Signalling air remaining when diving with single tank divers.

[Kronek]

I may be missing something. But when diving SM, why do you switch tanks every 500 or so PSI? Wouldn't breathing one tank down to say ~1000 psi then breathing the other down be more simple?

Unless it has something to do with the buoyancy of empty vs full tanks?

 

[MXGratefulDiver]

I may be missing something. But when diving SM, why do you switch tanks every 500 or so PSI? Wouldn't breathing one tank down to say ~1000 psi then breathing the other down be more simple?

Unless it has something to do with the buoyancy of empty vs full tanks?

Buoyancy is part of it ... the other part is making sure that you have adequate gas reserves to donate if you should need to. Keep in mind that your regs are each attached to a separate cylinder ... so if you breathe one tank down and then have to donate, one of you is going to have much less gas to use than the other.

Perhaps less of a concern on a dive where you can simply surface ... but SM was initially developed with overhead dives, and gas management, in mind ...

... Bob (Grateful Diver)

 

[Orlando Eric]

with the Zero to Hero DM's I have seen at some outfits they are confused as to how much Gas you have in say a 120 let alone any type of doubles or twins.

 

[DevonDiver]

I may be missing something. But when diving SM, why do you switch tanks every 500 or so PSI? Wouldn't breathing one tank down to say ~1000 psi then breathing the other down be more simple?

Unless it has something to do with the buoyancy of empty vs full tanks?

There's two factors that dictate how, and why, sidemount divers 'balance' gas levels in their cylinders:

1) To retain a sufficient reserve for themselves (tank 1) and for donation (tank 2).

2) To prevent gross miss-match in cylinder buoyancy characteristics, that would negatively impact on trim, stability and comfort.

A sidemount diver might choose to swap cylinders/regulators at 1/3 tank capacities (often retaining the final 1/3rd in each cylinder as a reserve), or they may swap at a more frequent rate, based upon set consumption; for instance, every 500psi. The specific approach adopted is often shaped by the nature and complexity of the diving they will undertake - and is especially relevant in overhead environment diving.

 

[supergaijin]

I'm surprised to hear that sidemount divers aren't taught how to deal with traditional single tank communication. In any case I agree that tapping your left tank and signalling the pressure followed by the right seems to make sense.

Any DM worth his salt should understand the signals.

I also believe that the onus is on the individual with 'non-standard' gear configurations, to establish this communication before the dive so that their buddy (and DM if using one) is made aware.

 

[MXGratefulDiver]

I'm surprised to hear that sidemount divers aren't taught how to deal with traditional single tank communication. In any case I agree that tapping your left tank and signalling the pressure followed by the right seems to make sense.

Any DM worth his salt should understand the signals.

I also believe that the onus is on the individual with 'non-standard' gear configurations, to establish this communication before the dive so that their buddy (and DM if using one) is made aware.

There are many "salt-free" DM's out there ... I've had issues with some of them who can't even comprehend a single-tank BP/W ... much less sidemount or doubles.

Tapping the cylinder and signalling pressure certainly works ... the question would then be how does one match their gas to the turn pressure of their buddy? An equivalency in usable gas would be one approach. As an example ... if turn pressure is with 1000 psi usable gas, that means (assuming a reserve of 500 psi) that the single tank diver turns the dive or begins their ascent at 1500 psi. For the sidemount diver, using double cylinders, this would be when the combination of their two cylinders has a sum of 1000 psi usable ... be that 1000 in each cylinder, or 1200 in one and 800 in the other ... again assuming a reserve of 500 psi per cylinder.

If it were me, I'd be explaining to my buddy ... or the DM if necessary ... that I will not be communicating my gas levels, but to avoid confusion will keep track of how much usable gas I have and be responsible for signaling when I reach the agreed-upon turn pressure.

As with any communication, it's only useful if it's understood ... and in this case, it may be easier and less confusing to simply say "don't worry about it, I'll let you know when I get there". You're not going to be able to teach gas-matching on-site to someone who's never heard of the concept before ...

... Bob (Grateful Diver)

 

[MXGratefulDiver]

Appears I'm on a roll tonight with questions that probably have obvious answers. This one has been bugging me since diving sidemount for the first time with my wife two weeks ago. She was diving a single tank backmount and I of course had two tanks sidemounted on my Hollis SMS 50. First dive really wasn't an issue as far as signalling air remaining as I always had air! Second dive however, I started the dive with about 2300 psi in each tank and once I got to 1500 psi in each it dawn on me that we hadn't discussed how I would signal my remaining air. I decided to treat it as if I was diving a single tank and signaled the pressure for the higher tank. Not sure though if that was the correct approach though.

That would not be my approach, since communicating how much air is in one cylinder says nothing about how much remaining air you actually have. In order for information to be useful it must convey an accurate picture of what you're trying to say, and it must be understood by the recipient. In this case, all you've told her is how much air you have on one side. Does she know enough about your configuration to then ask "so how much is in the other cylinder?" And if not, have you just confused her? Or given her an inaccurate impression of how much air you actually do have? If so, then by doing it this way you may be making matters more confusing than they already were, and creating a situation where she's likely to call the dive prematurely because she doesn't really understand that you still have plenty of air to continue.

So is there an agreed upon way of dealing with this situation?

As with most things sidemount, there are no agreed-upon ways of dealing with it. It's one of those situations where you should be discussing, before the dive, what your signals are intended to convey.

