Communicating remaining pressure on doubles(twinset)

[the_one_who_waits]

Let's say you are doing a 2-tank dive on doubles with a group where some are on singles and some (or only you) are on doubles, so you have the benefit of not needing swap tanks between dives. During the 2nd dive, if the leader/guide does a pressure check, how would you convey your remaining gas? Because if you signal what's on your SPG/AI-computer, there is the possibility it is interpreted as having significantly less gas (volume) than you actually do. While of course a good leader/guide should be able to differentiate between single and double pressures, and this all should be addressed pre-dive, it may not always be the case when there may be a lot going on and to keep track of.

Example scenario: everyone is diving same volume tanks and have the same SAC rate. You ended the first dive with 2000psi/140bar in your doubles. So you start your second dive with that much, and those on full singles start with 3000psi/205bar. After a while, the leader/guide does a pressure check all around. At this point, you and everyone has consumed the same volume of gas. Whereas your gauge may show 1250psi/85 bar, theirs will be at 1500psi/103 bar. Because of the reserve gas from the first dive you're carrying over, the two pressures are not drastically different such that the leader/guide may not to think beyond the erroneous conclusion that you are consuming gas very fast and remainder of the dive may need to be altered or ended much sooner.

Would you just go ahead and signal your remaining pressure and risk ending the dive ~20 minutes sooner? Could you signal twice your PSI to maintain semi-parity with those on singles?

 

[inquis]

On a guided dive, the turn pressure should have been discussed during the pre-dive briefing. If queried underwater, I usually sign, "pressure OK" (assuming it is above the equivalent turn pressure, of course).

 

[xan]

On a guided dive, the turn pressure should have been discussed during the pre-dive briefing. If queried underwater, I usually sign, "pressure OK" (assuming it is above the equivalent turn pressure, of course).

Same, I would clarify the gas strategy before the dive, and make sure the guide understands. I would then report accurately when queried about gas pressure, not try to "adjust" for doubles.

 

[Feye]

1. Discuss before diving and notice status.
2. Report exact pressure, and "Double" sign for situation.

Anyway, double tank/wing is very bulky, easy to identification.

 

[EFX]

First, NEVER lie about gas remaining. Let's say it's a group dive and the DM gets everyone's tank pressure. He decides everyone has enough gas to swim farther from the ascent point. But, you lied and you eventually signal low on gas. The DM can't take the rest of the group back to the ascent point because you don't have enough gas to make it back safely. Therefore, everyone surfaces and the boat has to come to pick up the group.

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.

 

[inquis]

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.

I think you mixed up consumption and remaining pressure. The example used the same SAC for all divers.

 

[gmerick]

I talk it over with the dive guide. What has usually been the case for most day-trip insta-buddy (same-ocean-buddy/) dives is:

I signal my usable gas remaining. Diving sidemount I add up both pressure readings, and subtract 500 psi (instead of surfacing with a minimum of 500 in one tank, I have to have 500 in each tank). At the beginning of a dive starting with 3250 in each tank, I'd be signaling 6000. Towards the end of the first dive with 2000 and 2200 remaining, I signal 3700. Toward the end of the second dive, with 1300 and 1000, I signal 1800.

For doubles, I'd do the same, but instead of adding two gauges, simply double your pressure and subtract 500.

At least this way the remaining air message remains consistent among all divers.

 

[boulderjohn]

I actually had this happen.

Shortly after recovery from a shattered collarbone, I was diving in Cozumel and planning to do decompression dives. I had worked on my shoulder mobility and was borderline ready. The dive operator suggested that I do two NDL dives while using full tech gear as a kind of dress rehearsal to make sure I was OK. Our boat was a mix of NDL and tech divers. The DM was appropriately advised as to why I was doing an NDL dive with steel doubles, an AL 80, and an AL 40. In his briefing for me and the two NDL divers with AL 80s, he told us how to respond to his request for air pressure. I assumed he was not including me.

As the dive went on I continually practiced gas switches, so I breathed off of all my tanks to varying degrees.

Late in the first dive, the DM stopped, pointed to one of the divers, and requested air pressure. The guy responded, and the DM made the same request to the second diver. When he was done, he turned to me and asked for my pressure. I was startled, and my immediate response was a WTF expression. He looked angry as he repeated the request emphatically. I then pointed to each individual tank, including each of the manifolded doubles, and signaled the pressure in each. Of course, my combination of tanks gave me more total gas than the three of them had combined at the start of the dive.

The look on the DMs face was priceless as he realized my situation. He never asked me again, either on that dive or the second dive.

People can figure it out, especially if you discuss it ahead of time.

 

[inquis]

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".

Changing your example slightly, say the single-tank turn pressure is 2000 psi (allowing 1000 psi for the "out"). That means you should only use 500 psi from your doubles (i.e., the same volume as everyone else). Starting with 2000 psi, your specific turn pressure would be 1500 psi.

(ETA: All this assumes everyone has the same tanks, but you have two of them. There's more math to do if your tanks are different sizes as well. There's a reason teams usually dive with the same equipment!)

Again, the above only applies to a specific type of dive. A discussion with the DM on the boat is important to gain a common understanding of just how desirable an at-depth return to anchor will be and how to report remaining outbound gas.

 

[EFX]

I think you mixed up consumption and remaining pressure. The example used the same SAC for all divers.

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.