Back Gas

Posted on 21. Jan, 2004 by Greg Mossfeldt in Gear

Back gas

by Sean Stevenson

Right and left post, as discussed below, refer to the diver's perspective while wearing the gear.

The long hose is the primary, and the second stage in your mouth is the one which is donated.

This accomplishes a few things:

1) This eliminates the possibility of donating a gas which is not appropriate for the current depth

2) This ensures that the diver in need of gas gets a fully functional second stage. By switching to your own backup, you are in a much better frame of mind to respond appropriately to a fouled, flooded or malfunctioning second stage, closed valve, etc., than is the out-of-gas diver

3) This ensures consistency in donation procedure, permitting this to become a conditioned action. Donation procedure in this case is identical, regardless of whether you are breathing back gas, stages or decompression gases.

4) This accomodates a diver who, under the influence of "tunnel vision" from an incipient state of panic, is apt to grab the first source of gas that is immediately apparent: the working reg in your mouth. In this case, the troubled diver obtains the long hose immediately, allowing you the requisite distance to immediately restore control to the situation.

5) This encourages preemptive action by permitting the long hose reg to be offered to another diver at the slightest indication of a possible problem, since it is trivial to do so and then, if the situation does not actually require donation, to revert to breathing this reg. Conversely, restowing elsewhere on the diver a long hose which is offered for donation but not needed may be any combination of time consuming, inconvenient, or difficult, if this regulator and hose are not immediately accessable and deployable as it is when breathing it. Consequently, a diver may be less apt to have this reg available to his buddy at the slightest indication of an impending problem. In consideration of the above (primary on long hose, primary as donated reg), this regulator is fed from the right post for the following reasons:

1) If this hose were fed from the left post, this would introduce the possibility of inadvertently rolling this valve closed due to contact with an overhead, cutting off the flow of gas to the donated regulator without indication. Sharing gas while negotiating a restriction in single-file (the purpose of the long hose) precludes having this hose on the left post, since doing so could be potentially fatal to the diver on the donated reg in this situation. Conversely, if the long hose is fed from the right post (and backup reg from the left), an inadvertent left post roll-off would be immediately noticed by the donating diver, who can re-open the valve, or if that is not possible, begin breathing from the BC inflator (tertiary backup - on right post) by depressing both the inflate and dump buttons simultaneously.

2) While not as important a reason as the preceeding comment, the primary reg on the long hose feeds naturally down from this post, across the front of the diver's body, behind the neck and into the second stage, as most standard second stages feed from the right side. Running this hose from the left post would necessitate an abrupt change in direction or tight bend in the hose to accomplish the same thing.

In consideration of the above (primary reg on right post), the LP inflator hose is also fed from this post for the following reasons:

1) By putting the LP inflator on the same post as the primary regulator, the diver is immediately alerted to a gas interruption to the inflator, as the regulator being breathed would also fail to deliver gas, and such a failure would then be acted upon accordingly. Placing the LP inflator on the post opposite the primary regulator introduces the possibility of a gas delivery failure to the BC which might only be discovered upon attempting to inflate - a potentially dangerous situation when you consider emergency scenarios which may necessitate immediate inflation.

2) In the event of a stuck inflator, response actions include disconnecting the LP hose to prevent uncontrolled ascent. While this is undeniably effective, it is not always possible, such as when ice forms around the quick disconnect, freezing it in place. A loss of intermediate pressure control on the first stage which feeds the inflator (HP seat failure), is quite likely to freeze this disconnect in place in cold water, necessitating an alternate course of action. In this case, the diver can dump the excess gas in the BC through use of the rear dump/OPV (on the left side on most BCs/wings) with the left arm to prevent a runaway ascent, while simultaneously reaching back with the right arm and shutting down the right post. Running the LP inflator from the left post would preclude this action.

3) Feeding the LP inflator from the right post necessitates running the hose behind the diver's head. This is advantageous because gas flow through hoses, particularly when the gas contains helium, is audible at depth, and can serve as an audible indicator of a possible problem (leak, etc.) if gas is noticed to flow when it should not.

4) As mentioned briefly above, the LP inflator can be used as a tertiary backup regulator by inhaling from the mouthpiece on the inflator fitting while simultaneously pressing the inflate and dump buttons. In the event that this is required, feeding this hose from the right post places it on the same first stage that feeds the donated reg, which provides a temporary means of supplying both divers with gas from the post which does not roll closed when in contact with an overhead, in the event that the left post valve is inaccessable, until such time as the closed valve can be re-opened.

The backup reg feeds from the left post, since the primary is on the right and the dependency must be split between first stages. As such, you also gain the benefit of running this hose behind your head (the audible indicator). The backup reg is hung on a necklace in a mouthpiece-up orientation that permits obtaining this reg without the use of the diver's hands.

The SPG is fed from the left post for the following reasons:

1) Since the probability of first stage failure is highest on the first stage with the greatest demand, the primary on the right post has the greatest probability of failure requiring isolation, in which case by having the SPG on the left post it remains relevant for the greatest length of time 2) If the left post has been inadvertently rolled closed due to contact with an overhead, or if the isolator happens to be closed (unintentional), the indicated pressure will fail to drop over time, serving as an indicator of the problem

3) When using a DPV (scooter), it is typically operated by the right hand, enabling the left to manipulate the LP inflator, drysuit inflator, nose for equalization, light head, reel, etc. For this same reason, having the SPG on the left allows the diver to consult it without having to let off the scooter trigger.


* January 21/03