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Did You Analyze Your Gas?

Italian tech instructor Eugenio Mongelli, founder of TEMC DE-OX, which makes digital gas analyzers, shares a story of a combat diver who went hypoxic on an oxygen rebreather and why it always pays to analyze your breathing gas; even when you’re filling your cylinder with 100% oxygen!



A Case of Hypoxia on an Oxygen Rebreather

by Eugenio Mongelli

In 2019, I was a member of an international diver team that was testing the most up-to-date military tactical equipment, including special dry suits, rebreathers, hi-tech ballistic protection, helmets, night vision systems, military Advanced Diving Vehicles (ADVs also known as diver propulsion vehicles), and many other real toys you see in action movies. 

One afternoon, a military diver decided to dive a light configuration with a fully closed oxygen rebreather, dry suit, dive computer, and standard accessories. The pre-dive check was smooth and successful, the oxygen cylinders fully filled, the soda lime filters newly replaced with new sofnolime. The atmosphere was of a fun, relaxed dive as when diving with a group of friends. 

The lake was calm, water temperature was 15ºC, and crystal-clear visibility—very good ambient conditions for that kind of dive. I was supporting the team by providing assistance. We began the dive with no problems, the pre-dive check was successful, and we performed a correct lung circuit purge for keeping air out. All the divers were maintaining a steady breathing rate, the rebreather lungs were regularly expanding, the oxygen was being added manually, and max depth was kept at 5 m/16 ft with the help of a line connected to a buoy. 

After twenty minutes, one team member started demonstrating signs of discomfort, as if he was in distress. He suddenly emerged and then lost consciousness. His face was blue, and he was not breathing. We immediately brought him to the lake shore removing his drysuit (actually cut it open)  and started CPR. Thankfully,we were able to revive him. One characteristic of the full oxygen rebreather is that if a diver has an accident diving with it, they tend to either die, or they survive with no permanent damage. There’s nothing in between. Full stop.

This diver luckily survived.  He was flown by helicopter to the nearest hospital for a medical screening where he spent the night. He was back with us the next day, smiling. But after the accident, we wanted to understand what caused the problem. I surmised it most likely was a caustic cocktail that had occurred because water entered the soda lime filter, affecting his lungs and causing loss of consciousness. There was no other logical reason, as the diver was trained in using oxygen rebreathers,which requires special attention.

Eugenio Mongelli at the event

The team felt some urgency to investigate in order to determine the cause of the incident and took the following steps:  First, we opened the rebreather and removed the canister, but we saw no sign of water flooding inside. The soda lime was dry, as were  the breathing lung and the mouthpiece with corrugates. Then, we speculated that the incident could have been caused by carbon dioxide poisoning, but the soda lime had not  been used before and had recently been replaced. Next, we wondered if it  might have been carbon monoxide poisoning, and because of this concern, we connected the oxygen cylinder to a DE-OX multigas analyzer. That’s when we discovered the frightening reality:  no carbon monoxide, no carbon dioxide, no helium, but only 80% oxygen. It seemed impossible. 

How could decanting certified pure oxygen from a big cylinder to a smaller cylinder lead to only 80% oxygen? The oxygen sensor must be at fault, we thought, so I replaced the analyzers with two other oxygen analyzers. The verdict was the same: 80% oxygen. The conclusion was that our diver had suffered from hypoxia when the partial pressure of oxygen went below the necessary amount for life support.  When he emerged, the oxygen partial pressure went even lower, causing him to lose consciousness.

If that was the cause, we were determined to discover how that could have happened. We tested the big oxygen cylinder, and it was 100% oxygen. Then I proposed that it could have been the booster used for pumping oxygen into the small cylinder. As the booster was driven by low-pressure compressed air, it could have resulted in a mixing of air with oxygen if compressed air leaked into the oxygen pumping chamber. Though unable to be certain, after checking all other options, we concluded that this was the most likely possibility.

TEMC DE-OX Easy O2 Analyser

How could a diver go hypoxic by breathing a nitrox gas mix with 80% oxygen? It’s a result of the nature of an oxygen rebreather, which has no O2 sensors. Oxygen is added to the loop by means of an add-valve that can be operated manually or automatically based on hydrostatic pressure. So, at a constant depth, the valve would add nitrox 80 in proportion to the amount of 100% O2 consumed by the diver and the CO2 consumed by the sorb. As a result, the FO2 and therefore the PO2 in the counterlung and loop would steadily decrease (the loop would gradually fill with nitrogen) until the gas became hypoxic. 

After experiencing this event, I definitely keep pushing all divers to analyze their gas cylinder, regardless of whatever dive and gas mix they are going to use. Gas analysis is not sexy, but it can save your life! 

Ed. Note: GUE Standard Operating Procedures: A diver should personally analyze & label all their tanks on the day of the dive.

