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Standard Gases: The Simplicity of Everyone Singing the Same Song

Like the military and commercial diving communities before them, Global Underwater Explorers (GUE) uses standardized breathing mixtures for various depth ranges and for decompression. Here British wrecker and instructor evaluator Rich Walker gets lyrical and presents the reasoning behind standard mixes and their advantages, compared with a “best mix” approach. Don’t worry, you won’t need your hymnal, though Walker may have you singing some blues.

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by Richard Walker. Header image by Derk Remmers. Images courtesy of GUE unless otherwise noted.

🎶🎶 Predive Click: “Sunday Morningby Maroon 5

In 1990, Pam Tillis released a track called “Don’t Tell Me What To Do.” I tried listening to it, but neither country nor western are my thing. I always preferred a song with a similar but harsher sentiment from Rage Against the Machine, but sadly the editor won’t print the words. [Ed.note: No f*** way!]

Anyway, What has this got to do with technical diving? 

Global Underwater Explorers (GUE) is perhaps best known for its standardised approach to equipment configuration. You know, where those mean GUE divers force you to dress a certain way just so you conform to the fashion. But then you discover that it all has some sort of logical reason, and the more you fight it the more it makes sense. 

“Fight the Power,” right?

Well, you may have heard that GUE also forces you to breathe only certain gases. Obviously this is just to make your diving more expensive, increase your training needs, and the like. 

NWA said it best, but I can’t print that either! 

Most divers get started by breathing the most famous standard gas of all—air—and nobody objected to that back in the day. When we dive air, we actually begin to see some of the benefits of a standardised system. Breathing the same gas on every dive means that we start to develop a familiarity for our no-stop times and any decompression requirements. Most experienced divers are familiar with the “120 rule” where your bottom time plus your depth in feet should remain lower than 120 to stay inside the no-stop times. 

Lynyrd Skynyrd explained it when they sang “Simple Man.” 

Who can deny that, when breathing air, we are rarely concerned that our buddy might be breathing different gases requiring a different decompression profile or a different maximum operating depth? It’s simple and it works. 

These advantages of air come at a cost. Every diver is a bit different, but it’s generally accepted that mental performance is reduced—potentially to the point of narcosis—beyond 30 m/100 ft. In the shallower ranges from 20-30 m/70-100 ft, the no-stop time gets quite short as well. 

A Best Mix?

Nitrox was developed for the sole purpose of increasing no-decompression times and/or decreasing decompression obligations. In the early days, two gases were promoted: 32% and 36%, but the dive community soon came up with the concept of “best mix.” This is what nitrox courses frequently teach today. You pick the depth that we are going to be diving, and work out the “perfect” gas for that dive to make the no-decompression time as long as possible. 

However, if you were to put three nitrox divers in a room and ask them to choose a gas for a 24 m/80 ft dive, you’d likely get four different suggestions. They’d all probably work just fine on that particular dive for the individual diver. 

Van Morrison told us to “Keep It Simple,” and he was right.

We’ve lost something from the days of air diving. That familiarity with no-decompression times has gone—there is no 120-rule for a best mix. When your friend calls you and tells you to show up for a dive, how do you choose the right gas for the dive to ensure you’re all on the same gas? One says 30%, the other 28%. Who’s right, which one is best? 

Nitrox 32 stage bottles

One Direction released a track called “Best Song Ever.” There is no such thing. Best mix can be considered in a similar vein. 

When John Lennon released “Imagine,” he was singing about an idea where the world lived in harmony. I don’t think he was really talking about technical diving, but the sentiment holds true. Standard gases help us remember the flexibility, familiarity, and simplicity of air, but we also retain all of the benefits of using mixed gases to increase no-decompression limits, reduce narcosis, and manage our exposure to oxygen. We do this by creating depth ranges for a small number of breathing gases, and if the dive is in that range, then the whole team uses this gas. 

If the depth ranges are wide enough, we end up with a very flexible tool that gives us the simplicity and familiarity back. It allows the use of “rules of thumb” for things like decompression planning (See: Rules of Thumb: The Mysteries of Ratio Deco Revealed), oxygen exposure management, and even gas blending. And everyone’s singing the same song!


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On to Standard Gases

There’s really nothing clever about the standard gases concept—using a single gas to cover a range of depths instead of a gas that is optimized for a single depth and typically a single parameter of the dive (usually decompression). Here’s how they look:

Standard Bottom Gases

Note: Meter and feet ranges rounded to even numbers for simplicity.

