able to use that data to keep ourselves safer.

View the Respiratory Physiology of Rebreather Diving research in full. For questions about gas density or comments about this and future articles, reach out to the author at RFogarty@DAN.org.

Gas Density Calculator

Here is a simple gas density calculator for you to download, created by Brendon Allen aka RainPilot, at ScubaBoard.com, that enables one to calculate the gas density of their bottom mix at their planned depth (in ATA). It also includes equivalent narcotic depth (END), and partial pressure of oxygen (PPO2) at the planned max depth.

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Reilly Fogarty is a team leader for its risk mitigation initiatives at Divers Alert Network (DAN). When not working on safety programs for DAN, he can be found running technical charters and teaching rebreather diving in Gloucester, MA. Reilly is a USCG licensed captain whose professional background also includes surgical and wilderness emergency medicine as well as dive shop management.

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Cave

Plan The Shoot, Shoot The Plan

Gas planning is an essential part of tech diving but how does it apply if you’re planning to conduct a photoshoot in multiple specific locations in the overhead environment of a cave? Arguably one of the most artful cave photographers today, and a high-level tech diver, Fan Ping explains how he calculates gas requirements when making pretty pictures in the dark!

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By Fan Ping. Header image: Bedding Plane at Jug Hole by Fan Ping.

Plan the dive, dive the plan. That’s something I have been hearing since the beginning of my diving career but never really mastered until I started my cave diving training with Global Underwater Explorers (GUE). I was surprised by how powerful dive planning can be as a tool, down to a minute, a meter and a few bars. Of course, there is flexibility, but the whole point is you will be aware of what is going to happen next, and have control over the entire process of the dive.

Planning can also apply to underwater cave photoshoots and filming. As a fulltime underwater photographer and director of photography (DP), I plan my shoots in the caves all the time and teach it as a part of my Underwater Cave Photography Course. It definitely makes my job much safer and more efficient. There are two parts of the shoot plan: diving and photography. They work together and can sometimes be complicated, especially when shooting at more than one location. I usually start with the diving part. Knowing exactly where I am going for the photo, I can easily calculate how much time and gas I am going to use to get to the location, and then recalculate a third so I know how much time I have to shoot the photo.

Then I plan the photo part, usually based on a sketch with lighting indicated. Having a sketch of the photo can be very helpful, as it tells me how many lights I am going to need in total and where to put them, both on location as well as when traveling with them. I will also know how much time I am going to need to place and retrieve them, and that adds to the total bottom time too, so I can have a relatively accurate time for actually clicking the shutter.

Plan The Shoot on CCR

It’s the diving part again after the shoot—whether to a second location or to the exit—and in the end, I will have a deco time and total runtime, so I can make sure we are not locked in the park and have somewhere to go for dinner.

Red is in, blue is out, shoot location is in yellow. Map created by Jeff Hancock, partially shown for planning purpose only.

Let’s start with a more straightforward example with one location on rebreather. My buddy Derek Dunlop and I planned a photoshoot at the fissure on Sweet Surprise line in Ginnie Springs. We wanted to scooter to the jump at 670 m/2200 ft on mainline in 20 minutes, drop DPV and sidemount bailout, then swim for another 200 m/656 ft to the shoot location in 20 minutes. The depth of the location is about 28 m/92 ft, which is also the maximum depth of this dive, and the average depth is about 24 m/79 ft before 6 m/20 ft deco, so it was well within our bailout radius, somewhat conservative considering the flow in this cave. (I have LP50 or 7.8L doubles + 1x sidemount 11L, Derek has 2x LP85 or 12L OC bailout. Assume we both have 11L x 2 x 200bar = 4400L OC bailout gas, SCR = 20L/min, ATA = 4, so we have 4400/20/4 = 55min to get back to the cavern. Swim speed = 10m/min, DPV speed = 40m/min, and it will take no more than 40 min in a real situation.)

We plan to shoot until the batteries of the lights die, which will take 40-45 minutes, plus 10-15 minutes to place and retrieve the lights, so it’s a 1 hour shoot at the location. That gives us a 150 minute bottom time plus 25-30 minutes of deco at 6m/20 ft (O2 setpoint: 1.2 bar), 3 hour total runtime.

Derek at the fissure. Most walls are very dark as it’s less traveled.

I usually use the GUE EDGE, i.e., GUE’s predive checklist, for planning, as it is a very good base to start with, no matter if you were trained with GUE or not, and it is very difficult to miss important information with it:

Goals: Photo at fissure on Sweet Surprise line. 

Unified Team: Derek diver # 1 and model, Ping diver # 2 and photographer.

Equipment match: Derek has 1 light on tripod, Ping has camera and 4 lights.

Exposure: Max depth 28m, average depth 20m; 20 min on DPV to jump, 20 min swim to shoot location, turn at 100 min. Total runtime 180 min.

