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What Happened to Solid State Oxygen Sensors?

The news in 2016 that Poseidon Diving Systems would be incorporating a solid state oxygen sensor in their rebreathers sent a buzz through the rebreather community. Galvanic sensors, along with their legacy-1960s “voting logic” algorithms to boost reliability, had long been considered the weakest link in closed circuit rebreathers. Many heralded Poseidon’s subsequent 2017 roll-out as the dawn of a new era in rebreather safety. Five years later, Poseidon remains one of two companies (the other strictly military) to have adopted optical sensors. Technology reporter and tech diver Ashley Stewart examines some of the reasons why.

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by Ashley Stewart.

Header image: Karst Underwater Research (KUR) rebreather divers at Weeki Wachee. Photo by Kirill Egorov

For years, it’s been said there’s a revolution coming for the closed-circuit rebreather— a new, more reliable, safer replacement for the traditional electro-galvanic oxygen sensor, widely considered the weakest component of rebreathers. In March 2017, that revolution looked to be just over the horizon. Poseidon Diving Systems began shipping an offboard solid state sensor to supplement the MKVI’s and SE7EN’s galvanic sensors and offered to license the technology to other manufacturers. Though Poseidon subsequently incorporated the solid state sensor into its SE7EN rebreathers, nearly five years have passed, and not much else has changed.

Poseidon remains the only manufacturer using solid state sensors in recreational rebreathers. No other companies have licensed Poseidon’s technology. Major tech diving manufacturers—including JJ-CCR and Divesoft—say they don’t believe the technology in general is ready for use in rebreathers. Some manufacturers worry that the sensors won’t function accurately in humid environments over a wide range of pressures, and they claim that addressing these challenges will be costly. Meanwhile, divers who tested Poseidon’s sensors offered mixed reviews, and even the inventor who sold the sensor validation technology patent to Poseidon believes they should be used along with traditional sensors. (Poseidon gives divers the option of combining the sensors).



Poseidon’s solid state sensor integrated into the SE7EN rebreather. Photo courtesy of Poseidon.

Oxygen sensors are the enabling technology that made mixed gas rebreathers possible, replacing rebreathers that could only be used with pure oxygen. In 1968, marine scientist Walter Starck introduced the first commercial CCR, called the Electrolung, which used polarographic sensors. The next year, BioMarine Industries launched its CCR-1000, the predecessor of the US Navy’s Mk-15/16. The unit was the first mixed gas rebreather to use galvanic sensors, which do not require a power supply. 

In addition to removing a diver’s exhaled carbon dioxide, a rebreather must measure and maintain a safe and efficient level of oxygen, as measured by the partial pressure of oxygen, or PO2, via oxygen sensors.

Measuring PO2 correctly is critical, and failures can be fatal. Too little oxygen can cause hypoxia and loss of consciousness, and too much can result in central nervous system toxicity and convulsions. Since sport divers began using CCRs over twenty years ago, both conditions have caused numerous drowning fatalities.

With the exception of Poseidon and military manufacturer Avon Underwater Systems, modern close circuit rebreathers have more or less used the same type of sensor since the 1960s. Rebreathers typically use three galvanic sensors, averaging the readings of the two closest sensors and ignoring the third in a protocol called “voting logic,” originally created by Starck in response to the sensors’ noted unreliability.

Even with this voting logic, however, the sensors can be unreliable (See “PO2 Sensor Redundancy” in Dive Deeper below). The galvanic sensors are cheap and time-tested, but they need to be recalibrated before every dive and expire after about a year. The new sensors—called “solid state sensors” or optical sensors—are expected to be more precise, reliable, and durable, though significantly more costly.

An illustration of luminescent quenching technology

Galvanic sensors are essentially wet-cell batteries that generate a millivolt current proportional to the PO2 in the loop. Conversely, Poseidon’s solid state sensor uses luminescent quenching, wherein a red LED light excites the underside of a special polymer surface, which is covered with a hydrophobic membrane and exposed to the gas in the breathing loop. A digital color meter then measures the responding change in fluorescence, which is dependent on oxygen pressure, and an algorithm calculates the PO2.

Experts more or less agree that the right solid state sensor could make rebreathers safer, but the market is split on whether the technology is ready for use in rebreathers and just how much better they’d have to be to justify the cost.

Field Test Results

Poseidon advertises its sensor as “factory-calibrated and absolute, delivering unsurpassed operating life, shelf life, and calibration stability.” Richard Pyle, a senior curator of ichthyology at Hawaii’s Bishop Museum who works with Poseidon-affiliated Stone Aerospace, has tested Poseidon’s sensors for years, initially as a passive offboard check against Poseidon’s traditional galvanic sensors. Later, in November 2019, he said he began testing Poseidon’s prototype with the solid state sensor as the primary sensor in the unit. “From my perspective as a rebreather diver, this is the most significant game-changing way to know what you are breathing,” Pyle said. “We will never go back to the old oxygen sensors.”

