Why call it SAC?

[Foxfish]

I've done a fair bit of freediving and spearing in the past and have in the last few years started using SCUBA. One of the things that I find slightly amusing is this thing called 'surface air consumption' even though it is depth dependent.

Anyone every tried deriving the equation for SAC? Just thought it sounded like a fun thing to do.

 

[TSandM]

"SAC" is air consumption, normalized to 1 ATA -- thus "surface air consumption". You then multiply it times the ATA at the depth you are to determine what your likely consumption is there.

 

[Foxfish]

I've never heard SAC quoted based on atmospheric pressure. In the equation you divide by the average depth. Seems to be a misnomer.

 

[Kern]

SAC= (bar used/time/pressure) x WC

The surface pressure is 1ATA, or close enough to 1bar.

You don't divide by the (average) depth, but by its pressure equivalent. Thus normalising your consumption to 1 ATA, or 1bar, or, the surface.

Knowing your rate of consumption at the surface, allows you to multiply by the ambient pressure of the depths you'll be diving, & so know how much gas you'll consume.

 

[Spg01]

Here are a few links that explain a few things. Knowing your SAC rate is very helpful when you get into gas planning. If you know gas consumption at the surface you can calculate gas consumption at different depths.

Which will let you know how long you can stay at depth

SAC Calculator

Surface Air Consumption Rate

 

[Kern]

Here are a few links that explain a few things. Knowing your SAC rate is very helpful when you get into gas planning. If you know gas consumption at the surface you can calculate gas consumption at different depths.

Which will let you know how long you can stay at depth

SAC Calculator

Surface Air Consumption Rate

As the OP is from Oz, those sites may be more confusing than helpful.

 

[Charlie99]

Anyone every tried deriving the equation for SAC? Just thought it sounded like a fun thing to do.

It's pretty straightforward.

Your lung volume is more or less constant independent of depth. Your breathing rate is more or less constant with depth (you need to breathe to get rid of CO2, so the higher ppO2 doesn't allow you to slow your breathing).

So the liters per minute of actual volume is independent of depth.

But the number of molecules in each liter is directly proportional to the absolute pressure.

Absolute pressure is 1ata at the surface + 1 more atm for each 33' of depth.
(The metric guys have it easy ---- 1bar at surface + 1 bar for every 10 meters of depth).

If you put all of the above together, you will see the correction factor to normalize your consumption at depth back to a common reference point of the surface is to divide by 1 + (depth/33').

 

[Foxfish]

The definition I got for SAC is (change in tank pressure x tank volume) / (dive time x average depth). Use absolute pressures for tank pressure and average depth. I've found that by using this formula I get a fairly constant air consumption.

If I work out the consumption rate as the total volume of air consumed referred to atmospheric pressure (ie. volume at start of dive less volume at end of dive calculated using the ideal gas equation at atmospheric pressure) divided by the dive time the consumption rate varies from dive to dive. Seems to me this is what should be called SAC rate.

 

[Kern]

The definition I got for SAC is (change in tank pressure x tank volume) / (dive time x average depth). Use absolute pressures for tank pressure and average depth. I've found that by using this formula I get a fairly constant air consumption.

If I work out the consumption rate as the total volume of air consumed referred to atmospheric pressure (ie. volume at start of dive less volume at end of dive calculated using the ideal gas equation at atmospheric pressure) divided by the dive time the consumption rate varies from dive to dive. Seems to me this is what should be called SAC rate.

This equation, SAC= (bar used/time/pressure) x WC, is just a running version of this equation, SAC is (change in tank pressure x tank volume) / (dive time x average depth). I use it because it makes the relationship between surface consumption in lt/minute & bar/minute very obvious.

I'v no idea why you think your SAC rate should vary much between dives, outside of variables related to physical stress, or why you want to use the ideal gas law .

 

[Foxfish]

I'v no idea why you think your SAC rate should vary much between dives, outside of variables related to physical stress, or why you want to use the ideal gas law .

From what I can gather the SAC formula is derived from the ideal gas law. You get big variations in SAC if you refer it back to atmospheric conditions as previously mentioned.