Diver Has 3400 Litres Of Gas In A

If solo diver B starts out with a partial fill that gives him a starting volume of a little more than 95 cubic feet, his allowable usable volume" will be about 64 cubic feet. Since he is using 3.42 cubic feet of gas every minute, his dive can only last for about 18 minutes before he violates the Rule of Thirds.

Key Concept

SAC is a constant and represents gas used at rest on surface. RMV is a variable and represents gas needed for specific dive.

Planning Appropriate Reserve Volume

For many divers in a buddy diving situation, the sort of basic plan outlined above allows for an appropriate degree of gas risk management. The Rule of Thirds is a well-established method used frequently to set aside what most divers consider an appropriate reserve of gas for most contingencies. The Rule of Thirds is a basic planning tool for all overhead diving and as such is a good starting point for general gas planning for sport solo diving. The main thing to remember is that you need an appropriate reserve of backgas - whatever flavour that back gas may be - to get you (and your buddy in team-based diving) back to the surface at or very close to your planned exit point following a slow ascent (your computer will dictate what this is but for the sake of argument let's say about 10 metres or 30 feet per minute) and with time to take a three to five minute (at a minimum) safety stop.

However, while following the Rule of Thirds may help keep team divers breathing, when we plan a solo dive, we have to consider some sort of contingency in the event of catastrophic failure somewhere in our primary gas system.

Carrying a Redundant Gas Source for Security

Modern scuba equipment is extraordinarily reliable but on occasion, something does go wrong. Straps break, plastic cracks, O-rings distort, bulbs burn-out and even the best quality, most well-maintained gear may suddenly decide to stop working the way it should. For the most part, equipment failure is a small "E" emergency and can be managed without a great fuss, and often the dive can continue. But failures in the system that delivers breathing gas to a diver are another class of emergency altogether.

For the solo diver without a buddy's gas supply to bail them out, any malfunction in the primary gas system requires access to a totally independent gas supply (IGS). We are going to discuss different types of IGS in the equipment chapter, but here we need to think about what volume of gas a diver needs to ensure a safe exit to the surface if a free flow, failed burst disk, frozen first stage, or extruded O-ring robs us of all the gas on our backs. Armed with this knowledge, a diver can make an informed decision about the best option for their IGS.

A solo diver - should not depend on bailout or redundant gas as part of your regular gas management plans

The simplest way to calculate the volume of redundant gas one needs is to carry a small independent cylinder fitted with an spg and regulator holding about one third of the starting volume for the dive! Using the examples above, this translates into about 700 to 800 litres for solo diver A and about 30 to 35

cubic foot for solo diver B. Any system that can hold this volume therefore would be a sound investment.

Another way to look at this is to work out how much gas we would need to make a normal ascent from 40 metres or 130 feet (the greatest depth open to sport divers) to the surface without sending our computer into convulsions. Let's say that this ascent, with stops, is going to take at least 10 minutes. That time represents about five minutes of ascent and a five minute safety stop. Frankly, this is a minimum ascent time from this depth but let's use it as a guideline. Now, if we use our average SAC rate (14 litres or 0.5 cubic feet), take the depth as 2.5 atmospheres (the mean depth) and a dive factor of 2 (the diver would be stressed after all), we would need about 700 litres or 25 cubic feet to execute that ascent!

Of course, not every dive will be to the maximum sanctioned depth, but we may as well carry a back-up system suitable for the worst-case rather than something inadequate for anything but a mad dash for sunlight.

One final and very critical point regarding the volume of gas in a solo diver's independent gas supply: That volume should NEVER be factored into the allowable useable gas volume calculations. Redundant gas is ONLY available to a diver in the event of an equipment failure bailout or emergency gas loss bailout and should NOT be considered in rationing back gas for a dive. If more gas is required than is available in one's main cylinder, use larger volume tanks. DO NOT depend on bailout or redundant gas as part of your regular gas management plans.

Review Questions

3. What is your SAC rate?

4. Using the average SAC rate (14 litres or 0.5 cubic feet) what is an acceptable RMV for a drift dive to 30 metres /100 feet in semi-tropical water with exceptionally good visibility?

5. How might the answer to the previous question be affected if the dive in question were conducted in cold water wearing a drysuit, carrying a large video set-up and working in current?

6. What fraction of a diver's starting gas volume represents a very sensible reserve for most emergencies?

7. A diver has 3400 litres of gas in a 14 litre cylinder at the start of a dive, approximately how many litres should they plan to have when they arrive at their safety stop?

8. A diver has 85 cubic feet of gas in a 95 cubic foot steel cylinder at the start of a dive, how many cubic feet should be put aside as a reserve if they follow the Rule of Thirds?

9. What is it about a pony bottle that helps manage the risks associated with complete loss of one's primary gas?

10. Why do we NOT our available gas include the gas in our pony bottle as part of when working the rule of thirds?

include the gas in our pony bottle as part of when working the rule of thirds?

Dealing with Equipment Issues

In this chapter, you'll learn about:

• Freeflowing Regulators

• Freeflowing LP Inflator

• Broken Mask, Fin Strap, Flooded Drysuit, and other nuisances

Equipment failure is inconvenient, usually expensive, disrupts an otherwise peaceful experience, and - in many cases - could have been avoided by simple inspection and a routine service schedule!

That's not to say that these habits will magically prevent the inevitable challenges associated with an equipment intensive activity like diving, but pre-dive gear checks - both visual and practical - and the "ready for use" signal from a qualified and certified equipment technician will help isolate and minimize the most annoying and potentially dangerous equipment malfunctions.

Regular equipment service and meticulous gear inspection coupled with a few in-water skills and learned responses to gear malfunctions, and the independent diver has raised the bar in terms of personal security simply and effectively

Let us look briefly at the various types of gear failure that a diver may be faced with and discuss ways to meet their challenge and resolve the problem.

Gas Losses

Gas loss refers to any loss of breathing gas and this category of nasty events, constitute the most serious threats to a diver's safety. We will look at options to deal with the special circumstances facing a solo diver suffering gas loss in the chapter on equipment. Let's look here at how we need to avoid and react to gas loss.

Any regular pre-dive inspection should pay particular attention to the bits and pieces of kit that hold, transport and deliver gas to us at depth. These include:

O-rings - are they the correct size and type and are they intact. Also are they perfectly seated (in place) and are they free of dirt and grease?

All hoses - are they sound with no visible bulges or distortions, and are they free of splits, tears and other damage?

Valves - were they serviced last time the cylinder was visually inspected and do they operate smoothly - no catches when opening and closing and are they easy to turn?

Regulator First Stages - annual service and inspection before each use for external damage. When in use do they function faultlessly?

Regulator Second Stages - annual service and set up. Are they free of cracks, holes in diaphragms and securely attached; and are mouthpieces intact and securely attached (perhaps not a source of major gas loss but it's inconvenient to say the least to have a second stage drop away from the mouthpiece when you're watching a fish swim by... or at any other time.

Table 1

Results of a study conducted by Advanced Diver Magazine to show length of time it takes to drain 2200 litres / 80 cu ft of air at various depths.

Depth in ATA

Burst Disk Failure

Ruptured HP Hose

Ruptured LP Hose

Freeflowing Regulator

0 (surface)

72 seconds

22 minutes

83 seconds

255 seconds

4 ata (30 m / 99 ft)

72 seconds

22 minutes

83 seconds

155 seconds

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Guide To Scuba Diving

Guide To Scuba Diving

Many people think that space is our final frontier and that is not entirely true. While it is more difficult to get to outer space, we probably know more about the various planetsand environments in space than wedo about what lies beneath the surface of our oceans.

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