Tuesday, 18 March 2014


This article is designed to allow the reader to set up their twin cylinders based on a Hogarthian/DIR setup.  The information below is based on how I dive and how my equipment is configured (unless teaching where agency standards dictate otherwise), however other methods are available.

On my previous blogs I’ve shown you how to choose and setup twin cylinders (here), wings (here) a one-piece harness and backplate (here), as well as discussing what a Hogarthian setup and DIR is.  Moving forward we’ll look at setting up a suit inflation cylinder.

Happy reading.

Why use a separate suit inflation cylinder? 
On my previous blogs I’ve mentioned that the wing, back plate and harness configuration is becoming much more popular, both for recreational and technical divers.  Also, the amount of divers that are diving closed-circuit re-breather (CCR) has also increased.  If you look a lot closer at some of these setups you will see a small cylinder (usually green, black or white) attached onto the main cylinders or the back plate.  This is what’s known as a suit inflation, or argon cylinder.

The 3 reasons for using an independent suit inflation cylinder are warmth, gas management and standardisation.

Suit inflation cylinder visible on the divers left side.
Picture copyright © David Jones

As most divers are aware, being cold increases the risk of a decompression illness.  Ok, but why have a separate cylinder?  For the majority of dry suit divers, from novice up to Advanced Nitrox and Decompression Procedures level, the suit inflation is fed directly from the main cylinder(s) which is absolutely fine.  When diving Trimix however, an alternative inflation method must be used.  This is because Helium is lighter than air, which means it’s less dense, therefore, not as warm.  

CCR divers also often use a suit inflation cylinder.  Because the gas within the re-breather is being ‘re-used’ they tend to use much smaller cylinders, often 3L.  Additionally, because of the potential third buoyancy source (counter lungs) some CCR divers tend to use their suit for buoyancy and leave the wing for surface use only.  As this can use up a large amount of gas the diluent cylinder is inadequate for breathing and suit inflation.  In addition, when diving deep and the diluent is now Trimix rather than air, the thermal issues again come into play.

When diving air or Nitrox, some Trimix divers choose to keep the suit inflation cylinder attached to their ‘rig’ as it standardises their equipment because it avoids adding and removing hoses and/or cylinders between dives.

Finally, instead of using a suit inflation cylinder some divers prefer to add a short low-pressure hose onto their stage cylinders instead.  This is perfectly acceptable and I have done this however there are a few things to consider:
1. Gas Planning.  When planning decompression dives you need to ensure there’s enough gas to complete the dive (including any eventualities).  If you plan to do this you need to ensure there is enough gas for your worst-case scenario plus your suit inflation.
2. Cylinder rotation.  If diving a number of stage cylinders there may be a time where you have to rotate them around to use them, or pass them off to a buddy if required.  If this is the case then you need to ensure you have another inflation option available.
3. High O2 mixes.  You should plan to use your leanest mix for suit inflation, especially if diving a heated under vest to reduce the risk of an oxygen fire inside your dry suit.  Although rare, these have been known to happen.

Argon or air?
Argon’s name was derived from the Greek word ‘argos’, meaning lazy, as it doesn’t react with much.  Argon is also much denser gas than Helium, Oxygen and Nitrogen therefore it’s a better insulator so why are the suit inflation cylinders usually filled with air?  There are two reasons; firstly Argon is only slightly denser than air so the thermal properties of the two gases are marginal and secondly, air is readily available.  So much so most divers decant into their cylinders from another rather than getting a separate fill.

The cylinder
There are a number of cylinders commonly available with the most popular being steel 6cf (0.85L) and 1L, or aluminum 1L and 1.5L.  Which is better; steel or aluminum?  As I’m sure you know, an aluminum cylinder compared to its steel equivalent will be lighter however it will also be larger which may be an issue depending on where you have it mounted.  Obviously the larger the cylinder capacity the longer it will last, but based on ‘normal’ use, one of the pre-mentioned cylinders should easily be able to do a days technical diving or a whole weekends recreational diving. 

Mounting options
Cylinders are generally mounted in 2 places; either on the back plate or on the main cylinders/re-breather.  My preferred method is under the left arm on the back plate.  The reason for this is because if it were on the right side it would interfere with the primary light and long hose, and if it were mounted onto the cylinders it may cause an obstruction or become damaged when in narrow spaces such as a cave or wreck.  The cylinder is generally mounted upside down to allow the diver access to the valve if required.

The regulator
The regulator attached to the suit inflation cylinder consists of a first stage, over-pressure valve (OPV) and dry suit hose.

