SAC Rates

[Kevrumbo]

A reasonable & nominal Surface Consumption Rate (SCR) to attain, IMHO --would be 22 litres/min.

For the common 11 litres/bar (AL80) tank, this would yield a pressure SCR of 2 bar/min (that is, 22 [-]litres[/-]/min divided-by 11 [-]litres[/-]/bar equals 2 bar/min).

To simply apply & utilize this value of 2 bar/min:
In 10 minutes of surface breathing (1 ATA), I would expect my SPG to decrease by 20 bar (check: 2 bar/[-]min[/-] multiplied-by 10 [-]min[/-] equals 20 bar delta)

In 10 minutes of nominal breathing at 30m for example (30m is 4 ATA), I would expect to consume 4 times as much as the SCR value above, so this would become 4 times 20 bar equals 80 bar delta down on the SPG.

Or in 10 minutes of nominal breathing at a shallower depth of 20m for example (20m is 3 ATA), I would expect to consume 3 times as much as the SCR value above, so this would become 3 times 20 bar equals 60 bar, etc . . .etc --for whatever depth you want to plan for.

(Much more easier and straightforward using the Metric System for Gas Planning )

 

[MXGratefulDiver]

It is important to note changing conditions will change you sac rate. Water tempeture is a big one. Unfamiliar gear, real poor vis, current or work load basicly anything that can cause you stress under water can change your sac rate. I tend to try and use a range rather than an exact number. Where I normally dive the water is very cold, I use dry suit, dive with doubles in a lake with generally poor vis my rate is .62 but when I go to there tropics I have a sac rate of .35. So when you are using sac rate to calulate air requirements you need to try and predict what things you may run into on that dive and ajust your calculations.

Absolutely! The analogy I use is with your car's "miles per gallon" ... there's a rated Highway and City statistic that you use to get an idea of the typical gas consumption rate based on conditions. Your actual "mileage per gallon" is typically somewhere in between those two numbers.

SAC rate is like that ... you have a "resting" and "working " consumption rate, based on conditions and how hard they make you work. Your actual "mileage" during a dive will be somewhere in between those two numbers. When I teach gas management I encourage my students to track their consumption over a period of several dives. They will observe that it varies from one dive to another.

Several factors can affect consumption rate, including current, water temperature, your physical well-being at the time of the dive, "stress factors" such as poor visibiilty, narcosis, weighting, technique and several other variables. The key to understanding how these variables can affect your consumption rate is to take measurements over a period of several dives, observe the trends up or down on a given dive, and consider what factors during that specific dive may have caused your consumption rate to go up or down. Looking at the actual measured data helps you determine by how much.

In so doing over a period of several dives, you can establish a pretty good idea of how a given set of conditions will affect how much air you breathe.

... Bob (Grateful Diver)

 

[MXGratefulDiver]

SAC rate is normally calculated considering the total underwater time, max depth and total gas consumption.
If your profile involves a deep dive that ends in a shallow part, your SAC will be very low. If your dive is from the start to the end at the same depth (like diving a wreck), the SAC rate will be representative.
My average is 0.61, but I did dives where I had problems with my weight, some others where I was cold, where my SAC was as high as 1.04 and very nice and warm dives where my SAC was 0.47.
I've also calculated a dive where I hit 0.28, but that dive was a deep one at the beginning ending in a shallow reef. I know that this is not a reality, but a trick of the maths involved.

Depth doesn't really affect your calculated SAC rate, because it's normalized for depth (by dividing the actual measured value by your pressure at depth, in ATA). But to get a reasonable number you need to consider average depth rather than max depth.

Deeper dives do tend to cause higher SAC rates ... but it's due primarily to narcosis or other stress factors such as reduced ambient light. Mental factors such as stress and narcosis do have a way of affecting your breathing pattern ... which can, in turn, affect your buoyancy control ... leading to a whole 'nother set of issues that can affect how much gas you consume.

It's important to think about the causes of increased gas consumption, so that you can anticipate those factors as part of your dive plan. This is particularly true of deep dives. Failure to do so is probably the leading cause of OOA and LOA emergencies at depth ...

... Bob (Grateful Diver)

 

[Rhone Man]

I've seen polls on ScubaBoard that, if I recall correctly, 0.5 cubic feet per minute was about in the middle of reported SACs. But as has been posted, SACs depend on many factors, including experience.

