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Shut downs are a pain in the bum for me, loads of straining and way too slow so the choice is invert or slob knob. Some people would have you believe that manual shutdowns are easy with a little practice, that’s because it was for them. I still do practice manual shut down every dive but I have only ever achieved ‘just about’ and never progressed to easy. I have tried mounting my tanks higher and higher until in the end I was diving like a bulldozer but all to no avail. The question in my mind is why are we using such poorly designed kit. If the valves were set at 45 degrees to the manifold it would be so much easier. If the central isolator valve was longer it would be a doddle to reach so why don’t they design them like that? Perhaps in the future they will. Truth is manifolded twin sets are a relatively new thing. Most modern kit takes the easy way out and just invert the whole thing.
Standard tank rigs were set up due to the need for twin hose regulators before modern first stage regs were invented. The practical aspects of diving a tank where the valve was at the top meant that the system persisted after the need for the location was gone. Having the valve at the top allowed the tank to stand up for easy kitting up and it did not require protection from bashing when the diver sits down so all in all it had more benefits than problems. Shut downs were never really considered an issue as divers were trained to ascend on a free flowing regulator so this detail was ignored.
Fact: Most divers last two years of UK diving then quit and stick to blue water stuff or finish diving all together. So it’s unlikely that a fair few of today’s divers were diving ten years ago. As a result a lot of divers are unaware of the rapid changes in that period. Deco and so called technical diving is still a very new thing. Twin sets were laughed at in Dover three or four years ago. Only the serious deep nuts used twin sets and independents were far more likely. Last year I dived off Lundy Island and both the available boat skippers looked at twin sets as unnecessary and dangerous. How you going to climb the ladder with that lot on your back I was asked. These days’ things are changing. In the last few years more and more boats are gearing up for twin set divers and trimix with lifts being fitted. “Got enough gas there mate?” is not shouted across Portland harbour as often as more and more divers get into twin sets and deco diving.
The isolation manifold became popular in the mid 90s and found wide spread acceptance around the end of the 90's and the turn of the millennium. Navy divers were already inverting their rigs as their twin set dive system was based on shut downs and tank equalisation and inversion made this easy. The inverted system was being widely used in cave diving and other overhead environments where gas management was a more critical part of dive planning than open water. With the general acceptance of decompression diving and Trimix in recreational diving, gas management became a major issue in open water for the first time. Deco was basically another overhead environment. As a result standards for gas management became part of training courses and the shut down drill was born.
The now standard twin set rig was not ideally suited to shut downs. The valves were in an awkward place to reach but to overcome the problem would impact on the whole industry in terms of standardisation of hose lengths for regulators dry suits and wing/ BCD inflation. As a result the problem has been ignored. The bulk of the diving industries customers are blue water single tank BCD divers. So the major manufacturers left it to the niche market companies like OMS and CD to cater for the small band of tec divers. This they did by bringing out remote isolation devices nick named slob knobs. They also brought out cages strong enough to support a twin set on and the recreational inverted rig was born. The cages were also popular for pulling the twin set forward to allow gripping of the isolation valves on conventional shut down drills.
Any new design piece of equipment entering the market place today has inverted cylinders with one notable exception. (The Halcyon Rebreather). ALL other SCR CCR systems and even the modular OC rigs are now fitted with inverted or lower back horizontal mounted tanks to allow easy access to valves.
It is obvious that it is easier to reach valves at your hips than it is to reach them at your shoulder blades and because it’s easier it is inherently safer. You can exert greater force on the valve at your hip than at your shoulder so if the valve is jammed open you stand a better chance of closing it. Also during the dive inverted tanks protect the valves and prevent role offs. This is where the valve tap is rolled shut by contact with the cave/ wreck shot line etc. Impact damage is also avoided, as reverse fin kicks are not so efficient yet that we reverse into any thing at any speed J
So if its blatantly obvious that inverts are safer why don’t we all use them? The first reason is house routing and kitting up. Owing to the nature of the beast the hoses run in between the set and the wing forcing the diver to rig the set before attaching the wing. Secondly the hoses have to be specially made, as there are no standard length hoses for inverts. The final issue is that the rig needs a sturdy valve guard to act as protector and resting point for the rig on the shore or boat. All this equates additional hassle and cost.
For these reasons divers like my self who can reach the left and right posts in a reasonable time but have difficulty isolating the set, opt for the remote shut down or slob knob.
