SAC vs Surface RMV

[Jax]

Steve Lewis has written, IMHO, the definitive guide on SAC - RMV for technical divers. It comes in two parts and is titled "Gas Planning 101". You can find it at Doppler's Tech Diving Blog

Okay, for those of you that said SAC and RMV is meaningless because there are no units . . . Surface Air Consumption is a rate and Respiratory Minute Volume is a rate, and those rates are in a volume per minute. Generally, these are cu ft / min or Litres / min.

For those of you that question the use of a constant, please read the two blogs on Gas Planning 101. (Edit | Find on this page | "Gas Planning 101" will take you to each)

From Steve Lewis' blog:

When working out gas volume requirements for a dive, I like to have one constant for each diver’s consumption rate on the surface. Once armed with this, all the other considerations such as depth, time, workload, temperature, narcosis, mental stress, fitness levels, how we feel today and what we had for breakfast can be factored in. The key piece of information is having a constant for surface air consumption in a state of rest.

It’s always seemed to me that Surface Air Consumption (SAC) is the perfect candidate. SAC is a unit measure of gas consumption on the surface, and since we need to have a constant non-variable figure to hang all the other factors from, SAC seems to win on several scores not least of which is its name.

And so, when I am planning gas volume requirements, I use SAC as a constant to describe an individual diver’s air consumption rate on the surface – and most importantly – at rest. This does away with the need to use an array of potentially confusing terms such as average SAC, resting SAC, swimming SAC and so on.

This is how it was spelled out in the SDI Solo Diver Manual.

Still, TDI's Guide to Advanced Nitrox and Decompression Procedures use RMV . . .

 

[Jax]

Any chance any of you RMV people with huge experience and data might try Steve Lewis' method and see how it marries up to your RMV data?

The base standard DF for a dive in familiar waters with little workload and the minimum of stressors is 1.5. For example, if Vlada’s dive were to fall into this category she would multiply her 60.5 litres per minute by 1.5 to factor in the dive factor: 90.75 litres per minute. But the planned dive with Joe will be in unfamiliar cool water, carrying a stage bottle of decompression gas. Joe suggests a DF of 2 for her. This translates into a volume of 121 litres per minute for Vlada’s dive. This is Vlada’s Respiratory Minute Volume (RMV) for this dive and will form the central strut of her gas management plan.

I put my dives in a spreadsheet and calculated the RMV. Then I put my SAC in and SWAGged the stressor multiplers. It seemed to work out pretty well for the dives with my air-integrated computer.

 

[MXGratefulDiver]

Any chance any of you RMV people with huge experience and data might try Steve Lewis' method and see how it marries up to your RMV data?

The base standard DF for a dive in familiar waters with little workload and the minimum of stressors is 1.5. For example, if Vlada’s dive were to fall into this category she would multiply her 60.5 litres per minute by 1.5 to factor in the dive factor: 90.75 litres per minute. But the planned dive with Joe will be in unfamiliar cool water, carrying a stage bottle of decompression gas. Joe suggests a DF of 2 for her. This translates into a volume of 121 litres per minute for Vlada’s dive. This is Vlada’s Respiratory Minute Volume (RMV) for this dive and will form the central strut of her gas management plan.

I admire Steve tremendously ... but in this case I disagree with him. The basis of my disagreement is five years worth of measurements, taken by every student who's ever been through my AOW class ... and the multiplier between resting consumption rate and working consumption rate has varied from a low of about 1.5 to a high of nearly 4.0 ... depending on the student.

The problem is that consumption rate is not a static number ... and the actual range between resting and working consumption rates is highly variable between individual divers. And without actually measuring *your* working consumption rate and comparing it to the resting value, you have no way of knowing what it really is. This is why I have my students do two completely different tests ... one under nearly static conditions, and one while working their butts off. Knowing what the high and low rates will be *for your body* under actual diving conditions gives you the best data for dive planning ... because you can then apply that data to the anticipated conditions, factoring in the stressors that you are likely to encounter during the dive. And in so doing you can better answer the question ... "Do I have enough gas for the dive I'm planning to do?"