Poseidon divers at 110 m/359 ft. Photo by John L. Earle

Pyle said he’s yet to fully analyze the data he’s collected to compare the performance of the solid state sensors against the galvanic sensors, but that  he’s had zero failures with the solid state sensors in the time he would have expected to have 50 to 100 failures with the galvanic sensors.

Likewise, Brian Greene, a Bishop Museum researcher who has tested the Poseidon sensors with Pyle, estimated that he’s made hundreds of dives with the solid state sensors without failure. But, not everyone has had this experience.

Sonia Rowley, an assistant researcher at the Department of Earth Sciences in University of Hawai’i at Mānoa, told InDepth that she experienced a variety of repeated failures when testing Poseidon’s system alongside Pyle beginning in 2016 and 2017, and Rowley dictated to InDepth specific dive logs detailing many of the failures. She wrote about her experience in the book “Close Calls.

Poseidon CEO Jonas Brandt said the company has tested the sensors since 2017 at different depths and temperatures, and that it has only seen one possible failure.


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Arne Sieber is a sensor technology researcher who said he developed the O2 sensor validation technology used in the Poseidon rebreather and sold the patent to Poseidon. Sieber is now researching uses for the solid state sensor including in the medical market. He told InDepth he believes the best way to incorporate the solid state sensors into rebreathers would not be to substitute one for the other, but to combine sensor types and design a rebreather that incorporates both. 

Traditional galvanic sensors have advantages over the solid state sensors, Sieber said—they’re cheap, simply designed, low-voltage, and time-tested. Also, while solid state sensors are very accurate at measuring low PO2, they become less sensitive at about 1.6 bar, and are more prone to incorrect readings of higher PO2 levels than galvanic sensors. As for whether the sensors can function in humid environments, Sieber said the sensors can work well in liquids, such as when used for blood analysis (though the sensors are used to measure much lower partial pressures of oxygen) and for measuring oxygen content in the sea. Liquid can delay the amount of time it takes a sensor to read a partial pressure, but it does not falsify the results, Sieber said. Of Poseidon’s system, Sieber said, “It’s a good start. It’s very important that someone starts. Someone always has to be the first one.”

Brandt said divers have the option of combining sensors in the company’s SE7EN rebreather, using either two galvanic sensors, two solid state sensors, or one of each, and said it could be argued that using one of each sensor is the most reliable.

Meanwhile, a catalyst may be coming to encourage the development and adoption of solid state sensors in Europe, Sieber said. European Union rules restrict the use of hazardous substances in electrical and electronic equipment, but galvanic sensors (which have an anode made of lead) have been granted an exemption in medical products because there is not a suitable alternative. The exemption is set to expire.

Poseidon’s solid state sensor sells to end users for as much as around $1,500USD, and its SE7EN rebreather units use a maximum of two onboard sensors. [Note: Poseidon sells the sensor for 6800SEK plus VAT from its website, which equates to 944 USD, some outlets in the states sell them for much higher]. Galvanic sensors, meanwhile, cost around $100USD, last one year and divers use three at a time. And, that’s just the cost of the sensors themselves: Manufacturers have to make significant investments in, and upgrades to, electronics systems to accommodate solid state sensors. 

Fathom rebreather lid showing its galvanic sensors. Photo courtesy of Fathom Dive Systems, LLC.

As for how long the sensors actually last, even the manufacturers don’t yet know. Poseidon has some from 2014, and they still work but have to be factory calibrated every two years. Galvanic sensors need to be replaced annually, while solid state sensors are expected to last much longer.

Brandt chalks the debate about its sensors up to competitiveness in the market. “I don’t think anyone likes that somebody cracked the nut,” Brandt told InDepth. Poseidon is ready to share the technology with other dive companies and manufacturers, Brandt said, but there have been no deals to date. “We wanted to raise the bar in technology and safety with the rebreathers, and to be honest, we haven’t said to anyone in this business that this technology is exclusive or proprietary.”

Market Interest

When the company first debuted its sensor, Brandt reported that companies like Hollis and Shearwater Research expressed interest in licensing the technology, but nothing has come so far of those discussions. Brandt did say one manufacturer reached out right before the pandemic. He declined to say which, but shared that it was a European company. Hollis brand manager Nick Hollis said his team recalls a conversation with Poseidon, but that it was back in 2014 or even earlier.

Shearwater director of sales and marketing Gabriel Pineda said the company is still interested in solid state sensors, but they see an issue with the price. “If you make the economic case of traditional galvanic sensors versus solid state or optical sensors, you have to dive a lot, and it takes a long time for these to make economic sense for a diver.”

Of course, Shearwater is not a CCR manufacturer, but the company is interested in seeing whether the sensors would be viable for use with its electronic control system that is used by a majority of rebreathers on the market. Shearwater currently has no immediate plans to license the technology from any manufacturer but Pineda said the interest remains. 

