Twinset Configurations - A novices guide

[Gareth J]

So you want to know about

Twinsets

Introduction
Over the years I’ve dived most twinset configurations, some by preference, some because of circumstance.

My general open circuit configuration has been a standard manifolded twinset with an isolator, prior to that an independent twinset. But I have been considering which configuration is the best. Before we get to this, let’s look at what options we are likely to see & there general pro’s & con’s.

Configurations
There are conventionally 7 configurations.

• Independent Twins.
• Manifolded Twins – no Isolator
• Manifolded Twins – Isolator
• Manifolded Twins – Isolator + slob knob
• Invertered Twins, Independent
• Inverted Twins, Manifolded
• Inverted Twins, Manifolded with Isolator

1. Independent Twins

Simple straight forward & easy to assemble, often a starting configuration.

Operation.
The cylinders must be breathed down in stages, to ensure sufficient air is available if a catastrophic failure occurs to allow the diver to surface safely.
The diver MUST switch regulators periodically. Sensible switch points are

• On reaching the bottom of the shot line. (Divers breath proportionally more on descent than at any other time, stress & apprehension. In addition, it is sensible to be certain that both regulators are functioning properly.)
• When the cylinder in use is 50bar lower than the one not in use.
• Often directly prior to ascent.

Con’s
a/ Two pressure gauges are required (one on each cylinder), an ‘obvious’ weak point. (saying that I’ve never seen one have a catastrophic failure in the water).
b/ At points during the dive the diver has no regulator in his mouth. (If he’s worried about this then he shouldn’t be diving).
c/ At alternate points during the dive the AAS will not be in the divers mouth (if you are a donate from the mouth enthusiast). Or the AAS is not secured (if you are against donate from the mouth,
d/ If there is a catastrophic failure on one cylinder, & its shut down, you are unable to ‘scavenge’ any remaining gas.
e/ The diver MUST remember to switch periodically (when there is no more than 50 bar between the cylinders), otherwise it no longer provides a ‘redundant’ air supply.
f/ The diver MUST monitor the air on BOTH cylinders.

Pro’s
a/ Straight forward
b/ A ‘shutdown’ is not critical, in the event of a failure.
c/ You will know if there is a regulator failure at a ‘safe’ point during the dive, i.e. on an intentional regulator switch.


2. Manifolded Twins – No Isolator

Generally a no, no. In the event of a catastrophic failure there is no redundancy. The configuration is basically one big cylinder.


3. Manifolded Twins – with Isolator

There are no significant ‘operational practices’. Which ever ‘preference’ the diver has (donate from mouth or not), the preferred regulator is always in operation.
It is often usual practice to remove one of the contents gauges (reducing a high pressure failure point). It is sensible to have the contents gauge fitted to the regulator NOT in use, if it’s not failing during the dive the isolator is closed or obstructed!

Con’s
a/ Unless actually checked during the dive, you are unaware if a problem develops with the second regulator.
b/ Only one cylinder is being monitored, so it is possible if the manifold is closed or obstructed to breath the unmonitored one to zero!
c/ You NEED to be able to shut down the central isolator ( & then the offending cylinder). If you wish to have a ‘redundant’ air supply. Otherwise, all the air will disappear when you need it most!
d/ Potential more failure points in the manifold, the high pressure side.

Pro’s
a/ No ‘risk’ of missing a regulator switch.
b/ Can ‘scavenge’ any gas left in the shutdown cylinder via the manifold.


4. Manifolded Twins – with Isolator & Slob Knob

All the points raised in the Manifolded twins apply.
A slob knob is a flexible extension attached to the Isolator Knob, allowing the isolator to be shut down without having to reach back.

Pro’s
a/ The slob knob makes closing the Isolator easier.

Con’s
a/ Slob Knob’s corrode, & can jam, they can also shear off at the connection point.


5. Inverted Twins – Independent.

A very unusual configuration. Same operational practice required as for independent twins.