Standardized multipliers MIGHT be adequate, but they're going to work just like your computer works when calculating your NDL ... which is to say they know nothing about *you* ... they are based on an idealized body that may or may not be adequate for your planning purposes.

In this application, standardized "baselines" have limited value. They're better than nothing, but I prefer working with actual measured data ...

... Bob (Grateful Diver)

 

[Jax]

I admire Steve tremendously ... but in this case I disagree with him. The basis of my disagreement is five years worth of measurements, taken by every student who's ever been through my AOW class ... and the multiplier between resting consumption rate and working consumption rate has varied from a low of about 1.5 to a high of nearly 4.0 ... depending on the student.

The problem is that consumption rate is not a static number ... and the actual range between resting and working consumption rates is highly variable between individual divers. And without actually measuring *your* working consumption rate and comparing it to the resting value, you have no way of knowing what it really is. This is why I have my students do two completely different tests ... one under nearly static conditions, and one while working their butts off. Knowing what the high and low rates will be *for your body* under actual diving conditions gives you the best data for dive planning ... because you can then apply that data to the anticipated conditions, factoring in the stressors that you are likely to encounter during the dive. And in so doing you can better answer the question ... "Do I have enough gas for the dive I'm planning to do?"

Standardized multipliers MIGHT be adequate, but they're going to work just like your computer works when calculating your NDL ... which is to say they know nothing about *you* ... they are based on an idealized body that may or may not be adequate for your planning purposes.

In this application, standardized "baselines" have little real value. I prefer working with actual measured data ...

... Bob (Grateful Diver)

Ummm . . . that is why I asked people to try it out (implied: on themselves) and measure against [their own] RMV however it was figured.

 

[MXGratefulDiver]

Ummm . . . that is why I asked people to try it out (implied: on themselves) and measure against [their own] RMV however it was figured.

OK ... and what I'm saying is that in some cases it will work out reasonably close, and it other cases it won't even be in the ballpark ... depends on the diver ...

... Bob (Grateful Diver)

 

[Jax]

Now I'm lost - I would expect some sort of variance, but that the variance be somewhat constant for each diver. You don't think that would be?

 

[MXGratefulDiver]

Now I'm lost - I would expect some sort of variance, but that the variance be somewhat constant for each diver. You don't think that would be?

Yes, but that's not what I'm reading into the discussion above ... which seems to be that one takes a "constant" SAC rate and applies some standardized multiplier to account for variances in conditions. That sorta works ... but I think there's a better way.

In another thread not too long ago, someone posted a graph they had made showing a data point of consumption rate for each dive over a series of dives. Taken as a whole, they showed a trend ... but the variance was different for every single dive *for that diver*.

This is more in keeping with my approach, and what I recommend to my students ... which is to track your consumption rates over a period of time, recording it and looking for trends. Those trends will give you a better idea of how to plan a given dive, based on measured data from previous dives using similar profiles and under similar conditions.

... Bob (Grateful Diver)

 

[Jax]

Yes, but that's not what I'm reading into the discussion above ... which seems to be that one takes a "constant" SAC rate and applies some standardized multiplier to account for variances in conditions. That sorta works ... but I think there's a better way.

In another thread not too long ago, someone posted a graph they had made showing a data point of consumption rate for each dive over a series of dives. Taken as a whole, they showed a trend ... but the variance was different for every single dive *for that diver*.

This is more in keeping with my approach, and what I recommend to my students ... which is to track your consumption rates over a period of time, recording it and looking for trends. Those trends will give you a better idea of how to plan a given dive, based on measured data from previous dives using similar profiles and under similar conditions.

... Bob (Grateful Diver)

I like the idea of looking for trends. I was thinking that using Steve Lewis' method, the trend analysis would give me Dive Factors that would work for me on my dives. It is the determination of that Dive Factor Multiplier that would adjust for each dive.

How many dives under one's belt before s/he settles out enough to have a good RMV, anyway?

 

[Cave Diver]

How many dives under one's belt before s/he settles out enough to have a good RMV, anyway?

Frequency and duration is likely to be more important than quantity.

 

[Jax]

Frequency and duration is likely to be more important than quantity.

That may be true, but what else can we vacation divers do? We get bursts of 8-10 dives 3 or 4 times a year.