Decompression: 30 minutes deco.

Gas: Sufficient OC bailout gas for each diver, 5.7L AL tank filled with oxygen to 200 bar.

Environment: Normal flow.

Filming at Jug Hole back in 2019.

Plan the Shoot—Open Circuit Edition

Here is another example of a short but multiple location photo shoot at Jug Hole in Ichetucknee Springs State Park, Florida, with my buddy cave diving instructor Joseph Seda as the model.

We planned to take a photo at the Diamond Sands restriction first, then an HDR panorama photo in the bedding plane right after the reaper sign, and a cavern shot if not too late.

The Diamond Sands restriction is only 80 m/262 ft on the mainline, but the flow in this cave is strong, and the bedding plane at the beginning is very low, so my swim speed would be about 8m/s, and it will take me 10 minutes to get to the first shoot location from the cavern. 

Average depth for this part is about 15 m/49 ft, maximum depth is 22 m/72 ft at the restriction. I have a very standard 20L/min SCR, so with 2 sidemount LP85 steel tanks (12L) I am going to use roughly 30 bar in each tank (5 bar/5 min with 12L doubles) before I can start playing with the lights.

Map created and authorized by Adam Hughes.

My tanks are filled with 32% to 260 bar (welcome to cave country!), so theoretically I have 260-30-30×2=170 bar to use for the first shot, with the depth of 22 m/72 ft, it gives me about 35 minutes before I have to turn the dive. 

Lighting is relatively simple here, just 1 light from the model’s back and 2 on the camera, so it will take only a couple of minutes to set up. Diamond Sands restriction is famous for the rolling sands in the flow when a diver passes, and that’s what we want in the photo, obviously from the exit side, and that makes my job easier, as Joseph will be the one placing the light in the back and coming back out of the restriction to pose. So he is diver #1, going in with two lights on tripod (one as backup and for shot two).

Going back to the bedding plane for the second shot only takes about 5 minutes, and getting out of the cave from there will take no more than 5 minutes too, which is about 15 bars in each tank. So usable gas for the second shot is 260-30-170-15-15×2=15 bar.

Diamond Sands restriction. Enlarge to see the rolling sands.

At 15 m/49 ft it gives me only 5 minutes, and I am supposed to get out of the cave with at least 50 bar in each tank, so we will have to shorten the first shot in order to get the second shot, which is a lot more complicated with 6 lights in total to light up the whole scene.

In the end, we got a shoot plan like this with GUE EDGE:

Goals:  Photos at Diamond Sands restriction and in bedding plane.

Unified team: Joseph diver # 1 and model + light monkey for shot 1, Ping diver # 2 and photographer + light monkey for shot 2.

Equipment match: Joseph has 2 lights on tripod, Ping has camera and 4 lights.

Exposure: Max depth 22 m/22 ft, average depth 18 m/59 ft; 10 min to location one, 20 min for shot 1; 5 min to location two, 30 min for shot two, 5 min to cavern.

Total runtime: 70 min.

Decompression: Minimum deco.

Gas: 260 bar to start, 170 bar to finish shot 1, 80 bar to finish shot 2.

Environment: Strong flow, restriction and sandy bottom at location one, very low bedding plane at location two.

*This calculation is relatively conservative, we have twice the amount of gas we need to get out of the cave at any point.

Plan for Safety

The purpose of planning the photo shoot is to make sure we don’t put ourselves in danger while being too focused on the camera in underwater caves. Open water photography is a lot less stringent in terms of planning; however, overhead environments require more precise ideas for how much time it takes to do the job, especially on open circuit. Good planning also makes the shoot more efficient by reducing unnecessary communication and setting up the scene as a team, which eventually leads to a safer dive. There is not one single photo worth a diver’s life, but there are countless caves that are worth diving with a camera

“There is not one single photo worth a diver’s life, but there are countless caves that are worth diving with a camera.”

Dive Deeper

InDEPTH: Cameras Kill Cavers… Again by Natalie Gibb

Here are some of Ping’s other stories:

InDEPTH: Close Calls: I Ripped My Drysuit a Kilometer Back In The Cave by Fan Ping

InDEPTH: Underwater Galaxy by Fan Ping


Fan Ping is a photographer and filmmaker based in Atlanta, Georgia, USA, and is dedicated to showing the beauty of the underwater world to people through his lens. He is specialized in combining artistic elements with nature and complex lighting skills in overhead environments, and this artistic style has brought him international acclaim, including awards from many major underwater photo/video competitions. You can follow his work on Facebook and Instagram: Be Water Imaging.


Be Water Imaging’s Underwater Cave Photography Course is a modular course that includes unique lighting skills and advanced photography techniques in underwater caverns and caves, and shoot planning is a very important part of the course. For more details please check my Be Water Imaging website, and contact Ping at: info@bewaterimaging.com.

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