Eugenio Mongelli was born in Rome, Italy, in 1965 and has been deeply attracted by the sea, diving, and scuba gear technology since childhood. Having learned to scuba dive in 1982, he first dived a closed oxygen rebreather in 1984 and then began using electronic closed-circuit rebreathers from the mid-1990s. Mongelli became a diving instructor in 1991 and is currently a free diving and scuba diving master instructor, and a rebreather and trimix instructor with FIPSAS, CMAS and TDI. 

Mongelli was shocked to discover aquaCORPS magazine in the early ‘90s, which opened his mind to new horizons in diving that have become his life path. A doctor in management engineering and a former naval officer, he founded the TEMC DE-OX company in 1996 with the intent of providing digital gas analyzers specifically designed for both sport divers as well as the military and commercial diving communities. Since then, he closely follows the development of technical diving in all aspects of training, learning, designing new gear, and has been an active part of the global community.  


No Direction Home: A Slovenia Cave Diving Adventure

Suffering from Covid lockdown, young, poetic Italian explorer, instructor, and gear-maker, Andrea Murdock Alpini, decided to take social distancing to the max! He packed his specially designed cave-van and set out on a three-week solo road trip to dive the water-filled caves lying beneath the Slovenian soil. His report and must-see video log, dubbed, “No Direction Home”—an homage to Martin Scorsese’s Bob Dylan docu—will likely satisfy those deeper urges for adventure. Did I mention the killer soundtrack? Kids don’t try this at home!




Text: Andrea Murdock Alpini

Photo & Video: Andrea Murdock Alpini

Ecco la storia originale così com’è stata scritta in italiano

Author’s note: I do not encourage other divers to conduct solo diving. The trip and the dives described in this article were conducted after significant training and experience.

Ed.Note: Global Underwater Explorers does not sanction solo diving.


That was the feeling I had last June 2020 when I left my home to begin a journey alone. Caves, abandoned mines, alpine lakes, and a few wrecks—that was my plan for a great adventure.  

The first COVID-19 lockdown had been in place for a couple of weeks, and I was afraid of going out and meeting people. Social distancing left an open wound. I loaded my wreck-van with plenty of stuff to survive alone for a long month traveling amongst rivers, lakes, mountains, and forests, and I was ready to practice scuba diving. 

At that time, tourist travel was impossible in Italy or abroad—anywhere  in Europe—because the coronavirus had locked the borders. I asked an editor in chief from a magazine—one whom I am used to sending articles to—to prepare a couple of official invitation letters for customs. For my trip, I converted my wreck van into a cave van. It was fully equipped with a 300-bar air compressor, helium, oxygen, deco cylinders, twinsets of different sizes, gas booster, fins, mountain boots, tent, camp burner, and brand-new dry suits, as well as thermal underwear to be tested for my company PHY Diving Equipment. 

I remember the day well. I was thrilled as I crossed the border between Italy and Slovenia. I had been restricted to nothing but a 200 m/650 ft walk from my house because of the pandemic restrictions, but with an eight-hour drive, I was free to enjoy walking into wild nature all alone.

The mental switch was awesome, and unexpected. I did make just one phone call from abroad. I talked to an incredible Russian who was the first guy I met in a small rural village in Slovenia. He had emigrated some years ago, and now he welcomed travelers by sharing his farmstead. 

However, once I arrived on site, I was not very welcomed by  the weather; instead, I was met by heavy rain. After the storm passed, I went out walking and filming with my phone. I had decided to record all of the trip. As luck would have it,  the rain returned again, and it never left me for the entire duration of my trip (almost a month).

My tour was articulated throughout Slovenia, Garda Lake (Italy), Austria, and South Tirol’s Alps, Tuscany’s caves, and finally I reached the central part of Italy—Appenini mountains and their peaks. I planned to reach two mines, but heavy rains stopped my dream. Excluding Slovenia, where I slept in a traditional bed, I passed all my time living in my tent. Cold weather and storms were my constant companions. 

I managed to see  a ray of light for just a few hours, I never had any chance to dry my equipment, and I warmed up inside my van. Every night I slept only a few hours because of loud wind noise or strong rain storms. Day-by-day I grew tireder and more feeble. One day, three weeks after I left home, I was in South Tirol descending a mountain when I decided to conclude my trip, and I returned home safe.

The goal of my trip was to tell scuba adventures from the surface point of view where the water is only a part of the context and not the objective. I made a mini-series film composed of three chapters. Each one brings you inside the scene. What  follows here is the first episode of the trip.

Social Distancing Beneath The Slovenian Soil

The first day of cave diving in Slovenia was very tricky and full of adventures. I had no idea how the second day would go. 

I left my accommodations around 6 a.m., after a good breakfast of cereal, dark chocolate with black coffee, dried fruit, and tasty Italian Parmesan cheese. I could not see anything from my window because what had fallen was not simply rain; it appeared to be an awesome flood. My plan for that day had been delayed.

I think that most parts of dry caves are condemned for hundreds of kilometers. So, I decided to check the weather forecast and water level conditions in caves close to the Croatian border. It would mean driving about four hours to see for myself whether scuba diving was allowed. I didn’t have to remind myself, I was alone here.

