Here are a few items you can include in your logbook to help you stay organized and honest, track progress, and work on self-improvement as a diver.
Because of your responses, we felt it important to put our previous article into context.
Buying your first rebreather is no easy task and as with any large investment you want to make sure you are making the right purchase the first time around so you do not spend that money twice.
This article explains several common myths and misconceptions surrounding rebreather diving, including: “rebreathers are complex” “A rebreather is only a tool to be used on the rare occasions when the dive is beyond the capabilities of open circuit.” And more…
The following are sensible suggestions of things divers should never do, based entirely on common sense.
In reality, Trimix is a risk management breathing mixture utilized by divers typically seeking to offset the consequences of diving normoxic air or nitrox mixtures at a planned diving depth by replacing much of the nitrogen and some of the oxygen with more benign inert gases like helium.
Fortunately, there is a way you can discover whether CCR diving may be for you without ever having to make that investment. It’s called the TDI Rebreather Discovery experience.
There is no fluff when it comes to CCR diving. Each step has value and should be followed as prescribed. And that’s not just my opinion, it’s the best, safest and most secure way.
by John E. Lewis, Ph.D.1:
This article could have dealt with Boyle’s law, partial pressures, oxygen toxicity, and how rebreathers work. Unfortunately, it would be three times as long and could (probably would) prove to be boring. Therefore, I chose a more direct approach. I have designed a single day of recreational diving and compared the experience of three divers equipped with different scuba devices ranging from a common open circuit aluminum 80 ft3 tank using air, an identical tank but one equipped with 32% nitrox, and finally the recently introduced Hollis Explorer semi-closed rebreather.
The Dive Scenario
The day’s diving takes place from a boat that is positioned in an area with a wide variety of depths ranging from walls with maximum depths over 30 metres / 100 feet to reefs as shallow as 18 metres / 60 feet. The Captain of the boat has decided there are to be three no-decompression (NoD) dives separated by two hour surface intervals and are to include a mandatory safety stop of five minutes at 3 metres / 10 feet. He also insists that the divers surface with no less than 35 BAR / 500PSI in their tanks.
This boat does not have a compressor, and it follows that all of divers must bring the tanks necessary for the three dives. The first dive is a multi-level dive beginning at 30 metres / 100 feet followed by a 15 metres / 50 feet second depth option. The second dive is also a multi-level dive to a maximum depth of 24 metres / 80 feet with a 12 metres / 40 feet second depth option. The third dive is to be at a single depth of 18 metres / 60 feet. In order to be able to visit the three sites that the Captain has chosen in the time allotted, he insists that for the first two dives the divers not exceed a 60 minute bottom time.
The Divers and their Equipment
Bob, by far the oldest of the three, carries on board three aluminum 80 ft3 tanks filled with air (21% oxygen). He has been diving for over 40 years, and he has developed the particularly low value of air consumption (SAC of 0.5 cu ft/min)2. Mike has three new aluminum 80 ft3 tanks that are prefilled with 32% enriched air mix and sport Nitrox labels. Ordinarily he has a considerably higher SAC, but for this exercise we have made his the same as Bob in order that we can see what Nitrox brings to the table. Nick, by far the youngest, has a brand new Hollis Explorer semi-closed rebreather that is equipped with a steel 28 ft3 tank filled with a 40% nitrox mix, and he too has two backup tanks similarly filled with 40% nitrox. Nick has a SAC of 0.75 cu ft/min, which represents a more common value among recreational divers.
All of the divers are equipped with dive computers, and with one exception, all have been programmed to reflect a predetermined value of oxygen content. The exception is Nick’s Explorer that has been designed to optimize the dive time by choosing a value of the oxygen fraction in the breathing gas such that the no-decompression limit (NDL) equals the capacity of the device. This of course is subject to the maximum operating depth (MOD) dictated by an accepted value of PO2 = 1.4 atm. In addition, the dive time based on the canister life of the Explorer is limited to 120 min.
The First Dive
The results of the first dive are shown in Table 1, where TBT refers to the total bottom time. The dive times that were controlled by NoD limits are labeled as ND, by scuba capacity as CAP, and 60 for the Captain maximum specified bottom time.
|Depth (fsw)||Bob (OC air)||Mike (OC Nitrox 32)||Nick (Explorer)|
|100||18 ND||30 ND||37 ND|
|50||22 CAP||12 CAP||18 (60)|
|TBT||45 min||47 min||60 min|
As can be seen, Bob, the traditional open circuit air diver, was seriously disadvantaged at the first depth of 30 metres / 100 feet where the other two divers have significantly greater bottom times. Note that if Nick with a more common SAC had been using the open circuit rig, his TBT would have been less than 30 min.
