Gas Management With Sidemount

[Razorista]

The most foolproof and effective way is to switch as often as you get the chance.
Every time you have a hand free for that, just switch without checking pressure or time.

With that method you just look at your spgs occasionally to check for stupidity, but have the tanks more or less equalized all the time without checking constantly.

Also use everything that happens during the dive as a reminder:
Reached bubble check depth - switch.
Reached bottom - switch
Reach an object - switch
Leave an object behind - switch

Cavedivers often recommend it this way with cavediver specific signals:
at every T - switch
at every turn -switch
Things like that...

You can also learn to identify the tanks behavior when close to empty as a warning sign and learn to guess remaining pressure close enough to do without spgs in a pitch.

 

[wetb4igetinthewater]

@DA_Aquamaster, you got what I was getting at.

Rather than use numbers, I'll use the variables. X is the starting pressure. Z is the maximum difference in air pressure that I want between my tanks.

Start: both tanks (left tank = LT, right tank = RT) at X psi
breath Z PSI from RT to , which is now Z PSI less than LT. switch to LT
breath 2Z PSI from LT , which is now Z PSI less than RT. switch to RT.
breath 2Z PSI from RT to , which is now Z PSI less than LT. switch to LT
breath 2Z PSI from LT , which is now Z PSI less than RT. switch to RT.

And so on.

I can take suggestions as to the value of Z. But the point I'm trying to make is that it makes no sense for one tank to always be equal and more than the other. They can swap that "role"

There are several different approaches that will work. Some are going to be more beneficial than others, depending on the type of dive you're doing. For most diving, I personally find using time to be a reasonable approach (for most dive profiles) ... for example, switch on 10-minute intervals. Why? Well, because we're more conditioned to look at our dive computers (particularly those of us who wear them on our wrist) than we are our SPG's ... and 10-minute intervals are easy to remember. Even if you are a little bit under or over it tends to even out over the course of the dive. And you always have the option to check your gauges if you're not sure ... the important thing is to keep the tanks somewhat even ... I prefer keeping mine within 200-300 psi of each other.

What system you use is less important than that you have a system, understand its limitations and applicability to the dive profile, and use it consistently. I personally find that easier to do when the "trigger" for switching depends on something I do out of habit anyway ... which is to look at my dive computer.

... Bob (Grateful Diver)

Bob,

You've never been in sidemount when we dove together. Do you have a wireless transmitter for each cylinder? I was thinking of going that route. I'm pretty happy with my Aeris A300 CS. Battery life sucks, but it doesn't lose the connection with the transmitter at all. I like being able to look at my wrist and see my air pressure. I was thinking of getting a second one (now the Oceanic VTX) and simply wear that computer on my other arm. I'd still have brass and glass SPG's) I think I'd prefer monitoring air pressure over elapsed time. I'm thinking about the time in the future when I'm proficient enough to task load myself with photography.

I'm hoping to dive with Tom C in the near future, as I got my SM rig from him, then dive with other folks who are getting into SM. The sooner I can take a class from Don P or Jay up on Vancouver Island, the better. But until I'm signed up for a class, I'm self learning as much as possible.

 

[MXGratefulDiver]

The most foolproof and effective way is to switch as often as you get the chance.
Every time you have a hand free for that, just switch without checking pressure or time.

My problem with that method is I rarely have a hand free ... the right one's almost always got a camera in it, and the left one's either got my light or I'm doing something with it. Switching frequently is one of those things that seems to me in like a good idea in theory, but in practice it'd be more effort than it's worth ...

... Bob (Grateful Diver)

 

[MXGratefulDiver]

Bob,

You've never been in sidemount when we dove together. Do you have a wireless transmitter for each cylinder?

I primarily dive sidemount for tech dives and when I'm going solo.

No ... I don't use an AI computer. I just have SPG's mounted lollipop style on my cylinders. If I want to look at a gauge, they're right up there close to my face ... all I need do is reach up and hook a finger around the gauge to pull it out far enough away from my body to see it.

... Bob (Grateful Diver)

 

[wetb4igetinthewater]

I primarily dive sidemount for tech dives and when I'm going solo.

No ... I don't use an AI computer. I just have SPG's mounted lollipop style on my cylinders. If I want to look at a gauge, they're right up there close to my face ... all I need do is reach up and hook a finger around the gauge to pull it out far enough away from my body to see it.

... Bob (Grateful Diver)

I'm trying to decide if I want to get a couple Shearwaters and just go standard tec, or get the same AI computer. Cost is about the same. I am sure some folks will have strong opinions on that!

 

[kelemvor]

I primarily dive sidemount for tech dives and when I'm going solo.

No ... I don't use an AI computer. I just have SPG's mounted lollipop style on my cylinders. If I want to look at a gauge, they're right up there close to my face ... all I need do is reach up and hook a finger around the gauge to pull it out far enough away from my body to see it.

... Bob (Grateful Diver)

I tied some bungee to each of my gauges. That way they're pretty secure in front of my chest. I can glance down and see both gauges with only minor effort and no hands.

 

[Razorista]

My problem with that method is I rarely have a hand free ... the right one's almost always got a camera in it, and the left one's either got my light or I'm doing something with it. Switching frequently is one of those things that seems to me in like a good idea in theory, but in practice it'd be more effort than it's worth ...

... Bob (Grateful Diver)

Think of it differently:
Instead of moving your hand the long way to the spg (even lollipopped ones turn their back to you occasionally) you just switch.
The movement is more reflexive: spit out, pop in, breathe.
Saves time and effort and can always be done blindly.

With the camera I always switch before focussing on the objekt and again when I turn away after taking the pic or stopping the recording.
Since I do not move when focusing I do not have to clip off the longhose, if I have to move unexpectedly I almost reflexively switch to the longhose again.
I use a one handed camera and a video-light in the other hand, but the same would work with a large camera.

 

[Razorista]

I am sure some folks will have strong opinions on that!

Air integration is not important enough to have a strong opinion about in sidemount, but I will add my two cents anyway.
In my opinion it is mandatory and self explaining to use normal non digital pressure gauges as backups/primaries with the transmitters in the second port.

It is very useless however.
With two tanks you simply never need the information.
Totally unnecessary to use space on the dive computer display for something you absolutely do not need.

Pairing often sucks when the components age and batteries are replaced late.

The transmitters are also close to a hand carried compass when used on sidemounted tanks and have been reported to sometimes have a strong influence on it.

And then there is the issue that those things do not understand real diving and will beep and flash at you when you almost empty your tanks in the shallows finishing a typical after work dive.

 

[Jack Hammer]

At this point in the dive (turn pressure in both tanks and at max penetration), you've used 1/3rd of the gas in each tank and you have 2/3rd (2400 psi) left in each tank. So if you suddenly lost all the gas in each tank, you've still got twice as much gas as you used for the penetration left in the remaining tank.

Sorry, I'm having difficulty with your math here... can you elaborate?

 

[pinshootr]

Jack Hammer ; he posted his cave fill is 3600 so his thirds are 1200