The Basics
Webbing a Harness
Stages & Deco
Gas Choice
Single Cylinders
Scooter Prop

Stages & Decompression Cylinders

Stage regulator choice

For a number of good solid reasons, have them the same as your back gas regulators

  • If you have any regulator failure before the dive, you can strip and move parts to your back gas regulators and modify the dive plan. I am more concerned about travelling to a dive site and NOT been able to dive due to a failure than anything the dive can throw at me.
  • You can exchange regulators on hoses underwater as the regulator should be on the hose only finger tight
  • With all the regulators the same you get to feel when one is not right in that it is breathing funny .  If you change to a different manufacturers or different spec regulator during a dive you have no reference as to what this one is like and thus no indication of a potential failure about to occur.


The best performing regs you have need to go on deco as this is when you are tired (and probably fed up of breathing from a reg).  Also you are loaded with O2 and are now on a high O2 mix which is loading you even more.  As such you don't want any breathing resistance which may build up CO2 and possibly trigger O2 toxicity.  Remember with a good helium mix it will pretty much breath through any regulator so a bottom mix regulator could be of lesser performance than a deco mix - not that you'd do that anyway coz they are all the same - see above.

Feel and Colour coding

Some people say that a decompression regulator (for this read all the intermediate trimixs and nitrox and O2 regs that are on the various stages the dive calls for) should be a different feel and colour marked so that you can tell which regulator you are about to breath from. This is Iin my opinion one of these silly things - the discipline of turning on the stage cylinder that you need and knowing what you are about to breath is all that is required.   When we dive all of our deco regs are rigged the same.  And as such they can be put on travel mix or deco cylinders.  This makes it easier to get the dive ready.  We are not dependent upon getting this the right regulator on the right cylinder or something silly like that (this is what killed Bobby McGuire). When ascending look down at the cylinder necks to see which one you want as the MOD and mix is marked on the cylinder neck (isn't it??).   Turn on the one you want, pull out the regulator attached to that cylinder (following the knob to first stage to hose) and wrap the hose around the back of your head so all regs feed from the right hand side (remember they are all the same), take out the regulator on your long hose from the mouth and breath the stage.  If you have got the wrong one and it does not breathe more than one breath (so we don't have a toxicity issue)   you put the long hose reg back in your mouth and sort it out.  If the stage breathes its the right one,  so clip off the long hose to the right shoulder D ring and continue the deco.  We now have one reg in our mouth , one backup reg hanging below the chin where it always is and the long hose on the right shoulder D ring. You should then check your buddy(s) to ensure they have deployed the correct reg.  As they have the cylinder markinged with MAX DEPTH on the side in 2 or 3 inch writing (don't they ??), you will be able to see which stage regulator has been deployed by checking the writing.  If they have screwed it up you can assist them to get the correct reg and then later by giving them the name of the nearest golf club.   If your scehedule calls for another mix or O2, you turn on the relevant stage, free up the regulator, swap to it as before then stow the now unused nitrox/travel regulator away down the side of its cylinder and then turn off that stage as it is not now needed.

First Stages & SPG's

The uses of a turret style first stage is a good idea as it allows the hoses to point down the cylinder whilst it is stowed and to be swivelled around to point upwards (toward the divers head) whilst in use.  This allows a hose 4 to 5 inches shorter than a hose on a non swivelled first stage to be used.  For this reason the Apex TX50 or the G250/Mk20 make good stage regulators. Always use a SPG, for decompression regulators.  Many people consider that the regulator does not need a SPG as they gauge the cylinder on the surface and then plan to have enough decompression gas.  They belive after all what can you do if you run out and seeing that you are about to run out is irrelevant ....

However as the decompression regulators and stage bottle regulators are set-up in exactly the same way,  they are interchangeable until mounted to a cylinder, they will need a SPG as a stage is breathed during a dive to a fixed point. The best way to mount a SPG is via a short 6 inch hose as shown in the picture to the right. The gauge is bent back and permanently held in place with a piece of line - this does not damage the hose as it sets in the position.  What does damage a HP hose is the constant moving and bending that a free hose will go through during its life.

Stage Cylinder mounting

All stages mount on the diver's left hand side (unless carrying three, in
which case the third goes on the right side).  The stage cylinder necks are clipped to the shoulder D Ring.  With the deco cylinder neck held close to the shoulder - a standard octopus length hose will deploy over the back of the neck and into the mouth from the right hand side for deco. The bottom of the cylinder has a single steel band under which the rope harness is ran - as you can see the cylinder is not mounted to you via any metal to metal fixtures as these can jam or stick and possibly prevent you from removing the stage in an emergency. With this system the stage can be cut free from yourself with two easy moves (for a diagram of a stage harness click here). These two photographs are of a small 3l steel cylinder that is commonly used in UK waters as a decompression gas cylinder whilst nitrox diving. Exactly the same system will be used for larger travel / deco or stage cylinders as required.

Stage cylinder marking
All stages should be marked with the content and the maximum operating depth.  When you analyse the cylinder stick a piece of duct tape with the mix, MOD and date on the cylinder neck in a place where it can be seen by you as you wear it. In addition the cylinder should have its MOD in 3 inch high letters on the bottom as shown below. This is visible to your buddy(s) during a dive and they can uses this as a reference to ensure that in a multistage/deco mix situation you have chosen the correct cylinder

Why you don't back mount a stage / decompression cylinder

Consider two possibilities - the stage is carrying redundant back gas (in the case of many UK air divers this is often their ONLY redundant air source i.e. the 3l pony cylinder) or is carrying decompression gas (i.e. High % O2 or if done properly 100%)

  • Stage with redundant back gas

The reg should be pressurised to keep water out and turned off when not in use.  The reason it is turned off is to prevent a free flow blowing all the redundant air.  This cannot be done if back mounted as you can't turn your own valve on/off so it has to be left on all the dive

  • Stage with deco gas

Again the reg should be pressurised to keep water out and again it must be turned off when not in use.  As well as preventing a free flow gas loose, we also need to prevent someone breathing off it a depth due to an error and dying due to toxicity - this cannot be done if back mounted, same reason as before

In both scenarios above if the first stage fails a diver can still use the stage gas if, and only if, he has access to the valve (i.e. to turn it on to breath and off when breathing out) - this cannot be done if back mounted.

If its backmounted MOST systems used do not allow you, the diver, to remove and replace the stage by yourself if you need to get out a small hole in the wreck  or a cave.  Also consider becoming a key - the stage is on one side so you might fit in a hole going in, but you are the other way round when you come out - so the stage may catch/foul.  A side slung stage can always be removed to get you out a hole.

When it is back mounted the stage cylinder creates additional profile in that it is along side your cylinder thus increasing your drag.  When side mounted its in the lee of your shoulder and thus does not create additional drag

Finally hose routing.  If back mounted where does the reg go - where does the SPG go ?? How do you distinguish between these and your back gas regs/spgs.  When mounted as a side stage the SPG is visually seen as being attached to the stages first stage by a short HP hose, the regulator is held to the tank with bungees/car inner tube - so you know that this regulator and this SPG belong to this stage. If we did have to remove a side slung it comes away, all as one unit (i.e. no hoses attached to the diver).  If you had to take off a back mount pony - what do you do with the hoses, regulator and SPG ??

If we ignore   all the above - a small 3l pony may be a consideration for back mounting.   However as bigger cylinders are used due to dive requirements e.g. 10l, it can no longer go on the back so has to be side slung.  Thus why change a configuration and cause confusion ?

Copyright © 2001 [Gas - Diving]. All rights reserved.
Revised:2 July, 2001