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Back in the early 1990s this was a system that was designed for trimix dives on coral reefs to 300FSW, or so. In light of what has been learned since, Id be very interested in a DIR and/or Hog, cave, etc. critique of the rig.
DESCRIPTION
The rig design incorporates two 160 ft³ primary bottles. One is filled with bottom-mix (TRIMIX or HELIOX) and the other is filled with intermediate-decompression-mix (usually NITROX-32). Two independent regulators are attached to the decompression-mix cylinder with a dual (slingshot) valve. One side of the valve is maintained in the open position and the other is kept closed after pressuring its regulator to prevent the entry of water during the dive. The closed side is the backup and is opened and used only in the event of a primary regulator failure. The primary bottom-mix cylinder has a single regulator maintained in the pressurized, open state. A 30 ft³ cylinder containing identical bottom-mix is mounted valve down on the right side and is maintained in the pressurized, open state for use if there were a malfunction in the primary bottom-mix system. Besides these three cylinders, a 30 ft³ cylinder of pure oxygen for the final decompression stops is mounted valve down on the left side. This cylinder, equipped with a single oxygen clean regulator, is maintained in a pressurized but closed state. It is a backup supply. Primary oxygen is a stage bottle or surface supplied oxygen system at the boat. The backup oxygen is only opened only in the event the diver is unable to return to the boat during the final stages of decompression or if there is a failure in the primary oxygen system. A buoyancy compensator bladder is connected to the EAN-32 bottle and a backup bladder is connected to the primary bottom-mix bottle. Redundancy is increased by the divers option to use a buddys backup gas supplies with standard auxiliary regulator breathing procedures.
The EAN-32 and oxygen backup regulators are specially prepared, oxygen cleaned and maintained for oxygen service. Auxiliary regulators, similarly prepared are mounted on special adaptors on the two BC inflators (One for primary bottom mix and EAN-32).
FAILURE MODE ANALYSIS
Deep mix diving shares some commonality with cave diving. In both cases there is an overhead obstruction that prevents a diver from returning to the surface. For cave diving this obstruction is physical rock over the diver. For a deep reef dive the obstruction is virtual, the divers decompression ceiling. This ceiling is the shallowest depth to which the amount of absorbed inert gas will permit a safe ascent. Under normal circumstances, both overheads are the same to the diver, but in the event of an emergency the differences are significant.
A diver whose concern is a decompression ceiling needs a different redundancy design than a diver who must travel a distance through a cave before obtaining access to the surface. A diver lost in a silted up cave at ten feet of depth will die when the air runs out. A diver with a twenty-foot decompression ceiling will likely survive a short trip to the surface and quick reimmersion with a fresh tank with no ill effects. Each TRIMIX cave diver must carry a sufficient quantity of bottom-mix reserve so that if one of the dive team suffers a complete primary gas equipment failure at the furthest point of penetration both divers will be able to make a long horizontal swim to gain access to a depth where it is safe to switch to EAN. Solving this problem requires the use of the rule-of-thirds (one third going in, one third coming out, and one third for my buddy) and a set of crossed-over-doubles with two regulators and an isolation valve. TRIMIX dives in the ocean have a virtual ceiling that, by limiting bottom time, is always kept above 130-FSW where EAN-32 may be safely used. Complete failure of the primary bottom-mix system thus only requires a direct ascent to 130-FSW where EAN-32 may safely be used.
All rigs demand attention to redundancy. Design must ensure that equipment malfunctions may be dealt with under actual and psychological pressure. This design takes into consideration the malfunction of pieces of equipment during a dive and always permits the diver access to sufficient breathing gas to permit a safe ascent to the surface. The design process is complicated by the need to have access to different breathing mixes at different points in the dive. On a deep dive using separate bottom, intermediate and decompression gases, each gas system must have a backup. Three times as much gear is needed to arrive at the same level of redundancy as would be needed for an air dive.
There are three failure modes that must be considered for each system: No-Gas-Delivery, Too-Much-Gas-Delivery and Gas-Management-Error. A No-Gas-Delivery failure mode is characterized by a regulator that suddenly stops working, although there is still gas in the cylinder. This is perhaps the most serious potential crisis, but fortunately is rare with modern, well-maintained, two stage regulators. A Too-Much-Gas-Delivery failure would result from a free flowing regulator and a Gas-Management error could result from an inaccurate pressure gauge or human error.
