The Mad Penguin

Background

For SCUBA divers, safety and deco stops are an important part of the activity and should be performed properly and fully to maintain as broad a safety margin as possible to prevent DCS and other issues.  Most recreational dive clubs and operators also maintain a requirement that divers end up back on the surface with 500psi in order to maintain a reserve in case of emergencies.  This catches a large number of the infrequent incidents that can occur due to inattention, equipment failure or other causes.  As an additional step, many groups also employ some kind of extra gas supply available at the designated safety stop to allow divers yet another option in the event of an incident allowing them to complete their safety stop fully even during a full out-of-gas scenario.  One option is to run long hoses ending in second stage regs all the way from the boat to the anchor line to supply divers with air/oxygen.

The “Hang-Tank” is also known as a “stage” or “deco” “tank”, “cylinder” or “bottle” and is a self-contained cylinder and regulator configuration that is placed on the anchor line, or below the boat in some way at a depth that allows divers to complete their safety stop.  This piece of equipment has been around for a long time and so is nothing new.

Technical divers have and use their own and are self-sufficient, where many recreational clubs, boats or groups have a more communal approach since in the recreational realm this is meant as a contingency instead of part of the regular dive.  However when I’ve seen it in use, it’s often ad-hoc using someone’s 2nd cylinder and spare set of regs as opposed to a specialized set of equipment. This isn’t in itself wrong, but I think there are opportunities for improvement over that system.

What follows is a collection of my own suggestions and thoughts surrounding this important piece of gear.  It’s not meant to be the end-all and be-all, but may provide a sound foundation for anyone looking to set up a hang-tank for their group or personal use.  I would further like to suggest that no-one follow this guide blindly, but think through what I’ve laid out, and use what makes sense for you and your situation.  Also, I’d like to encourage everyone to think about improvements (which I would love to hear about).

Configuration

1. A 72/80 cu. ft aluminum cylinder should be designated and clearly marked for the purpose.
2. The tank should be designated and treated for nitrox use and filled with 36% (or similar) to maximize safety and off-gassing at 15’ without having to resort to special oxygen compatible regulators.  Divers using the tank would regard it as air, and use it as such.  No marking on the tank would denote it’s actual oxygen content except perhaps a maximum operating depth (MOD) of 100’  (arguments for or against the green/yellow marking for nitrox can be made, but that is ultimately up to the individual/group that owns the tank).
3. The hang tank kit should include a special set of regulators that are designated for the hang tank only and that travel with the divemaster kit.  The regulators should be assembled using a first stage and two second stages on 40” (octo-length) hoses with an easily readable pressure gauge on a short (6” High-pressure Hose) to minimize entanglement hazards and risk of damage.  Ideally, no LP hose should be attached, but if an LP hose is included it may be bundled as outlined below.  One might also consider having the regs detuned as well to help prevent free-flows due to current.
4. There should be a means of stowing the 2nd stage hoses on the tank when they’re not in use to keep them from dangling, but allow quick and easy access.  Bungee, rubber or elastic wraps around the tank would suffice.
5. A set of open-water & altitude tables should be affixed in to the tank for diver reference.
6. A re-usable slate and pencil should be affixed to the tank in a fashion that allows it to be removed easily and passed via the safety diver/swimmer to surface support & the dive master.
7. The hang tank should be affixed to the anchor line or on the regular route to the surface at 15’.  At a bare minimum, a line should lead divers to the hang tank without their having to go above 10’ in depth (ideally staying at or near 15’).   A length of bungee cord can be used in suspending the tank to absorb some shock resulting from rough surface conditions and minimize bouncing of the tank and divers.
8. The tank should have two to three 2 lb clip-on weights attached to it for the use of divers having difficulty maintaining their stops.  The weights might be marked brightly and clearly (pink/orange) and be used by any divers who are using the hang-tank to aid in identifying the fact that the tank is in use or there is something going on.
9. A strobe or glow-stick may also be attached to the tank to assist in its location.

Procedures

Preparation

Before diving begins, the tank is assembled using the usual procedures to check for odours in the gas, o-ring quality, oxygen concentration if nitrox is used, etc.  The valve is turned on and a couple breaths are drawn from each of the attached regulators.  The pressure of the contents is verified as adequate for the dive.  The valve is then closed leaving the regulator hoses pressurized.  Hoses are stowed using the elastic/bungee wraps on the tank and any tables/slates/weights attached either with clips or in pockets (preferred for light-weight items).

Deployment on the Anchor Line

The hang tank is swum to the anchor/bow line and attached securely to the line at a depth of 15’.  At this point, the valve is opened and closed to ensure the hoses are pressurized, but that in the event of a free-flowing reg due to current or other causes, the contents of the tank are not emptied.  A strong recommendation is made to have a dive team work together to deploy the tank.

Alternate Deployment

Ensuring the valve is closed and the hoses are pressurized, the tank can be dropped over the side of the boat on a hang line along with a guide-line attached to the bow/anchor line.

Use

Divers should not make a practice of using the hang tank except in circumstances where no-decompression limits have been exceeded, or a diver running low enough on air that they cannot reach the surface with 500psi in their own tank after completing a safety stop.  Upon an incident requiring use of the hang tank, divers make their way to the hang tank utilizing air-sharing techniques if necessary.

If the incident triggering use of the hang-tank is exceeding the no-decompression limits, then both buddies should make use of the tank while perfoming their stop to increase the safety margin.  Otherwise divers with sufficient gas in their own tanks should continue to use their own supply.

The regs are deployed to each diver as required and the valve opened.  The divers can then make the switch to the hang tank regs and stow their own regs.  The tables stowed on the tank can then be un-stowed and consulted.  Regardless if the divers have been diving air or nitrox up to this point, the tables are consulted as if the whole dive were performed on air, and the divers complete whatever safety/deco stops are indicated by the tables.  The only exception of this is if the divers suspect hypothermia onset (note: not simply discomfort due to cold).  Divers should attempt to maintain their depth such that their chest is at 15 ft.  Upon completion of the proscribed stop, the divers switch back to their own regulators or air sharing as required and ascend in a controlled manner to the surface ending their dive.  Preferably, the valve on the hang-tank will be closed when the divers finish with it, but this is reduced to a suggestion to lower task loading on final ascent.  The incident must be reported to the divemaster along with pertinent details of the incident including the length of the actual safety stop performed.

Retrieval

All divers must be accounted for before the diver assigned to the task retrieves the hang-tank.  This may require the diver assigned to surface from their dive (having performed their safety stops) to check with the divemaster before submerging again to retrieve the tank.  A strong recommendation is made to have a dive team work together to retrieve the tank.

Training

The use of the hang tank should be included in a course (most likely at the advanced level) including actual in-pool practice where possible.  An orientation covering the features, placement and use of the hang tank should be held regularly with divers (ie. at the beginning of a dive day).

This entry was posted on Tuesday, April 7th, 2009 at 23:43 and is filed under My Stuff, SCUBA Diving. You can follow any responses to this entry through the RSS 2.0 feed. Responses are currently closed, but you can trackback from your own site.