Con’s
a/ Two pressure gauges are required (one on each cylinder), an ‘obvious’ weak point. (saying that I’ve never seen one have a catastrophic failure in the water).
b/ At points during the dive the diver has no regulator in his mouth. (If he’s worried about this then he shouldn’t be diving).
c/ At alternate points during the dive the AAS will not be in the divers mouth (if you are a donate from the mouth enthusiast). Or the AAS is not secured (if you are against donate from the mouth,
d/ If there is a catastrophic failure on one cylinder, & its shut down, you are unable to ‘scavenge’ any remaining gas.
e/ The diver MUST remember to switch periodically (when there is no more than 50 bar between the cylinders), otherwise it no longer provides a ‘redundant’ air supply.
f/ The diver MUST monitor the air on BOTH cylinders.
g/ Custom hoses required (to make them long enough)
h/ Risk of damage to regulators when removing equipment. (Very good cage required).

Pro’s
a/ Straight forward
b/ A ‘shutdown’ is not critical, in the event of a failure.
c/ You will know if there is a regulator failure at a ‘safe’ point during the dive, i.e. on an intentional regulator switch.
d/ Regulators are protected when in an overhead environment.
e/ Snag point (regulators) is now at the bottom of the cylinder, less likely to snag.

6. Inverted Twins – Manifolded, no isolator.
Very unusual configuration. Generally a no, no. In the event of a catastrophic failure there is no redundancy. The configuration is basically one big cylinder.


7. Inverted Twins – Manifolded with isolator.

Based on the original Navy configuration.

Con’s
a/ Unless actually checked during the dive, you are unaware if a problem develops with the second regulator.
b/ Only one cylinder is being monitored, so it is possible if the manifold is closed or obstructed to breath the unmonitored one to zero!
c/ You NEED to be able to shut down the central isolator ( & then the offending cylinder). If you wish to have a ‘redundant’ air supply. Otherwise, all the air will disappear when you need it most!
d/ Potential more failure points in the manifold, the high pressure side.
e/ Risk of damage to regulators when removing equipment. (Very good cage required).
f/ Additional stage cylinders or equipment can impact can access to the cylinder valves & isolator.

Pro’s
a/ No ‘risk’ of missing a regulator switch.
b/ Can ‘scavenge’ any gas left in the shutdown cylinder via the manifold.
c/ Shutdowns are ‘easier’ than on a conventional manifolded twinset.
d/ Regulators are protected when in an overhead environment.
e/ Snag point (regulators) is now at the bottom of the cylinder, less likely to snag.


Conclusion
The two conventional configurations (independent, & manifold with isolator,) require a minimum of specialist equipment or modification to existing equipment.

The independent system has the distinct advantage that it is possible to assemble a twinset almost anywhere if they have diving cylinders, i.e. overseas dive facilities. The only restriction to this is the ability of the stab’ or wing. In the simplest case a buddy with twining bands.

A twinset with manifold & isolator is a dedicated setup, there for not always available to the travelling diver. One relevant point is that a diver that has difficulty reaching the isolator can always dive the set as independents (with the isolator closed), thus requiring them to only shut off the valve for the failed regulator. Giving them plenty of time to open the isolator if there is any gas remaining in the shutdown cylinder.

Potentially the ‘ideal’ setup is the inverted manifold with isolator arrangement. Access is normally very easy, even for those with restricted movement (damaged shoulder joints). There is a minor risk that stage cylinders or other additional equipment can impede access to the valves, but this is normally very easy to resolve.
The downside is that it often requires special hoses to reach from the lower back to inflators, regulators etc.

It should be noted that it is harder to switch from a manifold with isolator rig to independence (or dive with the manifold closed). If the diver has been diving with the manifold for any length of time they will have lost the instinctive routine of swapping regulators periodically. This is more likely to result in failures to switch regulators at times of high stress or task loading, & is something to be VERY aware of.



I look forward to your views & feedback - applogise if this has been done before.

Gareth

[Mal Bridgeman]

Hi Gareth

It's a good piece.....there are bits with which, I personally disagree which is bound to be the case depending on one's chosen option. So rather than pick it to shreds , a question. When you say Pros & Cons .... what are you using to compare that to?

For example you list as a con of Manifolded Twins

Originally Posted by Gareth J

Only one cylinder is being monitored, so it is possible if the manifold is closed or obstructed to breath the unmonitored one to zero!

