Communicating remaining pressure on doubles(twinset)

[jadairiii]

.... he turned to me and asked for my pressure. I was startled,

Why? DM has never met you, has no idea of your SAC/RMV, for all he knew you could be a total Hoover.

He probably didn't notice your tech master aura?

 

[inquis]

I meant to say the OP's gas consumption resulted in using 4000 psi of gas on the first dive versus 3000 psi for the other divers.

My understanding of the OP's example is that the OP used 1000 doubles-psi on the first dive and didn't elaborate on what everyone else consumed. For the second dive, OP consumed 750 doubles-psi and the others consumed 1500 singles-psi (equal volumes).

 

[boulderjohn]

Why? DM has never met you, has no idea of your SAC/RMV, for all he knew you could be a total Hoover.

He probably didn't notice your tech master aura?

He knew that I was a tech diver preparing for a dive to 300 feet for the following day using the gear I was planning to use on that dive. He knew that if I were indeed a total hoover, I would still have more gas than the three of them combined.

 

[rhwestfall]

I talk it over in advance with the dive guide.

^ this

 

[jadairiii]

He knew that I was a tech diver preparing for a dive to 300 feet .....

You could have just stopped there. That alone means it would be impossible for you to burn through your gas. Impossible! Simple minded DM.....how dare he.

 

[sea_ledford]

Since this is the Advanced forum, @EFX raises a good point that if this is an "out & back" dive where it's highly desired to return to the anchor rather than surface, things change substantially. In such cases, you need to calculate your own turn pressure *based on using the same volume as the single-tank diver*. The reason being, if you were to lose all your gas (a remote possibility on doubles, but that's the standard assumption), you'd be sharing from one of them. This concept is called "gas matching".

But that doesn't really happen in an out and back with a bunch of divers. The guide will turn when the lowest one is at 1500(ish) psi to have them back at the boat by 500*. If someone has a catastrophic gas loss, the guide will air share and go directly to the surface (or the closest safe surface if conditions are dicey). It isn't like a cave dive where you have to get back to the entry point. It is certainly nicer that way all around, but boats can move. I'd rather have access to all the air in the world and have the captain have to drop a mooring or pull anchor. And so would the captain.

*Yes, I know that is 1500psi out and 1000psi back. It just works through a combination of current and swimming back faster than swimming out. I've taken probably 15,000 people diving, and I've had to air share maybe 5 times. Only one of those was a catastrophic gas loss, resulting from a bad seat that caused the 1st stage to freeze closed in the first 10 minutes of a dive.

 

[the_one_who_waits]

You mentioned that on doubles your gas consumption for a single dive is 4000 psi compared to 3000 psi for the rest of the group. This suggests that your weight and/or trim is not good diving on doubles. I would fix those problems first with a buddy before going on another group dive. Some men have high gas consumption rates and if that's you, and assuming your weight/trim is correct, then I would suggest diving with single 100 ft3 tanks (rent or bring 2) for the boat group dives. Then you won't be inconveniencing the group and pissing off the DM.

I meant to say the OP's gas consumption resulted in using 4000 psi of gas on the first dive versus 3000 psi for the other divers.

A set of double tanks filled to 3000psi doesn't equal 6000psi, it will still be 3000psi. For simplicity, the example assumed everyone has the same SAC rate so weight/trim isn't relevant. Everyone starts with 3000psi. The single-tank divers consumed 2/3 of a their tank, ending with 1/3 reserve (1000psi). A doubles diver that consumed the same volume of gas would have consumed 1/3 of their starting volume, leaving 2/3 remaining (2000psi).

It perhaps is better understood by looking at this by total volume. Let's say one tank is 90cubic feet:

	Single-tank	Doubles
Dive 1 Start	90	180
Dive 1 End (60cf consumed)	30	120
Dive 2 Start	90	120
Dive 2 End	30	60

 

[Vicko]

I mean if the dive leader knows what he is doing he is asking you your pressure a number of times during the dive. He won't just track the turn pressure but how fast everyone is going trough the remaining gas.
I absolutely hated divers who just signalled ok, that gives very limited information that is not exactly useful.
I often had to change the dive plan during the dive because someone is going trough their gas a bit faster than the rest or the group as a whole was going trough it a bit slower than expected so we could afford to spend a bit more time at depth or around a nicer part of the dive.

 

[inquis]

If someone has a catastrophic gas loss, the guide will air share and go directly to the surface (or the closest safe surface if conditions are dicey)

Right, and in some advanced dives, the "closest safe surface" is *at* the anchor. Yes, they could surface anyway, but between current and the delay recalling other divers in the water before unhooking, they might be looking at hours before getting picked up -- assuming they are successfully located.

The guide will turn when the lowest one is at 1500(ish) psi

Yes, and "lowest" needs to include the diver on doubles who is limited to the same volume as everyone else. If the doubles diver has the highest SAC, but you turn based on someone else hitting the turn pressure, there won't be enough to support the doubles diver if their doubles evaporate.

Again, this is a consideration for a dive where it is vastly preferable to return to the anchor. It may not be absolutely mandatory as with a cave dive, but it could be mandatory "enough" to plan for it in some cases.

 

[inquis]

I absolutely hated divers who just signalled ok, that gives very limited information that is not exactly useful

It's plenty of info for a fixed dive plan. OTOH, if you're trying for something more open-ended (e.g., go to spot X on the wreck, then check if we can extend to Y), then I agree. This would come up in the pre-dive discussion, of course, and underwater responses should be made accordingly (with suitable post-dive feedback if they weren't).

In the OP's case, if directed to give actual pressure, I would signal that immediately followed by "x2" as a reminder of the unusual situation. Even DMs can forget relevant details (as evidenced by @boulderjohn's humorous tale).