Using an SCR range for gas planning

[Peter Guy]

Mr. Sherwood wrote

Trace, You had more than enough gas!!!! Remember gas is a TEAM resorce.

Would you please expand on this as regards RB (Rock Bottom)? In a 2 man team, each would need RB, correct? BUT, do each need RB in a 3 man team or is 2/3's RB each sufficient since "gas is a TEAM resource?"

My (poor) recollection is that one may NOT consider a 3 man team as having more gas than a 2 man for purposes of RB.

[Note -- I just looked at your info and saw you and I share Birthdays. I think you are the first person I've met with whom I share a BD.]

 

[lamont]

Off topic gas planning discussion moved to parent tech diving forum:

http://www.scubaboard.com/forums/technical-diving-specialties/302613-gas-planning-discussion.html

 

[rjack321]

Mr. Sherwood wrote Would you please expand on this as regards RB (Rock Bottom)? In a 2 man team, each would need RB, correct? BUT, do each need RB in a 3 man team or is 2/3's RB each sufficient since "gas is a TEAM resource?"

My (poor) recollection is that one may NOT consider a 3 man team as having more gas than a 2 man for purposes of RB.

You don't cut into rock bottom for a 3 person team. You would have to switch the OOA diver off to the 3rd teammate at just the right time to avoid 2 OOA divers. You might have to switch the OOA diver to another donator as is - if your combined consumption exceeds your guesstimate (or start blowing off deep stops, bent being more desirable than drowned).

Rock bottom is just a more refined way of making assumptions about how much gas you need to get home, e.g. other agencies/instructors just assume 1/3rds which may or may not apply. There's no penalty for bringing "too much" gas back to the surface.

 

[Kevrumbo]

You don't cut into rock bottom for a 3 person team. You would have to switch the OOA diver off to the 3rd teammate at just the right time to avoid 2 OOA divers. You might have to switch the OOA diver to another donator as is - if your combined consumption exceeds your guesstimate (or start blowing off deep stops, bent being more desirable than drowned).

Rock bottom is just a more refined way of making assumptions about how much gas you need to get home, e.g. other agencies/instructors just assume 1/3rds which may or may not apply. There's no penalty for bringing "too much" gas back to the surface.

Well ideally . . .you don't cut into a Contingency Gas Reserve (i.e. the terminology I was taught which takes into account your deep stops ascending to your first deco stop). But I agree, your combined consumption may exceed your guesstimate on a gas share, 2-man or 3-man team. More significantly, you may have other emergencies in an overhead as well which may force you to encroach on the Contigency Gas Reserve (i.g. lost line, lost buddy, silt-out etc.)

I like to calculate both a full Contingency Gas Reserve, and a Minimum Gas Reserve/Rock Bottom (i.e. the absolute bare minimum amount of gas on a 30'/min ascent to get to the first deco stop --preferably upon exiting the overhead especially if it's a wreck), and figure that into 1/3rds (or 1/6ths penetration) planning. It gives you the option of spending a little more time inside should you decide, for example, to extend a search for a lost buddy. . .

 

[Kevrumbo]

A reply I posted on another forum might have some benefit here.


The reality is no one has a fixed SCR so learning exactly what your SCR is on a given day has little practical meaning. A better approach is to look at SCR as an ideal range that you want to be in. Most skilled divers should find themselves around a .5 to .7 SCR. This sounds like a huge range but when you start converting to PSI its not that huge.

To greatly simplify gas planning what I did was place three tables in my wet notes. The first table is my rock bottom calculation for a given depth on double 80s.

Second table is the time I could stay at a given depth within my SCR range on double 80s taking into account my rock bottom.

The third table is my consumption rate in PSI for a given depth in my SCR range on double 80s.

NOTE: DID THESE NUMBERS ON THE FLY WITHOUT LOOKING AT MY WETNOTES. MY BE SOME ISSUES WITH THE MATH.

The Rock bottom table would like something like this;
depth PSI on 80x2
100 600
120 1000
130 1100
140 1300
150 1400


Now use the SCR range of .5 to .7 to figure out how much PSI you use every 5 mins at depth on the tanks you use.