To me, though, thats a pro because I get two pieces of info from it .... one is if my isolator is closed and two how much gas I have.

Looking forward to others' reactions.
Mal

[Rob Dobson]

A very worth while thing to write up (green incoming).

One of the main draw backs of independent cylinders IMO is the gas planning implications. You need to plan around the loss of your highest pressure cylinder so the dive turn pressure becomes a significantly limiting factor.

[Woz]

Might be an idea to stick this in the "articles" bit on YD

[Ant P]

Originally Posted by Gareth J

2. Manifolded Twins – No Isolator

Generally a no, no. In the event of a catastrophic failure there is no redundancy. The configuration is basically one big cylinder.

This isn't right is it? If there is a problem with either post you can shut down the offending post and still access the cylinder's gas through the manifold. You don't need to isolate. Different matter if there is a problem with the manifold - but even an isolator wouldn't save you in that case.

Cheers, Ant

[Gareth J]

Woz, done (posted to articals as well).

Mal, the comment

Only one cylinder is being monitored, so it is possible if the manifold is closed or obstructed to breath the unmonitored one to zero!

It is possible to 'miss' that the contents gauge is not moving, especially if you are suffering narcosis or during times of high task loading.
If the contents gauge is on the cylinder you are breathing it is possible to be unaware that the other cylinder is empty & that the manifold is either closed or blocked.

I once breathed a twinset to zero on one side without realising what was going on, due to a blocked manifold, narcosis & high task loading & repeating the route we had previously done before taking in the camera gear. Stupid mistake, I should have been aware of the problem before it bit me in the ass - it's a real shock to run out of gas when you have a contents gauge sitting at 180bar!

Cheers

Gareth

[e14724]

I dive a 15/12 and a pony, in the near future however I my go down the twinset route as my diving progresses and requirements change. Having read a number of posts and dare I say debates(arguments?) I have a question regarding hoses. If using independent twins is it wise to feed your drysuit from one and your BCD from another to aid in bouyancy? How are the hoses setup? I ask about independents as this, as has been mentioned, would be my first steps.

Excuse my ignorance ive not really had the opertunity to ask before.

[Gareth J]

Originally Posted by Ant P

This isn't right is it? If there is a problem with either post you can shut down the offending post and still access the cylinder's gas through the manifold. You don't need to isolate. Different matter if there is a problem with the manifold - but even an isolator wouldn't save you in that case.

Cheers, Ant

Ant

The problem with fitting a non-isolated manifold is that manifolds interface below the cylinder valve - so if you close the cylinder valve the manifold is still accessing gas from that cylinder, even though the cylinder valve is closed.
If the manifold fails, you dump all the gas from both cylinders.
With an isolator you can shut off the damaged manifold from at least one of the cylinders, thus preserving the gas in the other cylinder.

Adding the isolator adds a new failure point on the high pressure side, fitting the manifold reduces the impact of this potential failure.

Isolated manifolds ensure that you can isolate each half of the configuration, worst gas a manifold failure will only dump gas from one of the cylinders not both if a isolator is fitted. In the event of a first stage failure the manifold allows you to access gas from the effected cylinder via the isolator.

EDIT - PS also see Liams comment about cylinder O-ring below.

Gareth

[Woz]

When diving indies or twins my configuration is identical:

Right post:
FSR 1st stage with-
Main TX40 reg (yellow cover) on 2m hose. Hose down wing loops.
SPG
Wing inflator

Left post:
FSR 1st stage with-
Necklaced ATX100 (black)
SPG
Drysuit inflator

I tend to not be quite as anal in worrying about shutdowns- just dive with the isolator closed and swap regs. If you need to scavenge the gas then open the isolator at your leisure rather than having to keep practising all the bloody time at being slick when you really don't have to.

[liamm]

Originally Posted by Ant P

This isn't right is it? If there is a problem with either post you can shut down the offending post and still access the cylinder's gas through the manifold. You don't need to isolate. Different matter if there is a problem with the manifold - but even an isolator wouldn't save you in that case.

Cheers, Ant

But if a neck O ring goes then it'll all go quickly from both tanks, an isolator would allow you save a bit of gas here.