Gas Management on Double-Tank Side-Mounting

Hello,

I am interested in getting into side mounting in the future. Thus, I have begun self-teaching myself side-mount by watching a lot of online video demonstrations, course outlines, and power point uploaded by scuba instructors. Of course, I will take a side mount course in the future.

However, I am currently having quite a bit of difficulty with Gas Management and the Rule of Thirds. I am learning these two skills from power points online. (Sidemount) (Padi Sidemount Diver Course Presentation)

I came up with my own test question.

Tank A: 3000 PSI
Tank B: 3200 PSI

Tank A's turn around time: 2000 PSI (3000 PSI divided by 3 = 1000 PSI. 1000 PSI x 2 = 2000 PSI)

I breathed 300 PSI to balance Tank A and Tank B. (A question through, on both power point examples, the power point taught me to breathe 600 PSI to balance both tanks. But wouldn't breathing 600 PSI make it unbalanced because Tank 1 would be 2700 PSI while Tank 2 would be 2600 PSI?)

2000 PSI - 300 PSI = 1700 PSI

1700 PSI / 2 = 850 PSI

3000 PSI - 850 PSI = 2150 PSI

So, 2150 PSI is my turn around point?

So, hypothetically speaking, I start with 3000 PSI. I use 850 PSI to get to my destination, leaving me 2150 PSI. I then use another 850 PSI to go back to wherever I came from, leaving me 1300 by the time I surface.

Another test question.

I came up with my own test question.

Tank A: 2800 PSI
Tank B: 3200 PSI

Tank A's turn around time: 1800 PSI (2700 PSI [2700 is divisible by 3] divided by 3 = 900 PSI. 900 PSI x 2 = 1800 PSI)

1800 PSI - 500 PSI = 1300 PSI

1300 PSI / 2 = 650 PSI

2800 PSI - 650 PSI = 2150 PSI

So 2150 PSI is my turn around pressure!? That just doesn't feel right. Ok, let's say I start the dive with 2800 PSI. I breathed 650 to get to my destination, leaving 2195 PSI. I then use another 650 PSI to go back to my starting point, leaving 1545 PSI by the time I surface. That is 1545 PSI. I can still go to my destination and back AGAIN and still have 245 PSI left.

Did I do something wrong or having that much gas by the end of the dive the whole point of Rules of Thirds.

Deviance,

I haven't done any of the PADI courses so am not sure how they teach this, but to me logic says if you are breathing each tank in sequence to keep balanced then you are using and breathing down your total gas supply and you should calculate the gas as a total volume when looking at turn times and reserves.

So if you start with two tanks at 3000/3200 in your first example you will reach your "third" for your turn point when you have used up ((3000+3200)/3) of your gas.

Therefore you will reach your turn point at when you have used 2066 psi in total and have 4,132 psi remaining across both tanks combined. That means you use 2066 going out, 2066 coming back and have 2066 remaining for surfacing. That is 2066 remaining in total - not in each tank.

So approximately 1033 psi per tank remains as a buffer. (this could be 800 in one and 1266 in the other and so on - but total will equal 2066).

Apply the same to your second example and you get ((2800+3200)/3) = 2000 used - so 4000 remaining across both tanks as you turn point, and 2000 total gas remaining on surfacing.

Diving separate tanks as side-mount like that means you have to do the maths, but if you do the maths before the dives totals and record turn points on a slate then it is easy to sum the gas across both tanks to let you know your progress.

I'm prepared to be shot down if this is wrong, but makes more sense to me. - P

---------- Post added October 3rd, 2013 at 12:32 AM ----------

Addition - just thinking this through again this means that you will still have double the usual reserve, compared to diving a single tank, left for surfacing, but if you didn't do this then you would have no redundancy.

If a third is what you need to surface and exit safely then you need to retain a third in each cylinder as a safety - other wise if you plan to have the equivalent of a third of a single tank left, rather than a third of the total volume, then you will not have enough air left for safety in the event of the total loss of use of either cylinder. - P

Wow, this brings back some memories – this was actually the original Divers Network sidemount presentation circa 2009 and a slightly modified version with another logo on the front! It was used to support a PADI Distinctive Speciality long before the official PADI course was available and had quite a few mistakes, obviously whoever ‘borrowed’ the file didn’t check it too well before uploading!

In terms of rule of third the main thing to remember is that this is generally an overhead environment rule or specifically for dives in which the start and return point/profile are the same/similar. Rock bottom gas management is much more appropriate for most dives.

The easiest way to make a quick calculation for rule of thirds in unbalanced cylinders to find the average tank pressure, i.e. for 3000 psi and 3200 psi (assuming equal tank sizes) it would be 3100 psi – then use this for the basis of calculating thirds – Find the nearest number easily divisible by 3, in this case say 3000, and take 1/3^rd off the average cylinder pressure – i.e. 3100 – 1000 = 2100 psi in both tanks. I’d normally switch between tanks every 300 psi or so, it’s good to balance the tanks sooner rather than later but it’s a good plan to spend so time breathing from the lower pressure tank also to check everything works fine.