The only gas, carried by the diver, that can be safely breathed below 130-FSW is bottom-mix. EAN-32 and oxygen may not be relied upon as deep backups. Therefore, the bottom-mix backup regulator must be maintained in the open state to allow the diver immediate access to safe backup gas in case of any failure mode. A slingshot valve would provide adequate regulator redundancy for the bottom-mix system. This results in the same redundancy deemed acceptable by cave divers. But this approach effectively doubles the probability of a Too-Much-Gas-Delivery failure. Such a failure would result in the rapid loss of the primary, and now only, bottom-mix supply. By using a separate backup bottle, with the valve open, an adequate redundant supply of bottom-mix is always accessible to the diver to provide a means of ascending to 130-FSW.
In contrast, a slingshot valve on the EAN-32 cylinder is a good choice since only regulator redundancy is required. Any bottom-mix with 12% or more oxygen can be breathed at 20-FSW, or deeper, without fear of hypoxia. Since EAN-32 is only used between 130-FSW and 20-FSW, the bottom-mix system provides a short-term emergency backup for a No-Gas-Delivery or Too-Much-Gas-Delivery failure. The diver can use bottom-mix while closing the primary EAN-32 slingshot valve (if necessary) and opening the secondary slingshot valve. Then the diver may switch back to EAN-32 from the secondary regulator. Mismanaging EAN-32 is unlikely since there is considerably more gas then needed. In fact, the cylinder has 160 feet³ of gas, enough to perform the required decompression almost three times over.
Primary gas for oxygen decompression is a staged bottle or surface supplied system located at the boat. The diver carried backup oxygen bottle is a redundant system and is used only if the diver is unable to return to the boat at the end of the dive or if the primary oxygen system malfunctions. In an immediate danger situation EAN-32 and even bottom-mix may be used as a temporary backup while the diver sorts things out.
Besides safeguarding against equipment failure, the rig is designed to reduce human error as well. To minimize the possibility of accidentally breathing the wrong mixture at the wrong depth all regulators are color coded to indicate the gas they deliver. The use of different regulator designs, side-breather vs. standard and left vs. right-handed also helps to differentiate mixes.
The rig design is not without weaknesses; all of the systems are at the mercy of a double failure, such as a blowout plug rupture in the primary bottom-mix combined with a regulator malfunction in the backup bottom-mix cylinder, which would leave a diver totally dependent upon the other diver for bottom-mix until they reach 130-FSW.
DESCRIPTION
The rig design incorporates two 160 ft³ primary bottles. One is filled with bottom-mix (TRIMIX or HELIOX) and the other is filled with intermediate-decompression-mix (usually NITROX-32). Two independent regulators are attached to the decompression-mix cylinder with a dual (slingshot) valve. One side of the valve is maintained in the open position and the other is kept closed after pressuring its regulator to prevent the entry of water during the dive. The closed side is the backup and is opened and used only in the event of a primary regulator failure. The primary bottom-mix cylinder has a single regulator maintained in the pressurized, open state. A 30 ft³ cylinder containing identical bottom-mix is mounted valve down on the right side and is maintained in the pressurized, open state for use if there were a malfunction in the primary bottom-mix system. Besides these three cylinders, a 30 ft³ cylinder of pure oxygen for the final decompression stops is mounted valve down on the left side. This cylinder, equipped with a single oxygen clean regulator, is maintained in a pressurized but closed state. It is a backup supply. Primary oxygen is a stage bottle or surface supplied oxygen system at the boat. The backup oxygen is only opened only in the event the diver is unable to return to the boat during the final stages of decompression or if there is a failure in the primary oxygen system. A buoyancy compensator bladder is connected to the EAN-32 bottle and a backup bladder is connected to the primary bottom-mix bottle. Redundancy is increased by the divers option to use a buddys backup gas supplies with standard auxiliary regulator breathing procedures.
The EAN-32 and oxygen backup regulators are specially prepared, oxygen cleaned and maintained for oxygen service. Auxiliary regulators, similarly prepared are mounted on special adaptors on the two BC inflators (One for primary bottom mix and EAN-32).
FAILURE MODE ANALYSIS
Deep mix diving shares some commonality with cave diving. In both cases there is an overhead obstruction that prevents a diver from returning to the surface. For cave diving this obstruction is physical rock over the diver. For a deep reef dive the obstruction is virtual, the divers decompression ceiling. This ceiling is the shallowest depth to which the amount of absorbed inert gas will permit a safe ascent. Under normal circumstances, both overheads are the same to the diver, but in the event of an emergency the differences are significant.