There is another less savoury reason why inverts and slob knobs are frowned upon. The dive god complex. Dive gods pick aspects of diving that are difficult to master or need loads of practice and say if you cant do it than you’re a crap diver. This of course is usually rubbish. I believe a method of diving, which is highly skill based and offers complexity to the diver will prove difficult or imposable under high stress situations and should therefore be avoided if at all possible. Shut down drills are worse than that as they can be very easy for some divers and impossible to others simply due to the way their arm and shoulder joints work. It’s like wiggling your ears. Some people can do it, some people can do it if they try real hard and some people just can’t do it.
There are ways around the problem such as extended rubber knobs and mounting the tanks much higher on the back of the diver and then getting trim sorted by adding weight low down on the set with V weights etc. but even these are not enough for some divers. Releasing the waste buckle of the wing and hoiking the set up whilst horizontal in the water always works for me but its time consuming and still a struggle.
I have found that in the summer months when I wear just a base layer and my 5mm neo suit I can reach all my valves. In the winter with a Thinsulate under suit on I cant. That said I have done practice shut downs where I could reach the valve but, on my unbalanced manifold I haven’t been able to exert enough pressure to turn the thing off.
It was all just to slow and too much effort so I opted for a Slob knob. I have run it for getting on for two years and it has never failed in the water. I have had one surface failure where the crimped end on the spring split but that was found by routine checking and it wasn’t terminal. Point is if a slob knob fails you can still perform manual shutdowns. You can use the hose of the slob knob to guide the hand to the valve and you can walk your fingers along the hose and pull the set forward with the hose to assist in the task. So total down side over a manual shut down errrrr none.
The failure of slob knobs and routing of internal springs etc etc. is a by product of poor maintenance. They should be regularly striped inspected and lubricated. If you don’t do this don’t complain if they fail along with all your other poorly maintained kit. The direction of the spring winding is such that it tightens on the shut down. As a result the alleged unravelling of the spring won’t happen in the shut down direction. Frankly it shouldn’t happen in the opening up direction as the coils are contained in a strong housing which if well lubed will keep it all together in your emergency. The problem will show its self in the post dive strip and maintenance cycle. This is exactly what happened with my split crimped end. The failure occurred after lending the twin set to another diver. When I got it back I striped it to lube the slob knob and I discovered the split its likely that they attempted to open a jammed open or shut isolator with the slob knob rather than by hand. This often happens as the gas fill man will fully open the isolator prior to filling the set and once there is 250bar in there the isolator needs a good twist to free it up. I always do this by hand and then just leave the valve one turn open.
The big benefit of inverts is that all the valves are easy to reach. With a slob knob you can guarantee isolation and saving the remaining gas in one cylinder but not in both. With inverts reaching the side valves is so quick and easy that the central isolator is virtually redundant. Some invert users use a slob knob for the central isolator to make that one easy to use as well. Best of both worlds. So it comes down to gas management. On a deep dive (for me that’s 50-70m) I am confident in always having enough gas in one tank to get me to my first gas switch depth. This is because I always plan my first gas switch mix to be a max PP02 of 2 to the top of the wreck. So I can virtually swap to travel gas at any point in the dive. When diving without travel gas I ensure that I will always have enough gas in one cylinder to get to the 6m or 9m gas switch stop on a deco dive or to the surface on a no deco dive. To this end you have to discipline your self to always carrying a totally independent gas source on deco dives. I don’t see a problem with that as I always minimise deco by using a rich deco mix even if it’s just a pony of 60% strapped to the side of the twin set. Using this gas management principle even if all I can do is isolate the manifold with the slob knob and I loose all the remaining gas in one cylinder I can still get home.
Inverts allow for greater reserves of gas. The bail out can be based on enough gas left in 1 & ½ tanks. I say ½ tank, as the free flow will obviously lose some volume of gas prior to the shut down. As a result they offer improved safety margins or longer dive times over a slob knob rig. The main reason I haven’t changed over to inverts is that my dives have been bottom time limited on deco commitment rather than gas or bail out. Most dives an hour on the bottom is plenty so on a 50m dive I am within tolerance on the available gas to gas switch depth. On deeper dives the deco quickly racks up so 40mins at 70m is enough for me as your looking at two hours deco running my gradient factors. Should I have a free flow at 70m I could isolate in less than 3 seconds using a slob knob. Left with 50 bar in one 12ltr tank I would have 5mins to ascend to my first gas switch depth of 66m at PP02 1.6 on 21/40 or air bail out. Failing that I have 5mins to ascend to 40m and switch to first deco gas of 32% To me that’s an acceptable level of risk.