Wheels were on the road and local conditions seemed quite good. I had checked the weather on my laptop and understood the risk. If I was lucky, I could dive; if not, I would have to drive back. I drove through Slovenia forest meeting no one. With less than an hour left to my destination, I came across an abandoned farm village, completely empty.

The dive inside Bilpa Jama was breathtaking. Now I was seated beside the cave shore preparing soup to warm myself. After a stunning solo dive, I was cold and wanted only to taste the peace of this magnificent place. While I was dipping  the spoon in my soup cup, I heard a faraway voice, a police woman calling me and asking me to stop eating and come quickly to her. 

After I did as I was asked, she started examining my passport, documents, and permissions. A few minutes later, a huge National Army truck reached us. The soldier had  an abnormal body shape, a man the size of a walking mountain in an Army uniform. Can you imagine how I was feeling in those moments?!

Well, in the end, everything went really well, and I now have a story to tell my grandchildren. 

Once the passport control was over, and they had checked that I did not cross  the border from Croatia to Slovenia illegally (customs was only a few hundred meters  from us), I had the chance to get back to my soup, which by then had turned cold. I warmed it up again, and I spent half an hour seated on a slippery stone covered with moss and lichens watching the beauty of the forest surrounding me.

On the way back to my accomodations in my cave van, I  played a new playlist. 

Four hours later, I approached my country lodge. I was really exhausted,  but I had to refill tanks and plan the next scuba diving days. Once I finished, I watched the forecast again. Unfortunately, it was growing worse, so I decided not to dive and instead get a surface break. Tomorrow I would drive, search, and catch info and GPS coordinates of caves. My tomorrow plans had turned into a sketching and surveying day. 

The Road To Suha Dolca 

I drove and walked for hours and hours, up and down the forest or on lonely roads in search of caves where I could return in winter or perhaps next year. During the last survey of the day, I watched a talented young guy playing a traditional concertina and thought, what a lovely atmosphere and a fitting way to close my hard-working day!

I decided to give a last gaze to Suha Dolca cave, my favorite one, on the way home. This was the third consecutive day I had arrived back at this spot. Observing it day-by-day, I tried to find the best moment to dive this cave.

Until now, it was inaccessible due to the strong flow. I wanted to dive here before leaving Slovenia. Tired and  driving slowly, I parked my van away from my accommodation. Since I had no lunch, I started feeling very hungry. A simple dinner was quickly served: dried fruits and a cup of hot noodle soup.

My ‘NO DIRECTION HOME’ trip was now at its peak. I had become a wanderer. I was alone in a wild country with, yes, an internet connection for historical research and checking the weather. That was the only technology I used. Aside from that, I lived simply. I walked, dived, wrote, and filmed my experience all with my mobile phone.

Rain was tougher than expected. I had hoped to stop for one day, not the two that it took. Following the surveys, the next day I started fixing my video equipment and saving photos and videos I had made on my hard drive.

I had too many ideas, no one clear till the end, and too many cave sketches and GPS points to reorganize; I needed a day to regroup. I just went out for a few hours to check Suha Dolca’s Cave conditions. On this day it seemed that the flow was getting more stable, and general water conditions were growing better. I had to be patient and wait one or two days more for the right conditions. I tried and failed to find a solution on my own, but the water always showed me the way. She told me to wait and to go back to where I came from. Step-by-step I walked the path again.  

The third video chapter of Slovenia Solo Cave Diving is the one I prefer, because I remember the indecision I felt, to stay or to leave. Solo trips are strictly linked to life’s decision.

The last day I was in Slovenia I left the accommodations and asked a new farmer, close to a different cave, if I could sleep inside his barn and dive the river hole on the following day. I was at the same place where I had dived the first day. He told me I could not stay in the barn due to the high risk of bears who live in the surrounding area. I jumped in my van again and I drove to the lake beside Suha Dolca’s Cave. 

I descended the path several times and brought all my scuba gear piece-by-piece. I decided to give myself a chance to dive my dream cave in the late afternoon. I had no other choice. Once I was inside the cave it was unbelievable, and I had a very nice dive even though I was really tired, and again I broke my light arms and camera housing. I resurfaced after the dive into a reed’s lake, which made me feel like a beaver.

I had conflicting feelings as I left Slovenia that same night after making a tricky and stunning dive. Bears, awesome forests, and rural areas were now all behind me. The cave-van played a new disc, I needed to shake off these feelings and look forward to my new goals: Garda Lake’s wrecks, South Tyrol’s stunning lakes, and finally Austria. In the country of green and wide grazing land I wish to dive surrounded by the amazing scenario of beautiful Alps mountains. 

At 9:30 PM I crossed the border again, and  Italy was straight ahead.

Andrea Murdock Alpini is a TDI and CMAS 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. Recently he published his first book entitled, Deep Blue: storie di relitti e luoghi insoliti.

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