Calculating turn pressures (gas matching) for a pair of cave divers.

[CaveSloth]

SSI. Yep. Just curious.

Cave Diving

 

[karstdvr]

I guess that's why they have waterproof cheat sheets. .

You would hope that gas planning and matching occurred before even getting in the water.

 

[kensuf]

You would hope that gas planning and matching occurred before even getting in the water.

I wouldn't. Here's why.

Let's say it is January and you do your gas calculations in the parking lot.

Your pressure gauge reads 3100 PSI. Your buddies pressure gauge reads 3200 PSI. You're both diving the same sized cylinders, so you just match based on pressure.

Diver 1 - 3100 PSI start, usable 1/3rd is 1000 PSI, turn pressure is 2100 PSI.
Diver 2 - 3200 PSI start, usable 1/3rd is 1000 PSI, turn pressure is 2200 PSI.

Now you go do your dive. While in the water, your cylinders warm up 30 degrees. This causes a change in pressure.

Diver 1 - Cylinders warm up to 3300. Usable 1/3rd is 1100 PSI, turn pressure is 2200.
Diver 2 - Cylinders warm up to 3400 PSI. Usable 1/3rd is 1100 PSI, turn pressure is 2300.

But because both divers did their gas matching in the parking lot, before they even got in the water, they dive to 2100 and 2200 PSI respectively. They've both busted thirds by 100 PSI.

Now the opposite situation is true in the summer, and obviously one could argue all day long that "thirds is for turds when diving in a two man team" (and I wholeheartedly agree with that sentiment), but here's why you NEVER do your gas matching in the parking lot.

 

[Manatee Diver]

On a cold day recently my tanks went from 4,000psi in the parking lot to 4,250psi when I was putting them on.

 

[Addison Snyder]

Meet back at the entrance with 500 psi. Same as wreck diving but use entrance instead of anchor line.

Not a cave diver, but isn't diving overheads without rule of thirds (or significant reason not to) pretty much suicide?

 

[O-ring]

Not a cave diver, but isn't diving overheads without rule of thirds (or significant reason not to) pretty much suicide?

Yep, sorry, this was a bad joke in an otherwise serious thread.

 

[jadairiii]

Yep, sorry, this was a bad joke in an otherwise serious thread.

I laughed

 

[jadairiii]

Before I get to gas management calculations, I first calculate Rule #1.

 

[karstdvr]

I wouldn't. Here's why.

Let's say it is January and you do your gas calculations in the parking lot.

Your pressure gauge reads 3100 PSI. Your buddies pressure gauge reads 3200 PSI. You're both diving the same sized cylinders, so you just match based on pressure.

Diver 1 - 3100 PSI start, usable 1/3rd is 1000 PSI, turn pressure is 2100 PSI.
Diver 2 - 3200 PSI start, usable 1/3rd is 1000 PSI, turn pressure is 2200 PSI.

Now you go do your dive. While in the water, your cylinders warm up 30 degrees. This causes a change in pressure.

Diver 1 - Cylinders warm up to 3300. Usable 1/3rd is 1100 PSI, turn pressure is 2200.
Diver 2 - Cylinders warm up to 3400 PSI. Usable 1/3rd is 1100 PSI, turn pressure is 2300.

But because both divers did their gas matching in the parking lot, before they even got in the water, they dive to 2100 and 2200 PSI respectively. They've both busted thirds by 100 PSI.

Now the opposite situation is true in the summer, and obviously one could argue all day long that "thirds is for turds when diving in a two man team" (and I wholeheartedly agree with that sentiment), but here's why you NEVER do your gas matching in the parking lot.

Understand what you are saying, but with all things being equal, the differences will be small. To me gas planning out of the water is putting one of the most important things in a place where concentration can occur (less distractions), and then make tweeks in the water. You've seen it where two people who decided to do a dive, skip the S drill, and say my turn pressure ought to be "x", what is yours? Gas planning often is haphazardly mentioned in passing in the water- to me what is wrong with discussing at home, in the parking lot and tweeking it in the water?

 

[Jack Hammer]

Nothing wrong with gas matching on land as long as you check and readjust the plan, if needed, once in the water.