Surface Air Consumption (SAC) Rate Calculation - You suck!

Today’s post was supposed to be an exciting rendition of a (little) technical dive on a White Star Line ship wreck; unfortunately this isn’t the case.

The problem with diving in the sea is that it relies upon compliance of the weather. This weekend the weather chose to give me the middle finger, coupled with 8m swells, and so the great expedition was called off.

Very disappointing.



I was really looking forward to the dive and getting more tech experience; namely finding an acceptable method of donning a stage cylinder on a rolling boat, in addition to meeting some more fellow Irish tech divers.



I hate the way scuba involves the sea; so annoying.



Usually if this happens I would attempt to locate a buddy for the local wreck, and go diving anyway. Instead I took the hump; bought a box of beer, watched Season 1 of The Sopranos in its entirety, and ate lots of chips.

I appreciate there may be those of you thinking; “Thank Christ, no ludicrous ramblings this week,” but of course this is not the case, and I am glad to present a short post on Surface Air Consumption.



Surface Air Consumption (SAC) Rate


With the dive cancelled, I decided to log a few prior dives. I’ve slipped a little behind and was feeling a bit guilty; we all know the advantages of logging dives don’t we?



No? Click here immediately!



Anyway, when I log a dive I also calculate my SAC rate.



What is Surface Air Consumption?


“What’s a SAC rate?” I hear you cry; fear not – I shall enlighten you.

Surface Air Consumption is basically how much air a diver consumes on the surface, calculated in litres per minute, or cubic feet per minute depending if you’re from America or not.

(I’m not from America, so I’m working in litres and bar; sorry guys, but the principles are easily applied.)






For example; an average diver has a SAC rate of 25 litres per minute.





How to calculate Surface Air Consumption

The first step is to go diving; a square profile works best, but if your computer calculates average depth, all the better.



Monitor the following on the dive:

• Size of cylinder used
  
• Air pressure at start of dive
  
• Average depth of dive (if diving a square profile, max depth can create a fair approximation)
  
• Air pressure at end of dive
  
• Dive duration in minutes 

EXAMPLE

Diver takes a 10litre cylinder with 200bar for a 20m dive. The dive concludes after 20 minutes and surfaces with 80 bar in his cylinder.

• Litres of air used:

200bar – 80 bar = 120bar used on the dive
120bar x 10 litre cylinder = 1200 litres

• Average air used per minute

1200 litres / 20 minutes = 60 litres per minute at an average depth of 20m

• Compensate for depth

(Depth / 10) +1


(20m/10) + 1 = 3

• Therefore:

60 litres per minute / 3 = 20 litres per minute SAC rate



Easy, no?



Why calculate SAC rate?

I was in the dive shop one afternoon and I mentioned SAC in conversation. I received a look from another diver in the shop, along with the comment;

“Oh … you’re one of those divers…”

I don’t really know what one of “those divers” is, but, yes; yes I am.

SAC rate can be so easily calculated!

I calculate my SAC rate on every dive; I find it extremely useful.



If your SAC rate is elevated, it can help a diver determine factors during the dive that may have created discomfort, stress or exertion.



Knowing how much air a diver uses is also paramount in planning a dive.



EXAMPLE of my dive planning using SAC rate

My SAC rate is 15, the local wreck dive is 16m average depth, and I plan to finish the dive with 50 bar in my twin 7’s, currently filled to 230 bar. 

230 bar – 50 bar reserve = 180 bar useable gas
180 bar x 14 litres (twin 7l tanks) = 2520 litres


Air consumption at depth: (16m / 10) + 1 = 2.6 ATA
15 (SAC rate) x 2.6 = 39 (litres per minute at 16m)


2520 / 39 = 64 minutes



This figure can help determine if it is a decompression dive, or if I want to dive longer, consider a bigger twinset etc.



I also know that every 10 minutes I should be using 390 litres, which equals 30 bar (ish).


39 per min x 10 = 390 litres
390 litres / 14 litres (twin 7 tanks) = 27.85 bar


During the dive I can check if I am using my projected air consumption. If I am using more it may be an indication something is affecting the dive and alter the dive plan accordingly.



FACTORS THAT AFFECT SAC

• Size of the diver; smaller divers have smaller lungs, therefore a lower SAC
  
  
• Women generally have a lower SAC rate than men
  
  
• Smokers tend to have a higher SAC
  
  
• BMI. The larger diver requires more O2 than skinny folk, hence a higher SAC
  
  
• Healthy divers use less air
  
  
• How relaxed a diver is during the dive; stress or anxiety increases SAC
  
  
• Exertion; fining in strong current will make a diver out of breath and increase SAC
  
  
• Streamlining kit and reducing drag in the water can keep SAC down
  
  
• Flat trim also reduces drag and effects SAC
  
  
• Good buoyancy will help SAC, as a diver isn’t constantly inflating and deflating BCD and working to maintain depth
  
  
• Diving in cold conditions can increase SAC
  
  
• Badly maintained gear; any leaks, no matter how small, will have an effect on SAC as extra air is being used 

CONCLUSION


I love all things scuba, and I enjoy calculating my SAC rate as it is another aspect of the dive that continues, even after the underwater bit is over. 

I find it interesting to watch my personal SAC rate vary from dive to dive depending on the conditions. In Ireland it is usually 15 l/m on my local site. When I conducted a technical dive on a site I didn’t know it increased to 17 l/min.

I was able to determine the increase was due to the anxiety of diving a new site, diving with a new buddy, and falling from a rhib which I wasn’t entirely comfortable kitting up on.

As a result when I plan my technical dives I used a SAC rate higher than my usual, to compensate for any anxiety I may have on the day.



When I dive in warm water it can plummet to 12 l/m. This is because I’m in a single tank, wet suit, crystal clear water and not freezing my nads off.

Wifebuddy and I ARE DIVER enjoying low SAC rate in Sharm

• What’s your SAC rate? Include some details so we can compare scenarios. 

• For those who don’t know your SAC rate, do it next dive and post here with the result. 

It’s all part of the fun, and there’s no wrong answer!