Timing of preparatory gas switch actions (not actual gas switch)

[_Ralph]

Depends on how much 'stuff' is going on ... lights, DPV, cameras, etc


Typically, we'll start stowing and getting ready at the 3m before (and in the last minute of said stop), slightly staggered to verify that we are doing the proper things at a non-rushed pace.

When we hit the switch depth, we (one by one) verify ourselves and teammates and switch.

If moving between bottles, then the last minute of the previous stop is stowing and moving to the long hose, then starting the next switch procedure.

Rotations take a little bit more time ... and generally do them when convenient before you need the bottle.


_R

 

[kensuf]

Task loading: The accumulation of tasks that are necessary to perform an operation.

Perceptual narrowing: developmental process during which the brain uses environmental experiences to shape perceptual abilities. This process improves the perception of things that people experience often and causes them to experience a decline in the ability to perceive some things to which they are not often exposed.

Perceptual blindness: Inattentional blindness, also known as perceptual blindness, is a psychological lack of attention that is not associated with any vision defects or deficits. It may be further defined as the event in which an individual fails to perceive an unexpected stimulus that is in plain sight. When it simply becomes impossible for one to attend to all the stimuli in a given situation, a temporary blindness effect can take place as a result; that is, individuals fail to see objects or stimuli that are unexpected and quite often salient.

These things exist. We all have a certain amount of bandwidth between our ears, but when it's used up, we miss important things. To demonstrate perceptual narrowing and perceptual blindness I show all of my cavern and AN/DP students a video that demonstrates perceptual blindness by having an "obvious" item appear, 9 out of 10 miss the item and that's while sitting in the comfort of a dry room.

Where I'm going with this is things like trying to switch gases on the fly while ascending from one stop to the next provide little benefit but increase your task loading. Ascend to the stop, get your house in order, then make your switch.

Oh and as AJ said, failing to do a buddy check is stroke **** and laziness.

 

[LiteHedded]

having done a recovery of a fantastic diver who toxed because of sloppiness and complacency (his own and the team's failure to catch it), it's hard for me to stomach people justifying skipping steps in a gas switch. maybe that's just me.

not just at the deco gas switch. you need to be verifying these gases at EVERY switch before you get underway. including stages.

 

[DevonDiver]

I advocate forward-planning and preparation in every phase of a dive. That includes decompression gas switches.

On a typical open-water decompression dive, the diver has a significant period of time where they are ascending from the bottom depth, or on a prior stop, where they can plan and prepare for known deco gas switch at the next level. Doing so reduces their task loading on the actual gas switch and provides them with ample warning should a problem transpire when they pressurize the deco cylinder.

Here's the specific procedure I use:

1. On the ascent to gas-switch depth, the diver locates and verifies the (label of the) relevant deco cylinder. Their hand does not leave the valve handle of that cylinder.

2. With the other hand, they locate and free the regulator second stage of that cylinder, holding it in front of them.

3. They pressurize and close-off the relevant cylinder, confirming this on the SPG.

4. They then purge the second stage, confirming 1st-2nd stages are correct by seeing the cylinder SPG drop.

5. Reopen the deco cylinder valve and present to the team for confirmation of label and hose to the 2nd stage (in their hand)

6. Diver continues to ascent until reaching gas switch depth (diver monitors depth relative to MOD)

7. At the correct depth, they place the 2nd stage into their mouth. (*deco captain signals gas switch)

8. Team confirms (again) that the correct regulator/cylinder is in use.

*The only diver on the team NOT to follow that process would be the reel/DSMB diver (if used). They will reach switch depth, confirm the gas switch for their team-mates, then conduct the full switch procedure themselves under observation.

9. All divers simultaneously switch gas mix on their computers (if used).

10. All divers clean up now unused regulators, hoses, move and leash cylinders etc as necessary.

 

[divezonescuba]

Reopen the valve at the end of step 4?

 

[DevonDiver]

Reopen the valve at the end of step 4?

Well observed (my bad), have edited. Cheers

 

[kensuf]

An ascent from 30' to 20' and 20' to 10' has a large % in the reduction in pressure. Venting some gas from a wing and drysuit during the ascent is also a good idea to avoid going positive and blowing a stop. That's why I prefer to go to the stop depth, get your house in order, then make the gas switch.

 

[kensuf]

Obviously for warm water, wetsuit diving with aluminum cylinders you should hopefully be close to neutral and not need to vent little if any gas. Steel cylinders, drysuit, thick undergarments, maybe different.

 

[TrimixToo]

An ascent from 30' to 20' and 20' to 10' has a large % in the reduction in pressure. Venting some gas from a wing and drysuit during the ascent is also a good idea to avoid going positive and blowing a stop. That's why I prefer to go to the stop depth, get your house in order, then make the gas switch.

I think you mean volume, not pressure, yes? The rate of pressure reduction (PSI/foot) is uniform in the water column, but the rate of volumetric increase is markedly higher per foot when shallow.

 

[tbone1004]

I think you mean volume, not pressure, yes? The rate of pressure reduction (PSI/foot) is uniform in the water column, but the rate of volumetric increase is markedly higher per foot when shallow.

he meant pressure. Note he said % of pressure. You two are saying the same thing