The first stage can be basic, as it only requires 2 low-pressure ports.  A high-pressure port is not required as most divers refill it prior to diving.  If you wish to know its pressure before a dive most people use another regulator to check it.  Some manufacturers produce a specific first stage for this as it’s smaller and only has the 2 LP ports, however there is always the argument which states you should use a regulator similar to your back or stage gas to enable it to be swapped out if required.

On a normal diving regulator, if there were a problem with the first stage any excess gas would vent off via the second stage, normally known as a free flow.   Because there is no second stage any excess gas will go directly into the dry suit and potentially cause a rapid ascent.  To prevent this an over-pressure valve (OPV) is fitted.

Construction of a suit inflation setup
Purpose made mounting systems are available from companies such as Halcyon or DIRZone, or you can make your own.  If you choose to do the latter you will need the following (based on a 1L aluminium cylinder):
1.  Approx 16”/40cm of 2”/5cm webbing (based on 1ltr aluminium cylinder).
2.  Approx 12”/30cm of 5mm bungee.
3.  1x mushroom bolt and a wide washer.

For the regulator system you will need the following:
1.  First stage.
2.  OPV.

3.  22” LP hose (mine is fitted with a top hat to aid disconnecting).

In addition you will require the following items to complete the setup:
1.  1x Back plate.
2.  1x suit inflation cylinder.

And you may require the following tools:
1.  Lighter.
2.  Knife/scissors.
3.  Soldering iron.

4.  Material tape.

Approximately ¾”/1.5-2cm from the end of the webbing make a hole large enough for the mushroom bolt.  You can do this with either a knife and a lighter or a soldering iron to melt though the webbing.  Whichever method you use you should ensure that the webbing is melted all around the hole so it does not fray. 

Wrap the webbing tightly around the cylinder and mark the position of the other hole, which is made as before.  If you have any spare end on the wedding this can be cut off now and the end melted.

The webbing is now mounted to the back plate.  It should be positioned mid-two thirds of the way up on the divers left side.  Starting from the female end of the mushroom bolt add the washer….

….fold the webbing in half and thread the bolt through both holes….

….align on the reverse side (cylinder side) of the back plate then screw on the male end of the bolt.

The washer is technically not required however after a period of time the webbing can stretch at the holes and could result in one of the webbing holes coming off the mushroom bolt.  It happened to me but luckily it was on a pre-dive check where it could be rectified and not during a dive.  I have since added the washer to prevent this from happening again.

Once fitted it is worth double-checking the fit of the cylinder fitted to the back plate.  If it’s too loose or tight you may need to repeat this process as necessary.

Prior to fitting I prefer to add a strip of material tape to protect the cylinder as the cylinder can rub on the back plate as you can see from the pictures.

Next we add the bungee loop.  There are 2 methods.

Tie a loop approximately 5-5½“/12.5-14cm long (length may vary on cylinder diameter) by using an overhand knot.  Cut off any excessive spare leaving enough to grab hold of and burn the ends.

Pass the end of the loop through the bottom hole on the back plate, pass the knot through the loop….

…and pull tight.

Alternatively (and my preferred method).

Pass one end of bungee through the bottom hole on the back plate and tie an overhand knot.

Approximately 1”/2.5cm tie a second overhand knot make a second loop.

Once the bungee loop is in position we need to check the cylinder is mounted in the correct location as not to interfere with the right hip D-ring.  If it hangs too long then the bungee loop needs to be shortened. 

The mounting can be achieved in 2 ways.  The first is to pass the bungee around the cylinder and over the hand wheel.  This gives the advantage that if required the cylinder could be removed under water.

The second method is to loop the bungee over the cylinder valve.  I prefer this method as I feel it is more secure however the cylinder cannot be removed without first disconnecting the first stage from the cylinder.

Lastly we add the regulator.  The dry suit hose passes inside the harness.  When in storage I place the end of the hose though the bungee/o-ring on the harness (used to secure the LPI) to keep it out of the way as it makes kitting up much easier.

That’s it.  This how to was very simple I hope?

Your twinset should now start to resemble something like the picture.  If you’re not using a suit inflation cylinder then look out for my next blog that covers regulator configuration.

I hope you enjoyed this article and if you have any questions please ask.  I would however always recommend speaking to an instructor about your own individual equipment requirements.

The boring bit!
All opinions expressed in my articles are my own and may differ to other instructor’s and agency guidelines; by no means are they wrong and I would not wish to disrepute any of them.  This article is for information only and should not replace proper training.

Safe diving!

Timothy Gort
BSAC, PADI & SDI/TDI diver training

l Mob: 07968148261 l Email: tim@rectotec.co.uk l