GUE standard gases are designed to maintain a maximum PO2 of 1.2-1.3 bar for the working portion of the dive and an equivalent narcotic depth (END) ≤ 30 m/100 ft. PO2s are then boosted during ascent to a maximum of 1.4 bar for deep decompression, and 1.6 for 6m/20 ft decompression, using standard deco mixes. Note: standard gases are used for open circuit and closed circuit diluent.

The maximum PO2 promoted outside of GUE is normally around 1.4 bar for the working phase of the dive. There’s likely nothing wrong with that, but you are starting to get close to a pretty unpredictable zone. The higher the PO2, the higher the likelihood for Central Nervous System (CNS) toxicity. GUE prefers that the limits be more flexible than a Maximum Operating Depth (MOD)—who can forget the days when nitrox course students were told that if they hit 1.41 bar they would surely burst into flames, but 1.39 bar was perfectly safe? That doesn’t seem right, does it?

“I’m on Fire!” said Bruce [Springsteen]. 

A  mixing panel designed to mix standard gases. Photo by Francesco Cameli.

GUE’s approach is to make the depth range of a standard gas in the 1.2-1.3 bar range for the working portion of the dive. The idea that a diver will know, at the limit of the gas, that the PO2 is climbing toward a more risky level. However, if you drop below that depth for a short time, there will not be any serious consequences. Picking up a dropped camera, helping a dive buddy, or simply being less than perfect with your buoyancy control will not result in a dangerous oxygen exposure. [Ed.note: The US Navy specifies a maximum setpoint of PO2=1.3 bar for closed circuit dives].

The gases also limit the Equivalent Narcotic Depth (END) to approximately 30 m/100 ft. At that depth one can think clearly and solve most problems if one slows down and concentrates. But, the further one goes past this depth, the more difficult it becomes to solve simple problems correctly. 

When I get to this depth on a nitrox mixture, a strange thing happens. This little guy appears on my shoulder and starts whispering ideas in my head. Ideas that change the dive plan. Ideas that seem fantastic at the time. I used to listen to him and things would never go well. I’d not get the survey data, I’d fail to attach the shot line to the wreck as planned at the surface, and small stuff like that. I’ve learned to ignore him, because he’s an idiot. Unfortunately, at around 36 m/120 ft, he becomes a lot more persuasive, and I get weaker. His suggestions make even more sense, so I tend to follow them. I’ve never learned to ignore him at this depth. The only solution I’ve found is to make sure my END is never more than 30 m/100 ft. 

Nirvana sang “Dumb”. Becoming so is not an instruction, and it’s entirely avoidable!

These gases give a lot of flexibility across their range. If you drop your camera when you’re at 45 m/150 ft and need to drop to 50 m/165 ft to retrieve it, then it’s not likely to cause a problem. If you used 21/35% for a 40 m/130 ft dive, the no-decompression limit would be 10 minutes, but if you used a “best mix” of 28/35, then your limit would be 11 mins. If you did a more realistic bottom time of 30 mins, and used a 50% nitrox for decompression, then the two schedules would only be 4 minutes different. 

You always analyze and label your gas, don’t you?

What “Difference Does it Make?” as The Smiths once sang. 

Here’s a final advantage of using these particular standard gases. If you are lucky enough to have banks of 32% nitrox, then you can create all of the other standard gases by adding 32% onto the required amount of helium. So, to make a 21/35, you would take an empty set of cylinders and fill to 35% of their final pressure with helium. Then you add 32% and, lo and behold, you have 21/35. Same idea for 18/45, 15/55, and so on. Got half a set of doubles of 15/55 left over? Blow it with 32%, and you’ll get a 21/35. And that’s where efficiency comes in. You can use every last drop of your expensive helium in a pretty easy manner using 32% as a filler gas. 

Easy, like “Sunday Morning.”

GUE also standardised their decompression gases, but that’s another playlist.

Dive Deeper

InDepth: Rules of Thumb: The Mysteries of Ratio Deco Revealed by Richard Walker

InDepth: Rules of Thumb 2: Further Mysteries of Ratio Deco Revealed by Richard Walker

Alert Diver: Anatomy of a Commercial Mixed-Gas Dive by Michael Menduno (2018)

InDepth: Oxygen Exposure Management by Dr. Richard Vann (1994)

Alert Diver.Eu: Rapture of the Tech: Depth, Narcosis and Training Agencies by Michael Menduno (2020)


Rich Walker learned to dive in 1991 in the English Channel and soon developed a love for wreck diving. The UK coastline has tens of thousands of wrecks to explore, from shallow to deep technical dives. He discovered GUE in the late 1990s as his diving progressed further into the technical realm, and he eventually took cave training with GUE in 2003. His path was then set, and he began teaching for GUE in 2004. 