A diver whose concern is a decompression ceiling needs a different redundancy design than a diver who must travel a distance through a cave before obtaining access to the surface. A diver lost in a silted up cave at ten feet of depth will die when the air runs out. A diver with a twenty-foot decompression ceiling will likely survive a short trip to the surface and quick reimmersion with a fresh tank with no ill effects. Each TRIMIX cave diver must carry a sufficient quantity of bottom-mix reserve so that if one of the dive team suffers a complete primary gas equipment failure at the furthest point of penetration both divers will be able to make a long horizontal swim to gain access to a depth where it is safe to switch to EAN. Solving this problem requires the use of the rule-of-thirds (one third going in, one third coming out, and one third for my buddy) and a set of crossed-over-doubles with two regulators and an isolation valve. TRIMIX dives in the ocean have a virtual ceiling that, by limiting bottom time, is always kept above 130-FSW where EAN-32 may be safely used. Complete failure of the primary bottom-mix system thus only requires a direct ascent to 130-FSW where EAN-32 may safely be used.
All rigs demand attention to redundancy. Design must ensure that equipment malfunctions may be dealt with under actual and psychological pressure. This design takes into consideration the malfunction of pieces of equipment during a dive and always permits the diver access to sufficient breathing gas to permit a safe ascent to the surface. The design process is complicated by the need to have access to different breathing mixes at different points in the dive. On a deep dive using separate bottom, intermediate and decompression gases, each gas system must have a backup. Three times as much gear is needed to arrive at the same level of redundancy as would be needed for an air dive.
There are three failure modes that must be considered for each system: No-Gas-Delivery, Too-Much-Gas-Delivery and Gas-Management-Error. A No-Gas-Delivery failure mode is characterized by a regulator that suddenly stops working, although there is still gas in the cylinder. This is perhaps the most serious potential crisis, but fortunately is rare with modern, well-maintained, two stage regulators. A Too-Much-Gas-Delivery failure would result from a free flowing regulator and a Gas-Management error could result from an inaccurate pressure gauge or human error.
The only gas, carried by the diver, that can be safely breathed below 130-FSW is bottom-mix. EAN-32 and oxygen may not be relied upon as deep backups. Therefore, the bottom-mix backup regulator must be maintained in the open state to allow the diver immediate access to safe backup gas in case of any failure mode. A slingshot valve would provide adequate regulator redundancy for the bottom-mix system. This results in the same redundancy deemed acceptable by cave divers. But this approach effectively doubles the probability of a Too-Much-Gas-Delivery failure. Such a failure would result in the rapid loss of the primary, and now only, bottom-mix supply. By using a separate backup bottle, with the valve open, an adequate redundant supply of bottom-mix is always accessible to the diver to provide a means of ascending to 130-FSW.
In contrast, a slingshot valve on the EAN-32 cylinder is a good choice since only regulator redundancy is required. Any bottom-mix with 12% or more oxygen can be breathed at 20-FSW, or deeper, without fear of hypoxia. Since EAN-32 is only used between 130-FSW and 20-FSW, the bottom-mix system provides a short-term emergency backup for a No-Gas-Delivery or Too-Much-Gas-Delivery failure. The diver can use bottom-mix while closing the primary EAN-32 slingshot valve (if necessary) and opening the secondary slingshot valve. Then the diver may switch back to EAN-32 from the secondary regulator. Mismanaging EAN-32 is unlikely since there is considerably more gas then needed. In fact, the cylinder has 160 feet³ of gas, enough to perform the required decompression almost three times over.
Primary gas for oxygen decompression is a staged bottle or surface supplied system located at the boat. The diver carried backup oxygen bottle is a redundant system and is used only if the diver is unable to return to the boat at the end of the dive or if the primary oxygen system malfunctions. In an immediate danger situation EAN-32 and even bottom-mix may be used as a temporary backup while the diver sorts things out.
Besides safeguarding against equipment failure, the rig is designed to reduce human error as well. To minimize the possibility of accidentally breathing the wrong mixture at the wrong depth all regulators are color coded to indicate the gas they deliver. The use of different regulator designs, side-breather vs. standard and left vs. right-handed also helps to differentiate mixes.
The rig design is not without weaknesses; all of the systems are at the mercy of a double failure, such as a blowout plug rupture in the primary bottom-mix combined with a regulator malfunction in the backup bottom-mix cylinder, which would leave a diver totally dependent upon the other diver for bottom-mix until they reach 130-FSW.