So why bother to write all this down.
Simple, to encourage any one out there who is aware of the fact they struggle with shut down drills to seek another way of dealing with the problem and to fight the corner for the divers who like me who have already made the choice. It’s not a sign of weakness or ineptitude and its not dangerous. What is dangerous is persisting with a system that just about works for you most of the time and is very slow in execution. I often talk to divers on boats who ask about the slob knob and by their questions I realise they have been totally misinformed. A couple have asked me if I was concerned about the additional 0 rings failing even though it has none. Others have comented on the internal spring going rusty but I show them my two year old 150+ dive unit without a spec of rust on the stainless steel spring and that’s usually the end of that argument. One thing they cant argue with, I can isolate the manifold much faster than any one can do it manually on a conventional rig.
This year I did a Trimix liveaboard with 10 other divers. Five days diving with four dives over 60m and two hour plus run times. Seven of the divers I had not met before. Apart from one other I was the least experienced trimix diver on the boat. Out of the 10 divers on the boat 8 ran slob knobs and one ran a CCR with inverted tanks. Eight of the divers on the boat had 5 years plus trimix depth experience. All of them had experienced an equipment failure resulting in gas loss at some stage in their diving career. So it’s no surprise then that they all chose to dive a rig that could be shut down quickly. The chap without a slob knob or inverts claimed to be able to shut down easily on a standard rig so he never saw the point. So that’s 1 out of 10 who believed he could do it just as fast on a conventional rig.
They cant all be numptie divers. Perhaps being at 60+m focuses the mind on the speed of shut downs a little more than at 30m
My advice to any new twin set diver is to attempt manual shut downs first. If you have problems seek professional or highly experienced help in resetting you rig. If that fails or you know in your hart that’s it’s a major struggle or just too dammed slow. Invert or get a slob knob.
ATB
Mark Chase
Shut downs are a pain in the bum for me, loads of straining and way too slow so the choice is invert or slob knob. Some people would have you believe that manual shutdowns are easy with a little practice, that’s because it was for them. I still do practice manual shut down every dive but I have only ever achieved ‘just about’ and never progressed to easy. I have tried mounting my tanks higher and higher until in the end I was diving like a bulldozer but all to no avail. The question in my mind is why are we using such poorly designed kit. If the valves were set at 45 degrees to the manifold it would be so much easier. If the central isolator valve was longer it would be a doddle to reach so why don’t they design them like that? Perhaps in the future they will. Truth is manifolded twin sets are a relatively new thing. Most modern kit takes the easy way out and just invert the whole thing.
Standard tank rigs were set up due to the need for twin hose regulators before modern first stage regs were invented. The practical aspects of diving a tank where the valve was at the top meant that the system persisted after the need for the location was gone. Having the valve at the top allowed the tank to stand up for easy kitting up and it did not require protection from bashing when the diver sits down so all in all it had more benefits than problems. Shut downs were never really considered an issue as divers were trained to ascend on a free flowing regulator so this detail was ignored.
Fact: Most divers last two years of UK diving then quit and stick to blue water stuff or finish diving all together. So it’s unlikely that a fair few of today’s divers were diving ten years ago. As a result a lot of divers are unaware of the rapid changes in that period. Deco and so called technical diving is still a very new thing. Twin sets were laughed at in Dover three or four years ago. Only the serious deep nuts used twin sets and independents were far more likely. Last year I dived off Lundy Island and both the available boat skippers looked at twin sets as unnecessary and dangerous. How you going to climb the ladder with that lot on your back I was asked. These days’ things are changing. In the last few years more and more boats are gearing up for twin set divers and trimix with lifts being fitted. “Got enough gas there mate?” is not shouted across Portland harbour as often as more and more divers get into twin sets and deco diving.