He is an active project diver and is currently involved with: the Mars project, Sweden; Cave exploration team in Izvor Licanke, Croatia.; Ghost Fishing UK, Chairman and founder. He is also a full time technical instructor and instructor evaluator with GUE, delivering these services via his company, Wreck and Cave Ltd. He sits on GUE’s Board of Advisors and serves several other industry organizations. He also knows his music!

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Hal Watts: Plan Your Dive

Known for his deep air diving exploits back in the day, 86-year-old Hal Watts, aka “Mr. Scuba,” is one of the pioneers of early scuba and credited with coining the motto, “Plan Your Dive. Dive Your Plan.” He founded the Professional Scuba Instructors Association International (PSAI) in 1962, which eventually embraced tech diving, but never relinquished its deep air “Narcosis Management” training. Italian explorer and instructor Andrea Murdock Alpini caught up with Watts and teased out a few stories from the training graybeard.

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Interview by Andrea Murdock Alpini 
English text by Vincenza Croce

Hal Watts, Terrence Tysall, and Bill Stone in March of 1993.  This was the last stop in the U.S. for a test dive of the Cis-Lunar Mk-4 rebreather prior to Stone’s San Agustin expedition (1994) for its first real sump dive.

“Plan your dive, dive your plan,” is a common refrain in diving, but it’s easy to forget the meaning of this phrase has changed over time.

The underwater explorers of the early days learned to plan their dives with watches, depth gauges, and US Navy tables. Back then, decompression tables were the Bible for divers—something miraculous, halfway between alchemy and physiology. Those trail-blazing divers defined what it meant to “plan” a dive.

But, at the time, the term “technical diving” did not exist; divers breathed air on the bottom as well as during decompression. Only after many years was oxygen added, followed by the famous jump into the hyperbaric chamber.  

Later came new innovations after a few decades of experiments: hyperoxygenated binary mixtures, the NOAA tables, Heli-air (i.e. the addition of helium in tanks loaded with air), the change in the speed of ascent, new molecules to be studied, new physiological and narcotic effects, and their consequent impacts on humans and their psyches.

In a very short time, diving traditions underwent a metamorphosis. The spool and the coral tank became a proper reel, the ascent bin and the plastic bag disappeared in favor of the buoyancy control device (BCD), the surface marker buoy appeared—and then, even later on, wrists were adorned with underwater computers instead of decompression slates.

Divers later renewed and revolutionized a niche discipline, transforming it into a sporting phenomenon and a vocation. Faced with imminent change, there is often nothing that can be done when an anomalous wave arrives; you cannot stop its irresistible force with the wave of a hand. And thus was the American revolution of underwater technique, where the means of exploration—read mixed gas and scooters—have become the end.

The self-proclaimed originator of the “plan your dive, dive your plan” motto was 86-year old Hal Watts, the founder of American didactic Professional Scuba Association International (PSAI) and a diving pioneer who once held the Guinness Book of World records for deep diving. Though the use of trimix grew in popularity, Hal continued to believe in deep air, in the ancient technique of coral fishermen. He supported wreck and cave diving—with decompressive mixtures and new configurations through PSAI; but, above all, he believed (and continues to believe) that deep air, if properly practiced, is a discipline with unique logistics, hidden dangers, and irresistible charms that can take you to a parallel world.

Hal Watts speaking at aquaCORPS tek.93 Conference

First of all, Hal, what was the dive that changed your way of seeing scuba diving? I mean, a dive that was like an epiphany, a dive which changed your point of view on a technical matter?

Hal Watts: Wow, you sure are really trying to test my old man memory. Now I’ll have to review some of my old logbook entries. 

The first scuba dive that really got my attention as to just how serious and dangerous scuba diving can be was on December 2, 1962. I was diving with Bob Brown, co-owner of Florida State Skindiving School in Orlando, Florida. I was a member of a dive club in Orlando known as Orlando Sport Diving Club. Bob and I had heard of a sinkhole in Ocala known locally as Zuber Sink as well as Blue Sink. Years later, I later leased the property and renamed it as Hal Watts’s 40 Fathom Grotto, and I eventually purchased the Grotto in mid-1979.

We had never talked to anyone about the sinkhole; therefore, we had no idea about the visibility or the depth. Up to this point, I had constructed my favorite BCD, using a large white Clorox plastic jug, which we tied to our twin tank system. We put air into the BCD from our “Safe Second Stage” mouthpieces. 

Bob and I tied our safety line to a tree on the bank of the sink and reviewed our dive plan. I am reminded of the motto I came up with, many moons ago—Plan your dive, dive your plan. 

We all know that motto. I didn’t realize that it was you who coined it.

