SAC rates for "Everyday Joe's"

[Eric802]

So, my only knowledge about SAC comes from Subsurface. I sync my dives from my Peregrine, plug in my starting and ending PSI and tank size, and it gives me a number. Assuming the PSI numbers I put in are accurate…

a) the SAC number should be accurate, right?
b) the number represents surface air consumption, so some math will be needed to estimate gas use at a given depth? and
b) as a practical matter, how do we utilize the info?

 

[Jcp2]

So, my only knowledge about SAC comes from Subsurface. I sync my dives from my Peregrine, plug in my starting and ending PSI and tank size, and it gives me a number. Assuming the PSI numbers I put in are accurate…

a) the SAC number should be accurate, right?
b) the number represents surface air consumption, so some math will be needed to estimate gas use at a given depth? and
b) as a practical matter, how do we utilize the info?

Given an ascent rate of 10 meters/minute, you can figure out how much time it will take to surface from any given depth. Let's say you are at 30 meters. Then it will take three minutes to ascend to the surface without a safety stop. The average depth is 15 meters, for average ATM of 2.5. So with a SAC rate of 1 cu ft minute, you will use up 7.5 cu ft of gas. If you are supporting a buddy with the same SAC because of a regulator failure, the you will both use up 15 cu ft of gas. If you are both panicking, then you both might use up double that, or 30 cu ft of gas. If you are each using AL80s, which only really hold 77 cu ft of gas at 3000 psi, then you need to reserve 30/77*3000 psi in each tank + 500 psi for safety. That means that you should start ascending even if nothing goes wrong at 1200 + 500 = 1700 psi, leaving 1300 psi for the descent and dive at depth. 1300 psi is 33 cu ft of gas. With a SAC rate of 1 cu ft/min, you would use up 4 cu ft/min of gas at 30 m. So you have 8 minutes at depth before you and your buddy should start going back up to have enough gas to account for a gas supply failure on either part. This is far less time than what your computer will tell you as your NDL time.

If both of you have better SAC of 0.5 cu ft/min, then you would need less gas for the ascent. It would work out to 15 cu ft for both of you on the emergency ascent, so the reserve you would need would be 15/77*3000 + 500 = 1100 psi, leaving 1900 psi for the dive. That would be 48 cu ft of gas. At 30 m, you would use up 2 cu ft/min so 24 minutes at depth, which would be greater than the NDL time the tables or your computer is likely to calculate on air.

Of course, these calculations do not take into account time at depth to solve the problem, or 3 minute safety stop, or a variable ascent rate with 20 meters/minute from 30 m to 20 m and 10 meters/minute from 20 m to safety, and then from safety to surface.

 

[Bowers]

The poles online are generally very skewed toward regular and professional divers. As a norm my ow students have an rmv between 0.6 and 1.5 cfm. In my opinion 2 cfm is an airhog, 1.5 cfm is in need of some control and less than 1 cfm is fine for the diver described by the op.

 

[Superlyte27]

I'm after some information on SAC rates for what I will call "Everyday Joe's" - Everyday Joe is someone who works behind a desk, doesn't run or workout, and is about 10-15% above optimal bodyweight for their height, but is a reasonably experienced diver.
Any info would be appreciated.

240lb 5’11, .32 the other night in buford

 

[jadairiii]

I'm after some information on SAC rates for what I will call "Everyday Joe's" - Everyday Joe is someone who works behind a desk, doesn't run or workout, and is about 10-15% above optimal bodyweight for their height, but is a reasonably experienced diver.
Any info would be appreciated.

Everyday Joe's SAC on a 45' rec drift dive or Everyday Joe's SAC on a 100' dive in a smoking current or Everyday Joe's SAC on a tec dive in a smoking current or high flow cave?

Since this is a tech forum, you can answer your own question by seeing how many Everyday Joe tech diver dive with CCR's.

What people will tell you on a forum and what you see in real life is completely different. A better question to ask is the size of the tanks the Everyday Joe is tech diving with or CCR. Size of tank is directly proportional to how bad their SAC is....when they transition to a CCR it is even worse.

 

[RedSeaDiver2]

Everyday Joe's SAC on a 45' rec drift dive or Everyday Joe's SAC on a 100' dive in a smoking current or Everyday Joe's SAC on a tec dive in a smoking current or high flow cave?

Since this is a tech forum, you can answer your own question by seeing how many Everyday Joe tech diver dive with CCR's.

What people will tell you on a forum and what you see in real life is completely different. A better question to ask is the size of the tanks the Everyday Joe is tech diving with or CCR. Size of tank is directly proportional to how bad their SAC is....when they transition to a CCR it is even worse.

SAC rate stands for Surface Air Consumption - that rate is calculated before factoring in currents, caves and depths

 

[inquis]

a) the SAC number should be accurate, right?
b) the number represents surface air consumption, so some math will be needed to estimate gas use at a given depth? and
b) as a practical matter, how do we utilize the info?

a) for that dive, yes. Most people will vary depending on conditions/activity. Build your database by diving more!
b) yes, @Jcp2 gives a good example to calculate "Rock Bottom" (reserve gas). Use the absolute pressure at a given depth or average depth when changing depths.
c) Subtracting that off the starting gas, you can get an idea of max bottom time (start-rockbot) / RMV / (absolute pressure at depth).

The quantity Subsurface gives you is more commonly known as Residual Respiratory Minute Volume (RMV), having units of volume per time. Multi-level dives are done per level and summed up. (SAC more commonly has units of pressure per time, which is specific to a given tank.) Yes, they're both "consumption", but that's the prevailing vocabulary.

You might play with the Dive Planner in Subsurface (under the Log menu). It's a great way to establish feasibility of a plan more complex than "swim around until reaching Rock Bottom for the depth I'm currently at".

 

[scubadada]

SAC rate stands for Surface Air Consumption - that rate is calculated before factoring in currents, caves and depths

This discussion has all taken place in the Average Gas Consumption thread, posted above. You might want to read it.

You don't really want SAC, which is generally reported as pressure/time such as psi/min and is cylinder dependent. You want to use RMV that is reported as volume/time such as cu ft/min and is cylinder independent. Both are surface consumption. There is ongoing confusion regarding nomenclature.

The RMV from a dive reflects the gear used and the conditions encountered on that dive. RMV can be used for estimating the gas requirement for a dive based on the average depth and dive time.

It seems like many do not understand this

Calculate your RMV over a number of dives and see if that might be helpful to you

Sorry, I see that @inquisit posted while I was typing and that my post is somewhat redundant. RMV is respiratory minute volume.

 

[RedSeaDiver2]

BTW the reason I am gathering this info is to get a rough idea of how much bailout different divers might need on a project with a standardised dived profile that will be repeated day after day.

 

[lowviz]

I am gathering this info is to get a rough idea of how much bailout different divers might need

How do you want to measure the reserve, in cuft/liters or psi/bar?

Is everyone diving the exact same tanks?