Question SAC rates change by dive

[sgmartz]

Hi all, one phenomenon I have noticed is not first dive is always a higher SAC rate than later dives. Even when I’m diving regularly, the change in SAC is significant. Primarily a vacation driver in various locations. Is this just my nerves calming? Or other? Any other suggestions? Thx

 

[KatieMac]

Your first dives are probably deeper.

 

[tbone1004]

Hi all, one phenomenon I have noticed is not first dive is always a higher SAC rate than later dives. Even when I’m diving regularly, the change in SAC is significant. Primarily a vacation driver in various locations. Is this just my nerves calming? Or other? Any other suggestions? Thx

How are you calculating your SAC rate and how wide is the spread? SAC rate is highly variable. First dive being higher can sometimes be a trend for new divers or those that have been out of the water for some time as you are getting comfortable again, conversely the first time is often a shakedown dive and is pretty chill in comparison to subsequent dives. Assuming that you are comfortable in the water and your equipment, the biggest factor that affects SAC rate is workload. If you're hiking in 65f weather in shorts and a t-shirt you're probably relatively chill, but if you crank the temp up or you put a 30lb backpack on then your workload increases and you'll find yourself breathing harder. The same applies in diving whether that workload is caused by inefficient technique, current, or actual work being done in the water that causes your body to have a higher metabolic demand.
SAC is tied back into RMV and the two variables there are respirations per unit time and the volume of those respirations. As your workload increases the respiration count will naturally increase along with the volume of those inspirations. A lot of divers with inefficient technique will breathe a lot deeper than they need to as part of buoyancy compensation but they don't slow their respiration rate down accordingly.

Your first dives are probably deeper.

Depth within recreational limits is not a factor in SAC rate

 

[scubadada]

Hi @sgmartz

Are you saying that your first dive of a trip is higher than subsequent dives or that your first dive each day is higher than repetitive dives?

When you say SAC, are you talking about pressure/time/atm (psi/min/atm) or are you talking about RMV, volume/min/atm (cu ft/min/atm)? The former is cylinder dependent, the latter is not. This only matters for comparison if you do not always use the same size cylinder. RMV is more universally comparable among different divers. Definitions of SAC and RMV vary among divers, knowing the units is the most reliable

Question Thread 'Definitions for SAC and RMV?'

Feb 5, 2023

This topic comes up over and over. Let's see what you think. Please vote once for SAC and once for RMV or vote for #5 and elaborate in your post or #6 if you don't know what this is all about.

• scubadada

You have probably seen the longstanding Average Gas Consumption thread. Where do you currently fall in the spectrum and what is your variability? @tbone1004 made some good comments. Exertion, being cold, and anxiety are probably the most important variables in dive to dive gas consumption variation.

Thread 'Average Gas Consumption'

Jul 3, 2016

There has been extensive discussion lately regarding average gas consumption. A poll may help give us a better idea of what the normal distribution of RMVs is.

1 cu ft = 28.32 liters
1 liter = 0.035 cu ft

<0.3 cu ft/min = <8.5 l/min
0.3-0.39 cu ft/min = 8.5-11.0 l/min
0.4-0.49 cu ft/min = 11.3-13.9 l/min
0.5-0.59 cu ft/min = 14.2-16.7 l/min
0.6-0.69 cu ft/min = 17.0-19.5 l/min
0.7-0.79 cu ft/min = 19.8-22.4 l/min
0.8-0.89 cu ft/min = 22.6-25.2 l/min
0.9-0.99 cu ft/min = 25.5-28.0 l/min
>1.0 cu ft/min = >28.3 l/min

• scubadada
• Replies: 620
• Forum: Basic Scuba

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[tursiops]

Depth within recreational limits is not a factor in SAC rate

It might be if one is a bit nervous on a deeper dive, or has not adjusted their buoyancy correctly.

 

[tbone1004]

It might be if one is a bit nervous on a deeper dive, or has not adjusted their buoyancy correctly.

depth is not a direct contributing factor to SAC rate in recreational depths. It could be an indirect factor with your examples, but I was responding to a comment that implied it was a direct contributing factor, and it is not. Obviously when we start to get beyond standard recreational limits there is gas density to contend with and that can make it a bit fuzzier if you're doing real deep air or you get into some weird mixes, but within the <130ft/40m range depth is not a directly contributing factor

 

[tursiops]

depth is not a direct contributing factor to SAC rate in recreational depths. It could be an indirect factor with your examples, but I was responding to a comment that implied it was a direct contributing factor, and it is not. Obviously when we start to get beyond standard recreational limits there is gas density to contend with and that can make it a bit fuzzier if you're doing real deep air or you get into some weird mixes, but within the <130ft/40m range depth is not a directly contributing factor

We're in Basic Scuba, so I'm not sure the subtle, undefined differences between "direct" and "indirect" factors are really important. My point was simply that if the inexperienced diver goes deeper, their SAC can easily increase.