I'm curious because when we hit the all-inclusive resorts that have the mandatory DM "follow me" dives you have to signal pressure once you reach 1500 psi and then the whole group ascends once the first diver hits 1000 psi. I don't want to be the cause of the whole group coming up to early on the 2nd dive since 1000psi in each of my tanks would really be 1500 single tank equivalant. Or am I doing the math wrong?

See my previous post on "usable gas". Think in terms of how much gas you have to maintain in reserve, and deduct that from your combined total to determine your usable gas. Assuming you're using the same size cylinders as everyone else, your usable gas is the amount in BOTH of your cylinders minus the reserve in EACH. Since the generally-recognized standard of reserve in recreational diving is 500 psi, that means that you'll need to reserve that amount in EACH cylinder. In this case, you need a total of 500 psi more in your 2 cylinders to account for the fact that you have to have a reserve in EACH, than the single-tank diver does, since they only need to account for reserve in one tank. Therefore you'll be reporting your pressure when your combined (both tanks) pressure is 2000 psi and beginning your ascent when the combined pressure in BOTH of your cylinders is at 1500 psi.

But the real issue here isn't so much gas planning as it is communication ... and that's something that you need to establish prior to the dive, in order to avoid confusion and stress on the part of those you're diving with.

... Bob (Grateful Diver)

 

[boulderjohn]

But the real issue here isn't so much gas planning as it is communication ... and that's something that you need to establish prior to the dive, in order to avoid confusion and stress on the part of those you're diving with.

Only a few weeks ago I was in a similar situation to the OP of this thread. I had had a very bad year of diving, with a bad injury keeping me completely out of the water for several months. I planned a dive trip for soon after the doctor told me I should be sufficiently recovered, and that trip included two days of deep technical diving. Because there was a question (due to the nature of my injuries) as to whether I would be able to perform certain technical skills well (like a valve shut down), the dive operator and I agreed that it would be a good idea for me to do a day of pure recreational diving while wearing technical gear and practicing those skills in the open water.

So I did a two tank recreational dive in Cozumel, with the legally required DM and a couple of other divers. The DM was told what I was doing and why. The DM and the rest of the team were in normal AL 80s. I had double (back mount) steel LP 85s overfilled to 3200 PSI and an AL 80 carried as a deco bottle--meaning I had more gas than the rest of the group, including the DM, combined. In the dive briefing, the DM explained his elaborate system for communicating PSI remaining. I listened as I always do, but I assumed that that part of the briefing did not pertain to me. As the first dive progressed, I practiced gas switching a few times, so I was breathing off of each of the three tanks for a while. When we were pretty well into the dive, the DM pointed to one of the other divers and signaled for PSI. The diver responded. He did the same with the second diver, and the diver responded. When he turned to me and gave me the same signal, I at first looked at him with a "You've got to be kidding me look," but he repeated the signal, somewhat insistently. I then responded by pointing to my doubles and giving the PSI and then pointing to the deco bottle and giving the PSI, indicating I was now down to about the same amount of gas the three of them had started the dive with. As we got closer to the expected end of the dive and the other divers were approaching ascent time, he did the same thing with everyone, including me.

Just following required protocols, I figured.

Back on the boat during the surface interval, I didn't say anything to him about it--his rules, I figured. The other divers, though, had a good laugh about it talking with me. It had not occurred to them how much gas I was carrying. It was a good chance to talk with them abut the differences in tank volumes between the LP 85s and the AL 80s, etc. The DM was not too far away, and he must have been listening, because on the second dive he not only never asked me for my gas amounts, he even let me stay down longer after the others had gone to the surface.

I agree, then, that communication of what is important is the key. In my case, a simple solution would have been for me to indicate that I had checked my pressures lately and found nothing unexpected by merely returning an OK signal rather than go through the elaborate procedure he required of the others.

 

[Jerry_]

While a few years ago I usually had buddies that were diving the same volume and pressure (within maybe a margin of 20% variation), I nowadays lead dives where this margin can be much wider (100% and more), be it due to the number of tanks, different sizes or tanks allowing for different pressure (f.i. 200 to 300 bar).

Yet, for the sake of a comprehensive communication, we don't want to change the two basic messages ("I am at half bottle" and "I hit the agreed bottomline").

What we agree for communication, especially in groups with 2 or more configurations is (1) a bottomline and (2) indicate when they reach half the volume between the starting pressure and the bottomline. In more technical dives a 3rd indication is used in terms of percentage, not absolute pressure (1 finger=10%, finger up=plus, finger down=minus, related to main gesture in (1) or (2))

This allows to lead a divegroup according to waypoints and identify potential problems, while allowing all to have a safe dive.

While it not specifically applies to double SM versus single BM, it applies to any configuration of tanks, including the previously mentioned.

Having a DM looking at the pressure gauge is always ok, but the understanding of possibly upcoming problems is not guaranteed if done differently.

However - as always - the keypoint is to agree prior to the dive, how communication shall take place.

 

[Karl_H]

Maybe just dive a single tank like everyone does on those mandatory DM dives. Why haul 2 when you don't have to.

Just as a general comment on this I'd say it's never a bad thing to carry too much gas unless you really take it too far. There's not many things I'd rather had over spare gas, except for maybe common sense - on that note...

Most sidemount divers set-up their tanks with a single regulator on both tanks, diving with a single reg on a single tank isn't really ideal and I'd say the hassle of reconfiguring regulators would outweigh the hassle of carrying the second tank which really doesn't take much effort?

Solving the communication issue seems a much simpler solution than having to reconfigure regs or bring a second set-up to me so it seems a valid question. Maybe I'm missing something?