The isolation manifold became popular in the mid 90s and found wide spread acceptance around the end of the 90's and the turn of the millennium. Navy divers were already inverting their rigs as their twin set dive system was based on shut downs and tank equalisation and inversion made this easy. The inverted system was being widely used in cave diving and other overhead environments where gas management was a more critical part of dive planning than open water. With the general acceptance of decompression diving and Trimix in recreational diving, gas management became a major issue in open water for the first time. Deco was basically another overhead environment. As a result standards for gas management became part of training courses and the shut down drill was born.
The now standard twin set rig was not ideally suited to shut downs. The valves were in an awkward place to reach but to overcome the problem would impact on the whole industry in terms of standardisation of hose lengths for regulators dry suits and wing/ BCD inflation. As a result the problem has been ignored. The bulk of the diving industries customers are blue water single tank BCD divers. So the major manufacturers left it to the niche market companies like OMS and CD to cater for the small band of tec divers. This they did by bringing out remote isolation devices nick named slob knobs. They also brought out cages strong enough to support a twin set on and the recreational inverted rig was born. The cages were also popular for pulling the twin set forward to allow gripping of the isolation valves on conventional shut down drills.
Any new design piece of equipment entering the market place today has inverted cylinders with one notable exception. (The Halcyon Rebreather). ALL other SCR CCR systems and even the modular OC rigs are now fitted with inverted or lower back horizontal mounted tanks to allow easy access to valves.
It is obvious that it is easier to reach valves at your hips than it is to reach them at your shoulder blades and because it’s easier it is inherently safer. You can exert greater force on the valve at your hip than at your shoulder so if the valve is jammed open you stand a better chance of closing it. Also during the dive inverted tanks protect the valves and prevent role offs. This is where the valve tap is rolled shut by contact with the cave/ wreck shot line etc. Impact damage is also avoided, as reverse fin kicks are not so efficient yet that we reverse into any thing at any speed J
So if its blatantly obvious that inverts are safer why don’t we all use them? The first reason is house routing and kitting up. Owing to the nature of the beast the hoses run in between the set and the wing forcing the diver to rig the set before attaching the wing. Secondly the hoses have to be specially made, as there are no standard length hoses for inverts. The final issue is that the rig needs a sturdy valve guard to act as protector and resting point for the rig on the shore or boat. All this equates additional hassle and cost.
For these reasons divers like my self who can reach the left and right posts in a reasonable time but have difficulty isolating the set, opt for the remote shut down or slob knob.
There is another less savoury reason why inverts and slob knobs are frowned upon. The dive god complex. Dive gods pick aspects of diving that are difficult to master or need loads of practice and say if you cant do it than you’re a crap diver. This of course is usually rubbish. I believe a method of diving, which is highly skill based and offers complexity to the diver will prove difficult or imposable under high stress situations and should therefore be avoided if at all possible. Shut down drills are worse than that as they can be very easy for some divers and impossible to others simply due to the way their arm and shoulder joints work. It’s like wiggling your ears. Some people can do it, some people can do it if they try real hard and some people just can’t do it.
There are ways around the problem such as extended rubber knobs and mounting the tanks much higher on the back of the diver and then getting trim sorted by adding weight low down on the set with V weights etc. but even these are not enough for some divers. Releasing the waste buckle of the wing and hoiking the set up whilst horizontal in the water always works for me but its time consuming and still a struggle.
I have found that in the summer months when I wear just a base layer and my 5mm neo suit I can reach all my valves. In the winter with a Thinsulate under suit on I cant. That said I have done practice shut downs where I could reach the valve but, on my unbalanced manifold I haven’t been able to exert enough pressure to turn the thing off.
It was all just to slow and too much effort so I opted for a Slob knob. I have run it for getting on for two years and it has never failed in the water. I have had one surface failure where the crimped end on the spring split but that was found by routine checking and it wasn’t terminal. Point is if a slob knob fails you can still perform manual shutdowns. You can use the hose of the slob knob to guide the hand to the valve and you can walk your fingers along the hose and pull the set forward with the hose to assist in the task. So total down side over a manual shut down errrrr none.
The failure of slob knobs and routing of internal springs etc etc. is a by product of poor maintenance. They should be regularly striped inspected and lubricated. If you don’t do this don’t complain if they fail along with all your other poorly maintained kit. The direction of the spring winding is such that it tightens on the shut down. As a result the alleged unravelling of the spring won’t happen in the shut down direction. Frankly it shouldn’t happen in the opening up direction as the coils are contained in a strong housing which if well lubed will keep it all together in your emergency. The problem will show its self in the post dive strip and maintenance cycle. This is exactly what happened with my split crimped end. The failure occurred after lending the twin set to another diver. When I got it back I striped it to lube the slob knob and I discovered the split its likely that they attempted to open a jammed open or shut isolator with the slob knob rather than by hand. This often happens as the gas fill man will fully open the isolator prior to filling the set and once there is 250bar in there the isolator needs a good twist to free it up. I always do this by hand and then just leave the valve one turn open.