The Second Dive
After a 120 minute surface interval, the Captain has moved the boat to a new dive site where the maximum depth is 24 metres / 80 feet. Again Bob is at a disadvantage at the first stop. However, it is interesting to note that Mike who gained eight minutes over Bob at the greatest depth, lost five minutes in total bottom time. Nick with the Explorer greatly surpasses both Bob and Mike at the first depth even with the imposed 60 minute TBT.
|Depth (msw)||Depth (fsw)||Bob (OC air)||Mike (OC Nitrox 32)||Nick (Explorer)|
|24||80||30 ND||38 CAP||55 (60)|
|TBT||TBT||47 min||42 min||60 min|
The Third Dive
Finally the boat is anchored above a reef that has a constant 18 metres / 60 feet depth, and the Captain has told the divers that they no longer need to adhere to the maximum 60 min bottom time. All of our divers have switched to their clean tanks, and Nick has renewed the Explorer’s canister absorbent. The result is that with no constraint on bottom time, as can be seen in Table 3, Nick has more than twice the bottom time with the Explorer over the open circuit divers.
|Depth (msw)||Depth (fsw)||Bob (OC air)||Mike (OC Nitrox 32)||Nick (Explorer)|
|18||60||46 CAP||46 CAP||112 (IDEAL)|
|TBT||TBT||51 min||51 min||117 min|
We don’t see dramatic differences during the first two dives largely because of the boats 60 min bottom time limit. However, Mike by using Nitrox has had more time at the greatest depth as well as Nick using the Explorer. It is worth remembering that Nick has an average diver’s SAC of 0.75 cu ft/min whereas Bob, the elder, and Mike by caveat was granted the same low SAC rate of 0.5 cu ft/min. Finally, on the third dive where the Captain removed the 60 min bottom time cap, the Explorer had more than twice the bottom time as the open circuit divers. It is worth noting that while the Hollis Explorer is semi-closed, during the entirety of these dives the exhaled gas never reached 10% of that of the open circuit divers.
Based on this example, the Hollis Explorer rebreather has a significant advantage over open circuit divers even those with exceptional breathing control that is 2/3 the SAC of the average diver such as Nick. The Hollis Explorer is not accurately characterized as “no bubbles.” However, the undeniably aesthetic appeal of quiet that is less than 10% of open circuit divers is of considerable potential value to any diver in addition to the increased ability to interact with wild live.
In 1989, the term “dive computers” was first coined to describe expensive and exotic devices that were known by no less than 27 different names. Less than ten years later, the term was common place and the majority of divers dove with one. I will be surprised if another ten years go by before the same cannot be said of the expensive and exotic devices known as … rebreathers, and for recreational divers, rebreathers similar to the Hollis Explorer have the potential to be the standard for the future.
1This article is an updated version of an original article by the author that appeared in the magazine Discover Diving in February 1997.
2Surface Air Consumption
by Mark Phillips:
War is one of mankind’s greatest failings and perhaps one of the greatest instigators of invention.
In war men fight. Ships sink. When men learned how to extend their time underwater to salvage sunken vessels it was their nature to consider the concept of an underwater warrior. Those soldiers would have to be comfortable in most any condition of water. They would have to be able to swim great distances and still have the strength and stamina to carry out their mission and still escape unharmed.
They would need specialized breathing equipment that would allow them to breathe underwater. In order for the apparatus to allow for long range penetration of enemy held areas, it had to reuse the exhaled breath of the diver while preventing exhaust bubbles from escaping and giving away the position of the diver. It had to offer extended time underwater, be light weight, versatile and dependable. It had to be a rebreather.
On the morning of June 6, 1944, Operation Overlord commenced. It was an event like none other and was divided into many parts, each of which had to work with the others to be successful. It was also the beginning of Operation Neptune and began the invasion of France at Normandy. It was D-Day. It was the largest amphibious invasion in world history.
Those landing from water were to land on one of five beaches code named Utah, Omaha, Gold, Juno and Sword. The hard thing about such an invasion is that it is hard to hide. When those occupying the land do not wish to be invaded, they resist.
Fortifications, hedgehogs, steel and concrete spikes, some steel tetrahedral, mines and other hazards had been placed on the beaches and in the water. Guns were placed on heavily fortified bunkers perched on the hillsides. Fortified machine gun nests had overlapping fields of fire. And the guns of Pointe Du Hoc could rain down hell on both Utah and Omaha beaches and sink vessels at sea 15 miles out.
Before the landing craft could land the fortifications blocking their way had to be destroyed. Naval demolition teams were responsible for those obstacles underwater and the Army engineers above water. But plans never quite work out the way they are supposed to.
The weather changed. Conditions worsened but once started Operation Overlord could not stop. The degrading weather cost them time and high tide was missed. Because the tide was out when the demolition teams made it to their objectives, most of the obstacles were out of the water. The naval group took those seaward while the army teams placed explosives on those closer to land. On D-Day, they were not all referred to as frogmen. Those from the Royal Navy were Landing Craft Obstruction Clearance Units. More commonly called Lockyews.
|Landing Craft Obstruction Clearance Units|
The ‘LCOCUs’ were a vital part of the D-Day invasion forces in Normandy. Four Royal Navy and six Royal Marines units comprising 120 divers wore newly developed neoprene suits with ‘blast proof’ kapok jackets underneath, helmets, breathing apparatus and fins.They laid the foundations for the Very Shallow Water (VSW) and beach clearance techniques in use today.Those remaining after the war were eventually incorporated into the Clearance Diving Branch.