PSI 5 min usage at depth on 80x2;

Depth(ATA) / PSI every 5mins SCR .5 to .7
1 surface / 50-70
2 (33) / 100-140
3 (66) / 150-210
4 (99) / 200-280
5 (132) / 225-350
6 (165) / 300-420


Then take the above two tables to figure out how much bottom time you have at a target depth on the tanks you use within the SCR range of .5 and .7

Bottom time on 80x2 within SCR Range including min deco.

Depth .5 SCR .7
100 65mins 45mins
120 50mins 35mins
130 45mins 32mins
140 35mins 25mins
150 33mins 23mins

So now a days when I plan a dive I ask myself where is my SCR going to be today? If I haven't dove in a while I might plans towards the .7 side. If I have been diving every weekend and I feel great then I might plan towards the .5 side.

During the dive all I have to do is remember the ranges I should be at every 5 mins and check my gauge every 5 mins. So If I do a dive to 130 and planned around .7 SCR when I check my PSI at 5 mins and found that I used 250 psi then I know I am well within my planned ranged.

Doing it this way will also help you get better at knowing how much PSI you should have before looking at your gauge. So using the 130 range again if I go in with 3000 psi after 5 mins I should be able to guess that I am around 2750 PSI when I check my gauge.

Obviously I did this for deco dives and I have an advantage in that double 80s covert to PSI so nicely due to their tank factor but it can easily be done for recreational dives. Takes some initial work but once its done and in your wet notes its done.

Another point to using ranges also helps to know where your buddies should be on gas consumption as well. You will generally know who has good consumption versus who is a hover so this should help knowing what your buddies should come up with when they do their calculations.

Mark

(Hijacking back to the OP . . . )

Mark, within your system, keep it simple and instead just remember your nominal SCR for 0.5 cf/min, then translate it to a usable pressure DCR for your particular working depth. If you're working hard, just add 40% of that nominal value in your head before confirming it with your SPG.

Look at your first table example: Can you see the 40% relationship between your nominal 0.5 SCR and the higher SCR of 0.7? --

Depth(ATA) / PSI every 5mins SCR .5 to .7
1 surface / 50-70
2 (33) / 100-140
3 (66) / 150-210
4 (99) / 200-280
5 (132) / 225-350
6 (165) / 300-420

(Make it easier to do cognitively and more conservative: use 50% instead. A stressed condition turns out to be 100%, same as 1.0 cf/min).

IOW, use a known nominal baseline of 0.5 for reference (instead of referring to ranges), and add in percentage factors of that baseline based upon actual exertion levels experienced during the dive.

[And believe me it's way easier in metric because of the one-to-one correspondence between depth in ATA and the delta SPG reading in bar. . .]

 

[vioch]

Thanks all for the input.

And what is taken into account by DIR divers for deco gas amount?
Should back gas cover any lost deco gas or should each gas cover the loss of the next one?

 

[coldsmoke]

Thanks all for the input.

And what is taken into account by DIR divers for deco gas amount?
Should back gas cover any lost deco gas or should each gas cover the loss of the next one?

Each gas covers the loss of the next one. That's why sometimes an additional deco gas is introduced - it's more from a rock bottom concern than from a significantly increased deco benefit.

Hunter

 

[rjack321]

Thanks all for the input.

And what is taken into account by DIR divers for deco gas amount?
Should back gas cover any lost deco gas or should each gas cover the loss of the next one?

backgas should cover 1 lost deco gas.
we don't plan for multiple lost gases (plan for lost backgas or 1 lost deco gas regardless of how many deco gases are brought)

 

[vioch]

So
Each gas covers the loss of the next one
or
backgas should cover any 1 lost deco gas
?

And how is the amount of each 1 deco gas calculated? I mean is there any additional amount to calculated mandatory one?

 

[rjack321]

So
Each gas covers the loss of the next one
or
backgas should cover any 1 lost deco gas
?

And how is the amount of each 1 deco gas calculated? I mean is there any additional amount to calculated mandatory one?

Backgas covers either:
buddy's lost backgas
your lost deco gas

You need to calculate rock bottom and a lost deco gas plan and reserve the larger amount.

Calculate your deco gas requirements using your resting SAC. GUE adds 50%, UTD has a less rigid approach (its not a strict +50% its assuming you have the gas somewhere to cover loss, a combination or backgas and buddies deco gas. aka use your judgement on how to CYA)