Rock bottom relies on the gas reserve you need (normally 1/3^rd but it is affected by the tank sizes/SAC rate of anyone else in your team) plus the gas you need to get to the surface/gas switch at the end of the dive, or can be calculated as the gas needed to get yourself and a team member to the surface/gas switch from the bottom of the dive with en elevated breathing rate (typically around 30 l/min).

There’s plenty of other references for rule of thirds and rock bottom calculation on here – the main point for my post was to say there are mistakes in those presentations (I actually put together the updated version when I was involved with Divers Network).

Karl

Karl - glad you posted that - after I replied last night I went through the presentation and was not overly impressed, I then looked at similar presentations on slide-share and found two other versions of the same one, both slightly different, and both with what I think must be mistakes. Both carried the PADI logo's but clearly they had just been copy and pasted with new backgrounds and logo's attached and I did not think much of the content.

I have been toying with doing the PADI speciality course but if that is still the presentation part of the course material I shall not bother I will look for someone else doing an independent course.. Have fun - P

I wrote an article on sidemount gas management in X-Ray magazine that may be helpful. It opens section five.


X-Ray Mag #56 | X-Ray Magazine

Phil_C:

I have been toying with doing the PADI speciality course but if that is still the presentation part of the course material I shall not bother I will look for someone else doing an independent course..

Click to expand...

PADI haven't released course instructor materials (other than a basic instructor manual) for the sidemount course/s yet. It's been more than a years wait. That's why instructors are sharing/promulgating private resources around in the interim.

The Sidemount student manual is expected sometime soon. We can hope that offical presentations/videos will be released at the same time. Let's see... We can also hope they'll be nice and professional, because private efforts aren't guaranteed error free. But hey, if someone puts resources online/public for free access/download, beggars can't be choosers.

Thanks DD - then I will at least wait until there is something official from PADI before I do their course - in the meantime I have got hold of copies of the side mount book by Jill Heinerths and Brian Kakuks, and also the one by Jill on cave diving. Both of these cover rigs, tanks and set ups in some detail with options presented together with the pro's and con's of each.

I am not sure anyone is realistically ever going to ask to see a side-mount card to let me dive the configuration so I am going to throw myself in and see how it goes - my regular buddy (a PADI and CMAS instructor) is happy to dive with me and to work things through so I am going to start there. - P

So working on a Min Gas scenario, which for GUE (unlikely sidemounters, I know) is 1200 litres for a 30m dive, or 50bar in twinset, based on 2 divers breathing 30lpm. What is the correct protocol for having the gas available in Sidemount? 50 bar min in each cylinder (assuming 12l)? Although this is the same amount of gas, when all gas is accessible you can have one diver breathing 25lpm and the other 35lpm and it is balanced out. With isolated cylinders you lose this benefit, so do you push up the min gas in each cylinder or leave as is?

Ste Wart:

So working on a Min Gas scenario, which for GUE (unlikely sidemounters, I know) is 1200 litres for a 30m dive, or 50bar in twinset, based on 2 divers breathing 30lpm. What is the correct protocol for having the gas available in Sidemount? 50 bar min in each cylinder (assuming 12l)? Although this is the same amount of gas, when all gas is accessible you can have one diver breathing 25lpm and the other 35lpm and it is balanced out. With isolated cylinders you lose this benefit, so do you push up the min gas in each cylinder or leave as is?

Click to expand...

The hijack seems more fun... If you know one diver has an especially high SAC rate, you'd use that as the basis for your rock bottom calculation and that would decide your reserve for you, surely? If you're positing that someone might breathe even more than the breathing-more-than-expected amount, you can't really assume that'll be evened out by someone else breathing a little less in the event of diving doubles, so your rock bottom calculations are blown and it's all gone horribly wrong anyway. The whole point of doubling usual SAC rates when working out rock bottom is that it's a worst-case scenario - if you believe a team-mate will breathe faster than that then I guess your options are (a) plan reserves accordingly or (b) find a team-mate who keeps better control of their breathing when it all goes to custard

I tend to work out what I need for the dive, including contingencies and rock bottom, and then just carry a lot more than that. Seems to work.

I have no problem with this thread getting hijacked. In fact, I rather enjoy learning how you guys calculate the turn around point.

Also, I wrote up an equation for calculating turn around point based on the teaching from the power points.

Tank A: X (Always the tank with lower pressure)
Tank B: Y (Always the tank with higher pressure)

Tank A Turn Around Time (TAT): (X/3) x 2
Balancing pressure between Tank A and Tank B: Y - X

Tank A TAT - Balancing pressure between Tank A and Tank B: Z

Z/2 = W

X - W = Turn Around Pressure

I am going to have this equation printed out, laminated and glued to my wrist slate.