It was back in the 1960s when I was writing course manuals for NASDS [National Association of Scuba Diving Schools] and opened up my Mr. Scuba dive shop.

Mr. Scuba’s Magic Bus!

But back to the dive at Zuber. I’ve failed to mention the fact that neither of us had been doing any dives below 30 m/100 ft. We followed the cave line down slowly, not paying enough attention to our depth. Before we realized it, we had hit the bottom, stirred it up, and had no clear water.

Lucky for us, I kept the cave diving reel in my hand, and Bob kept his hand on the line. I couldn’t see; however, I could feel Bob’s hand, squeeze his fingers tight on the line, grab his thumb, and give it the “thumbs up” signal. I don’t know how we managed it, but we were both able to use our NASDS safe second stages and add air into the Clorox “BCDs.” We were actually fated to begin an uncontrolled, too-rapid ascent. All of a sudden, we hit an overhead wall, which stopped our ascent at a depth of 9 m/30 ft. 

We looked at each other, and gave the OK hand signal. While decompressing, following the old Scubapro SOS mechanical computer, I started to pull up the loose line until the dive reel appeared. Wow, we sure had an awful lot of loose line floating around us. Were we extremely lucky? Of course, we were. Our problem was that we never planned our dive, and consequently, were unable to dive a plan.

After that dive, I worked with Scuba Pro and Sportsways to create the “Octopus,” or “safe second.” A while later, the octopus appeared for the first time in Scuba Pro catalogs. I was also the first to add a pressure gauge along with the Octopus.

Hal Watts set the world deep air record to 120m/390 ft in 1967

Ah yes, the “Safe Second.” That’s what NASDS called backup second stages, right? Sheck Exley (1949-1994), the legendary cave explorer with whom you were friends, was also credited with fitting a redundant second stage reg with a necklace. I want to ask you more about Exley, but first, I want to know: What are the best wrecks you ever dived?

This is really very hard to answer. I’ll have to list four, in the order that I dived them: the USS Monitor, Andrea Doria, Japanese wrecks located in Truk Lagoon, and the Lusitania in Ireland.

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The most important would have to be the USS Monitor, a submarine used during the Civil War. A group of well-known USA divers applied to the National Oceanic and Atmospheric Administration (NOAA) for a permit to dive the Monitor, as she was located in protected waters. In addition to myself, the group consisted of: Gary Gentile, attorney Peter Hess, and several other well-known expert divers. At first, NOAA refused. Then, Peter Hess filed proper papers asking that we get the NOAA permit. To that end, we presented my Deep Air training material to the concerned NOAA group. I appeared as an expert witness and provided NOAA staff and their legal representatives with my internationally accepted training material and my record of training several world record deep air divers. Our deep air training has been accepted worldwide with zero diving deaths. After that, we received the permit. 

Other than the Monitor, my favorite deep wreck dive would be the Lusitania, which is a very personal and proud story for me. The main reason is because venture capitalist Gregg Bemis owned the diving rights to the Lucey at the time. Gregg had contacted me requesting that I train him on PSAI Narcosis Management Level V, on air—which is 73 m/240 ft—and then train him on trimix so he and I could dive to 91 m/300 ft on the Lusitania lying off the coast of Ireland. 

When word got out that I had enrolled Gregg in my Narcosis Management Course, a well-known international course director (a personal friend of mine) called and told me, “Hal, do not teach Gregg deep diving.” 

PSAI’s ad in aquaCORPS Journal circa 1994 offering deep air training.

He told me that he had been training Gregg at his facility, and that he was a “train wreck.” “He is from a very well-off family in Texas, and if you cause him any injuries, you will be sued and put out of business,” my friend said. Well, guess what? Gregg completed the 240 Level V Deep Air course, then our PSAI Trimix course. My wife, Jan Watts, Gregg, and I went to Ireland to dive the Lusitania. He and I made an awesome 91 m/300 ft trimix dive to the deck.

Diving on the Andrea Doria with Tom Mount, Peter Hess, and several great wreck divers was also an awesome dive. Last but not least was a great trip to Truk to dive on some of the Japanese wrecks.

Please tell us about Sheck.  What was your relationship with him like?

Sheck and I became friends and made several dives together, and one of my favorites happened when Sheck, his Mary Ellen, my wife Jan, and I were diving at 40 Fathoms. Sheck wanted to practice gas switches during descents. Sheck was practicing, getting ready for a planned very deep dive (I think in Mexico with Jim Bowden). The four of us swam to the east side of The Grotto, slowly following the wall during our controlled descent, watching Sheck practice gas switching. 