The big benefit of inverts is that all the valves are easy to reach. With a slob knob you can guarantee isolation and saving the remaining gas in one cylinder but not in both. With inverts reaching the side valves is so quick and easy that the central isolator is virtually redundant. Some invert users use a slob knob for the central isolator to make that one easy to use as well. Best of both worlds. So it comes down to gas management. On a deep dive (for me that’s 50-70m) I am confident in always having enough gas in one tank to get me to my first gas switch depth. This is because I always plan my first gas switch mix to be a max PP02 of 2 to the top of the wreck. So I can virtually swap to travel gas at any point in the dive. When diving without travel gas I ensure that I will always have enough gas in one cylinder to get to the 6m or 9m gas switch stop on a deco dive or to the surface on a no deco dive. To this end you have to discipline your self to always carrying a totally independent gas source on deco dives. I don’t see a problem with that as I always minimise deco by using a rich deco mix even if it’s just a pony of 60% strapped to the side of the twin set. Using this gas management principle even if all I can do is isolate the manifold with the slob knob and I loose all the remaining gas in one cylinder I can still get home.
Inverts allow for greater reserves of gas. The bail out can be based on enough gas left in 1 & ½ tanks. I say ½ tank, as the free flow will obviously lose some volume of gas prior to the shut down. As a result they offer improved safety margins or longer dive times over a slob knob rig. The main reason I haven’t changed over to inverts is that my dives have been bottom time limited on deco commitment rather than gas or bail out. Most dives an hour on the bottom is plenty so on a 50m dive I am within tolerance on the available gas to gas switch depth. On deeper dives the deco quickly racks up so 40mins at 70m is enough for me as your looking at two hours deco running my gradient factors. Should I have a free flow at 70m I could isolate in less than 3 seconds using a slob knob. Left with 50 bar in one 12ltr tank I would have 5mins to ascend to my first gas switch depth of 66m at PP02 1.6 on 21/40 or air bail out. Failing that I have 5mins to ascend to 40m and switch to first deco gas of 32% To me that’s an acceptable level of risk.
So why bother to write all this down.
Simple, to encourage any one out there who is aware of the fact they struggle with shut down drills to seek another way of dealing with the problem and to fight the corner for the divers who like me who have already made the choice. It’s not a sign of weakness or ineptitude and its not dangerous. What is dangerous is persisting with a system that just about works for you most of the time and is very slow in execution. I often talk to divers on boats who ask about the slob knob and by their questions I realise they have been totally misinformed. A couple have asked me if I was concerned about the additional 0 rings failing even though it has none. Others have comented on the internal spring going rusty but I show them my two year old 150+ dive unit without a spec of rust on the stainless steel spring and that’s usually the end of that argument. One thing they cant argue with, I can isolate the manifold much faster than any one can do it manually on a conventional rig.
This year I did a Trimix liveaboard with 10 other divers. Five days diving with four dives over 60m and two hour plus run times. Seven of the divers I had not met before. Apart from one other I was the least experienced trimix diver on the boat. Out of the 10 divers on the boat 8 ran slob knobs and one ran a CCR with inverted tanks. Eight of the divers on the boat had 5 years plus trimix depth experience. All of them had experienced an equipment failure resulting in gas loss at some stage in their diving career. So it’s no surprise then that they all chose to dive a rig that could be shut down quickly. The chap without a slob knob or inverts claimed to be able to shut down easily on a standard rig so he never saw the point. So that’s 1 out of 10 who believed he could do it just as fast on a conventional rig.
They cant all be numptie divers. Perhaps being at 60+m focuses the mind on the speed of shut downs a little more than at 30m
My advice to any new twin set diver is to attempt manual shut downs first. If you have problems seek professional or highly experienced help in resetting you rig. If that fails or you know in your hart that’s it’s a major struggle or just too dammed slow. Invert or get a slob knob.
ATB
Mark Chase