Here is a link to a previous post of mine that shows how my SAC differs between diving wet and dry and over time. http://www.scubaboard.com/forums/basic-scuba-discussions/344311-sac-rates-6.html#post5385111

Yeah, that is about right - here is a link to a poll which we did about 3 years back. Middle of the range was 0.5 to 0.6

http://www.scubaboard.com/forums/advanced-scuba-discussions/264147-how-small-your-sac.html

 

[Ghanguss]

Mines been about 0.5 for a few years. Recently its dropped to < 0.3 on sites Im familiar with. I have lots of time to explore now

 

[Blackwood]

SAC rate is normally calculated considering the total underwater time, max depth and total gas consumption.

It's never calculated that way. Total-gas-used/max-ata*runtime is a calculation, but not of SurfaceAC rate.

The reason your numbers fluctuate so dramatically is not some trick of the math, it's that you aren't computing anything meaningful.

If you want to use max depth, your other variables must be gas-used-at-max-depth and time-at-max-depth.

 

[Jaydubya]

I've seen polls on ScubaBoard that, if I recall correctly, 0.5 cubic feet per minute was about in the middle of reported SACs. But as has been posted, SACs depend on many factors, including experience.

I think some people in the surveys might be stretching the truth a little. 0.5 cf/min is a good SAC rate, not an average SAC rate. I suspect the real average based on how much gas my buddies have used on the same dives to be in the 0.6+ range. For new divers, the average could be 1.0 cf/min. Getting weighting and buoyancy under control will drop SAC significantly. I recall mine went from 1.0 to 0.8 instantly after I did a proper weight check and concentrated on staying neutral the entire dive. The improvement down to the 0.5 range was gradual as I became more comfortable in the water and learned to swim horizontally, streamline my gear, not use my hands to turn, etc. I frequently get below 0.5 now, but I would never plan on it.

 

[JB]

If you let Suunto Dive Manager calculate your SAC it looks very good, about 15% lower than reality. It seems that their maths normalises to 0 degrees Celsius. Just beware of the methods involved in the calculation. The other logging software that I have tried such as divelog and macdive don't normalise for temperature, and give a comparable number that is higher than that obtained from Suunto Dive Manager. I wonder why they have chosen to correct for temperature? In warm clear tropical water Suunto reports 0.41 and MacDive 0.5 for most of my dives, which is quite a large difference.

 

[georice]

this is incorrect, your SAC rate is just that (your surface air consumption) you use it to estimate your gas use at different depths. From that you can determine your diveable time with a given amount of gas or your gas needs for a given amount of time.

SAC rate is normally calculated considering the total underwater time, max depth and total gas consumption.
If your profile involves a deep dive that ends in a shallow part, your SAC will be very low. If your dive is from the start to the end at the same depth (like diving a wreck), the SAC rate will be representative.
My average is 0.61, but I did dives where I had problems with my weight, some others where I was cold, where my SAC was as high as 1.04 and very nice and warm dives where my SAC was 0.47.
I've also calculated a dive where I hit 0.28, but that dive was a deep one at the beginning ending in a shallow reef. I know that this is not a reality, but a trick of the maths involved.

 

[vladimir]

I think some people in the surveys might be stretching the truth a little. 0.5 cf/min is a good SAC rate, not an average SAC rate. I suspect the real average based on how much gas my buddies have used on the same dives to be in the 0.6+ range. For new divers, the average could be 1.0 cf/min. Getting weighting and buoyancy under control will drop SAC significantly. I recall mine went from 1.0 to 0.8 instantly after I did a proper weight check and concentrated on staying neutral the entire dive. The improvement down to the 0.5 range was gradual as I became more comfortable in the water and learned to swim horizontally, streamline my gear, not use my hands to turn, etc. I frequently get below 0.5 now, but I would never plan on it.

It's possible that the respondents to an anonymous poll exaggerated their SACs, but it's hard to see what the motivation for that is. I suspect that the large group that was sampled by the poll--including women and people diving in tropical conditions roughly approximating a swimming pool--is a better representation of the average than you and a handful of buddies diving in more challenging conditions off the coast of North Carolina.

The poll has its biases, however, which would tend to skew the SAC down. I would expect the population sampled--Scubaboard members--to be more experienced than the population of certified divers. I would expect the respondents who actually know their SACs to have better-than-average ones. I would expect a non-response bias: people with lousy SACs choose not to participate. What I would not expect, however, is wholesale exaggeration on an anonymous poll.