Sheck Exley and Hal Watts at a NSS-CDS conference

After reaching our planned depth of 73 m/240 ft, we began our controlled ascent up to our first planned deco stop. During our last deco stop on our 4.5 m/15 ft platforms, I noticed that Sheck had a funny look on his face and was messing with his drysuit between his legs. I remembered then that he had told me that he had an attachment installed in the drysuit that would allow him to pee underwater. He was clearly in a bit of discomfort and Mary Ellen, Jan and I just floated nearby and watched.

I’ve heard that Sheck later used diapers, or just cut it loose in one of his old neoprene drysuits on his big dives, so evidently he didn’t get that early p-valve to work. What about your friendship and job collaboration with Gary Taylor, your brother-in-arms and a co-owner of PSAI?

Andrea, get comfortable, since this question will take some time to properly answer.

I first met Gary in Miami, which is where we became friends when I was staying in his home and taking Tom Mount’s nitrox course.  I have a photo of Tom, Gary, and me gas blending on the floor of Tom’s garage. During the course, Tom was still using his worn-out hand written paper flip charts as his notes.

Gary was impressed with my deep air program and offered to put together an updated slideshow presentation for me to teach with. PSAI still uses an updated version of this system to date. Gary stayed with Tom until Tom thought he had sold IANTD [International Association of Nitrox and Technical Divers] to another individual. After that sale came about, Gary contacted me wanting to get more involved with PSAI. Being smarter than folks thought I was, I jumped at the chance to have Gary on the PSAI Team. Tom’s deal fell through, but Gary was totally involved with PSAI, and now is a partner and president of our agency. Thanks to Gary and Tom. 

Many, many years ago I was still taking some type of classes—I think regarding mixed gasses, maybe with Rebreathers—at Tom’s house. In fact, I was one of Tom’s instructors who did the final proofreading of one of Kevin Gurr’s manuals. Too far back to recall much about this mixed gas stuff—remember my reputation for being a deep air diver.

Tom Mount and Gary Taylor mixing up some trimix in the garage.

Speaking of the people with whom you’ve dived, was the aim of The Forty Fathom Scubapros Club?

Before I invested in a sinkhole in the Ocala, Florida, area—which was locally referred to as Blue Sink or Zuber Sink, and is now referred to as 40 Fathom Grotto—several diving buddies whom I had dived with and trained for extreme deep air diving—as well as cave exploring—got together and planned to dive The Grotto at least one Friday night per month. Within a short period of time, several other buddies joined our group, which eventually became known as The 40 Fathom Scubapro’s dive club. Each diver had to meet my requirements of training. 

Forty Fathom Grotto aka Zuber Sink
An early Sheck Exley mix course at Forty Fathom Grotto
An Eric Hutcheson drawing of Forty Fathom Grotto

Eventually, our group set specific personal requirements—being a good person, supporting our club safety rules, and making at least one 40 Fathom Grotto dive per month. We set a limit of 14 or 15 members. Three 40 Fathom members eventually set World Records for deep air: I was one, A. J. Muns, and Herb Johnson set ocean records, and later I set the air depth record for cave diving. Naturally, as time passed and we got older, our membership got smaller. It is notable that none of our club members have died during any scuba dive.

Finally, what led you to create the iconic motto, “Plan Your Dive. Dive Your Plan?” 

I used to be a private pilot, and we used to say, “Plan your flight, fly your plan.” This was back in probably 1961 when I had just started diving and there were so many instances where all the other divers would get in the water without saying anything. I’ve seen so many incidents and fatalities that could have been avoided through proper planning.

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Dive Deeper

ScubaGuru: LXD 029 : Hal Watts – Record Deep Diver & Technical Diving Pioneer

Netdoc: Netdoc chats with Mr Scuba, Hal Watts

InDEPTH: The First Helium-based Mix Dives Conducted by Pre-Tech Explorers (1967-1988) by Chris Werner

Alert Diver.Eu: Rapture of the Tech: Depth, Narcosis and Training Agencies

Professional Scuba Association International: PSAI History


Andrea Murdock Alpini is a TDI and PSAI technical trimix and advanced wreck-overhead instructor based in Italy. He is fascinated by deep wrecks, historical research, decompression studies, caves, filming, and writing. He holds a Master’s degree in Architecture and an MBA in Economics for The Arts. Andrea is also the founder of PHY Diving Equipment. His life revolves around teaching open circuit scuba diving, conducting expeditions, developing gear, and writing essays about his philosophy of wreck and cave diving. He published his first book, Deep Blue: storie di relitti e luoghi insoliti (2018) and IMMERSIONI SELVAGGE, the new one is on the way, out on fall 2022. 

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