A RN demolitions team was working Gold beach. When they arrived, they found that the obstacles they were to clear were underwater. Each of the hedgehogs they were to clear had to have 36 small charges placed at strategic positions so that the steel would blow into pieces with none more than 18 inches above the bottom. Each of these obstacles was covered with pressure sensitive explosives designed to punch holes in water craft.
Lt. Hargreaves described the experience:
“We must have been about four hundred yards from the beach when the firing first started, and they didn’t forget to inform us that they knew we were coming. When we finally got on the beach we discovered that we were being systematically sniped, not only with rifles but also by odd bursts of machine-gun fire – a most unpleasant experience”.
On another beach one of the men described his experience like this:
“We were spotted from a tower ashore and were subjected to pretty heavy mortar fire during which a petty officer was killed and two men were wounded. Later the R.A.F. blotted out the tower and things were more comfortable although shells still kept coming over. One shell destroyed our breathing apparatus, which we had not been using as the tide was low. When the water came up later, Leading Seaman A. Robertson and myself tried staying underwater by holding our breath. We blew about fifteen obstacles in this way, but we couldn’t keep it up. We carried on the next morning, after sleeping in a R.A.F. crater, where incidentally we were subjected to fire from an 88mm gun.”
Dennis Shryock was 21 years old when he landed on Utah Beach. Dennis had been trained as an explosives specialist and one of those elite men who were the forerunners of our modern day Navy Seals.
The machinegun fire was deadly. They did not have the protection of being underwater and had to wade to each of the obstructions to place explosives. He said the water “looked like pure blood.” But they did the job.
According to navy statistics, at Utah, the demolition teams lost six men and had eleven wounded. Omaha beach did not fare as well. They lost thirty-one and another sixty were wounded.
|Pointe Du Hoc |
The water was rough from the stormy wind and the unexpected rain soaked equipment that was intended to remain dry. It took the landing craft longer to reach the beaches than expected. The plan was to hit the beaches at high tide; for the troops to be able to take shelter in the bomb craters as they made their way up the beaches. But they missed high tide. Most of the obstacles placed to keep the landing craft away were on dry land. When they were able to land, the troops had to run through wet sand 300 to 500 yards just to get to the bomb craters. Landing craft that had been fortified with bullet proof plating caused the crafts to ride much lower in the water. Too many of them were swamped and sunk before they could reach shore. The majority of the soldiers on board, weighted by 70 pound of equipment, drowned. Those that survived had to face a wall of bullets and artillery shells. The guns at Pointe Du Hoc had to be taken. The 2nd Ranger Battalion had trained hard in preparation for this day. They had practiced climbing cliffs and had brought along firefighting ladders and rocket propelled grappling hooks to help make the 100 foot climb. But it had rained.The ropes were wet and the propellant used was calculated with dry rope. The ladders were hard to foot and difficult to climb. German machinegun fire was held to a minimum by sharpshooters on the ground but they could not stop them from dropping hand grenades in an effort to keep them from climbing. The 2nd Ranger Battalion clawed their way to the top using footholds in the mud and rocks and bayonets driven into the cliff side when necessary. It took them twenty minutes to make the climb and take the Pointe. They held it for two more days before reinforcements reached them. Of the 200 men that started, only 90 were left in fighting condition.
What exactly was the breathing apparatus used by the USN frogmen and the Royal Navy Landing Craft Obstruction Clearance Units? It was able to supply an extended amount of breathing gas underwater by reusing the exhaled breathe of the diver. It did not allow exhaust bubbles to escape and was stealthy. It was lightweight and maneuverable. It was a rebreather.
While only remotely similar to rebreather units today, the ones used in 1944 were many generations of development old. In fact, the Italians started the concept of underwater assault teams using specialized equipment. And they recognized the need as early as World War 1.
In 1918 two members of the Regia Marina (Royal Navy) literally rode a torpedo into a harbor and sank an enemy ship. At the time, they had no breathing equipment and had to guide the torpedo at the surface in order to breath. They sank the ship but were captured when they tried to swim away. These human torpedoes became more like mini submarines and were human guided. Obviously the ability to be under the water, able to breath and stay stealthy was an advantage.
By 1941 the Italian navy had both a surface unit that operated fast, explosive motor boats and a subsurface unit that used manned torpedoes. Within this group they also had assault swimmers. It did not take long before other countries developed their own versions.
The idea of an underwater warrior is older than modern history. The functional ability to use such a warrior has always been limited by the inability to breath underwater. Throughout history, man has found ways to extend his time underwater and by the turn of the 20th century, some of the first closed circuit rebreather systems had been experimented with and used. The rebreather units used on D-Day were rudimentary compared to the modern versions we see today. But at the time, they did the job and those who used them had to be beyond courageous.
On June 6, 1944 those underwater warriors had a mission to do and short of being killed or captured, that is what they did.
Mark Phillips is a retired 33 year career firefighter and Public Safety Diver; A Master Scuba Instructor an ERDI trainer, and Publisher of PSDiver Monthly, an Internet magazine dedicated to advancing the safety and knowledge of the Underwater Investigator.