Meanwhile, Poseidon’s solid state sensor CCR is still making headway, Brandt said. The current biggest buyer of the Poseidon units is the military (Brandt said three European Union countries’ forces are actively using the sensors). The sales have continued throughout the pandemic, and, over the past six months, Poseidon has started an upgrading program, allowing divers to add the new sensors to their old units. Poseidon is looking into a program Brandt compares to Apple Care, where customers can pay a fee for maintenance throughout the life of the sensor.

Solid state sensors replace numerous galvanic sensors which have a one-year life. Photo courtesy of Richard Pyle.

Meanwhile, Avon Underwater Systems is using three solid state sensors in its MCM100 military rebreather. Kevin Gurr, a rebreather designer and engineer who sold his company, VR Technology Ltd., to Avon, said the company uses the sensors “because of the increased safety and the decreased user burden as far as daily calibration.”

Gurr, who designed and produced the Ouroboris and Sentinel closed circuit rebreathers at his prior company VR Technologies Ltd., believes it’s the cost that has discouraged other manufacturers. “It shouldn’t be about cost at the end of the day,” Gurr said. “The digital interface is so much safer.”

Martin Parker, managing director of rebreather manufacturer AP Diving, said his company follows solid state sensor development but has yet to come across a sensor that meets its accuracy requirements. One such sensor using luminescence quenching can achieve good accuracy through a replacement disk the user must apply to the sensor surface after each use.

“Having been in the diving business for 50 years, we don’t believe it is on any diver’s wish list to have to re-apply every diving day a new component, as simple as that is to do,” Parker told InDepth. “With no easy external measure of accuracy prior to the dive, it is easy to foresee that many divers would ‘push their luck’ and use the discs for multiple days, then when they get away with it, they would encourage other divers to do the same… with the inherent risk of DCS or O2 toxicity.”

Parker said that he’s aware of two additional sensors under development, but neither has shown a working product yet. He declined to identify any of the manufacturers, citing commercial sensitivity. “Hopefully, we will get these to evaluate in the next 12 months,” Parker said.

Divesoft co-founder Aleš Procháska said he believes Poseidon’s approach to the sensor could “lead to success.” Speaking generally about solid state sensors rather than about Poseidon’s specifically, Procháska said his company isn’t yet utilizing solid state sensors because he believes the sensors are unable to function in humid environments with extreme water condensation and not applicable over a  wide range of pressures. To be able to use one of these sensors in a rebreather, Divesoft wants it to be durable in high humidity, consume less energy, and have a good price-to-lifetime ratio. 

“It is possible to build a CCR with the currently available O2 solid state sensor but not without sacrificing important properties of the breathing apparatus,” he said, such as size and energy. “Overall, the reasons why no one currently sells this technology on the market seems to be quite simple. It’s extremely difficult to come up with a suitable and functional principle that would lead to a cheap, small, and low energy consuming solid-state sensor. Despite this, I do believe that it’s only a matter of time until someone solves this one.” Asked via email about the status of DiveSoft’s own work on the technology, Procháska replied, “Well, as I said earlier, it’s just a matter of time,” adding the text, “Aleš smiles.”

JJ-CCR lid showing its three galvanic sensors. Photo by Kees Beemster Leverenz.

Halcyon COO Mark Messersmith said that divers are slow to embrace new technologies in general, and the current sensors just aren’t deficient enough to merit widespread adoption or the investment from manufacturers. “It’s not unlike many other technologies,” Messersmith told InDepth. “People are often slow to embrace a new technology if the existing technology is functional. The existing tech needs to be vastly deficient, and existing oxygen sensors are still largely functional.” 

The bottom line: Solid state sensors might very well be safer, but there isn’t enough incentive for the market to make them a reality. 

David Thompson, designer of the JJ-CCR, told InDepth they don’t use the sensors because he doesn’t believe the technology is ready yet and research in that area is extremely expensive and difficult for what he believes is essentially a small market. “Analog cells have a long history, and in the right hands are very reliable, easily available, and have a long history of working in a rebreather environment which is very hostile,” Thompson said, adding that high humidity and temperature in a rebreather is a challenge for any sensor. “I am sure it will be in the future, but that future won’t be here yet.”

Dive Deeper:

InDepth: Where Have All the Sensors Gone? Assessing the Global Oxygen Sensor Shortage

Rebreather Forum 3 Proceedings: PO2 Sensor Redundancy by Nigel A. Jones p. 193-202

Alert Diver: Oxygen Sensing in Rebreather Diving by Michael Menduno

Wikipedia: Electro-galvanic oxygen sensor


Photo by Daniel McMath 

Ashley Stewart is a Seattle-based technology journalist and GUE Tech 1 diver. Reach her via email: ashannstew@gmail.com, Twitter: @ashannstew, or send a secure message via Signal: +1-425-344-8242.

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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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