Using an SCR range for gas planning

[Sloth]

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

 

[Kevrumbo]

You can make it more simple if you use the metric system with double AL80's tanks (22 litres per bar).
An SCR of 0.7 cf/min*ATA Imperial converts approximately & conservatively to 22 litres/min*ATA Metric.

Divide 22 litres/min*ATA by 22 litres/bar and you have a pressure SCR of 1 bar/min*ATA. (And that's the math "trick": using an easy unity conversion value of 1 bar/min*ATA in metric, instead of 14.5 psi/min*ATA imperial).

To figure out your DCR, just multiply your depth in ATA by the pressure SCR above:
At 30m depth, same as 4 ATA: 1 bar/min*ATA mulitplied by 4 ATA equals 4.0 bar/min.
At 33m depth, same as 4.3ATA: 1 bar/min*ATA multiplied by 4.3ATA equals 4.3 bar/min.
At 36m depth, same as 4.6ATA: 1 bar/min*ATA multiplied by 4.6ATA equals 4.6 bar/min.
AT 39m depth, same as 4.9ATA: 1 bar/min*ATA multiplied by 4.9ATA equals 4.9 bar/min.
At 42m depth, same as 5.2ATA: 1 bar/min*ATA multiplied by 5.2ATA equals 5.2 bar/min.
At 45m depth, same as 5.5ATA: 1 bar/min*ATA mulitplied by 5.5ATA equals 5.5 bar/min.

Now all you have to do is multiply the particular DCR above by the time you spend at that depth:
At 30m depth for 10min: 4.0 bar/min multiplied by 10min equals 40 bar of gas used.
At 33m depth for 10min: 4.3 bar/min multiplied by 10min equals 43 bar of gas used.
At 36m depth for 10min: 4.6 bar/min multiplied by 10min equals 46 bar of gas used.
(And so on. . .)

In essence then, all you have to do is just multiply your depth in ATA by your time at that depth, and you'll know even before looking at your SPG how much gas in bar you've consumed in nominal conditions for that SCR of 22 litres/min.

So much easier to do in your head, on-the-fly, using the metric system. . .

 

[Sloth]

yeah ive thought of going to metric for this reason. So many people dont like it for whatever reason.

My main point is to get away from using a set SCR and instead use a range.

 

[Kevrumbo]

Sloth, I've got similar SCR ranges: 0.75 cf/min in cold water and 0.55 cf/min in warm tropical.
(Translated to volume SCR in metric: 22 litres/min and 15 litres/min.)

The warm water SCR is roughly 30% less than the cold water SCR, so all I do when tech diving in the tropics with double AL80's is use the pressure SCR and DCR evaluations above in post#2 for 22 litres/min . . .and just subtract out 30%.

Easy simple pre-dive gas planning arithmetic in Metric, and after some practice, you can do it real-time-on-the-fly as well without referring to your wetnotes anymore. . .

 

[rjack321]

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

Yeah the differences gets worse and worse the deeper you go. To the point where you can "do the dive" with one SCR and if you are "off" that day you are cutting the dive significantly short.

Up here on my teams we(I) cringe when we see SCRs in the 0.7+ range on tech dives (where its most obvious) - its the first indication of abnormal stress. We(I) do everything we can to keep our SCRs in the 0.5 to 0.6 range and when they aren't we work to modify the dive parameters next time in an effort to get folks more relaxed. Better tides, more experience, scooters, etc.

 

[Sloth]

Yeah the differences gets worse and worse the deeper you go. To the point where you can "do the dive" with one SCR and if you are "off" that day you are cutting the dive significantly short.

Up here on my teams we(I) cringe when we see SCRs in the 0.7+ range on tech dives (where its most obvious) - its the first indication of abnormal stress. We(I) do everything we can to keep our SCRs in the 0.5 to 0.6 range and when they aren't we work to modify the dive parameters next time in an effort to get folks more relaxed. Better tides, more experience, scooters, etc.

Very true but I wasn't trying to show the downsides to a high SCR. Just wanted to highlight a way for newer DIR divers to start thinking about gas management and planning.

I should have done this for recreational ranges to keep the "tech" part of it from distracting from my intent.

Mark

 

[rjack321]

Very true but I wasn't trying to show the downsides to a high SCR. Just wanted to highlight a way for newer DIR divers to start thinking about gas management and planning.

I should have done this for recreational ranges to keep the "tech" part of it from distracting from my intent.

Mark

True, in the rec range small changes aren't very significant (cause you are only multiplying by 2 to maybe 4 ATAs). I like Bob's way of doing SCRs for contingency planning purposes. Swim as hard as you can at 33ft for 5 or 10 mins. Convert to cf and use that of just a little more for calculating rock bottom. For many newer divers it comes out at 1.25-1.5 cf/min per diver.

 

[Kevrumbo]

Here's the simple quick way for Rock Bottom calculations, taking convenient advantage of and applying the system in which the majority of the world uses and understands. . .

For double 11 litre tanks (double AL80's) for a total of 22 litres/bar metric tank rating and a volume SCR of 22 litres/min, using an example NDL/Minimum Deco dive to 30m depth:

Just "tally the ATA's" from 30 meters to the surface:
4.0
3.7
3.4
3.1
2.8
2.5
2.2
1.9
1.6
1.3

Sum Total: 26.5 --this also happens to be the pressure in bar needed for one person to reach the surface.

[Note: Remember that magic unity conversion pressure SCR from post#2 of 1 bar/min*ATA? As you can see, to simplify the tally, the time at each depth ascending to the surface is for only 1 minute; in other words after multiplying 1 bar/min*ATA by 1 minute and by the depth in ATA, the depth in ATA now directly transforms and is equivalent to pressure in bar. So tallying up the ATA's above is equal to totaling-up the pressure in bar]

For two divers: 26.5 multiplied by 2 equals 53 bar.

For two divers stressed: 53 bar plus 30% of 53 bar equals 69 bar.

For two novice divers stressed: 53 bar plus 100% of 53 bar equals 106 bar.

And so your SPG Rock Bottom Ranges are 53 to 106 bar.


From post#2 above, you will consume 40 bar of gas in 10min at 30m depth; therefore in 20min you will have consumed 80 bar of backgas from your double 11 litre tanks. A full twinset is 200 bar, so you now have 120 bar left in backgas after 20min at 30m (and 20min is the NDL for 30 meters on Air mix) --enough left over should you have to gas share at the highest value of your Rock Bottom Range (106 bar).

 

[limeyx]

Here's the simple quick way for Rock Bottom calculations, taking convenient advantage of and applying the system in which the majority of the world uses and understands. . .
...

Simple if you have a PhD in math maybe ....I make RJack calculate mine in the truck on the way to the site

 

[b1gcountry]

Getting back to the concept for a second, I don't worry about ranges when I'm calculating my Rock Bottom. I just use the worst possible SCR I can would maintain under stress.

If I'm trying to calculate my bottom gas to see if I have the gas to do the dive, I just always use .5 since I can do the math in my head. I don't try to be too accurate. Things change. If I need to cut a dive 5 min short, so be it.

Tom