SAC Rate Changes with Depth

[Charred]

Hi all,

I just finished my second dive trip with a Perdix AI. It is constantly calculating my SAC rate.

My last two trips were in Cozumel and the dive profile is usually deep (80-90ft) and then slowly working shallower or just shallower (50-60 feet).

My average SAC is usually 15. However, I noticed that my SAC rate when deep is significantly better around 12 but on one deeper dive down to 110-115 I was below 10. As the dives get shallow, my SAC rises to 16-17. My SAC at the safety stops is usually 17 so higher than the dive average.

Why is my SAC rate better at depth then shallow? I have 2 thoughts but I won't bias replies.

 

[northernone]

I calm and relax further I am from the surface.
Ascending takes effort and motion. My SAC goes up when I'm working harder which is moving towards the deco stop.

If I settle in shallow, my SAC is the same shallow as deep (recreational depths)

Cameron

 

[charlier]

Interesting observation. My SAC rate remains the same through much a typical dive. That said, my SAC rate is usually the highest upon entry.

 

[Kevrumbo]

Hi all,

I just finished my second dive trip with a Perdix AI. It is constantly calculating my SAC rate.

My last two trips were in Cozumel and the dive profile is usually deep (80-90ft) and then slowly working shallower or just shallower (50-60 feet).

My average SAC is usually 15. However, I noticed that my SAC rate when deep is significantly better around 12 but on one deeper dive down to 110-115 I was below 10. As the dives get shallow, my SAC rises to 16-17. My SAC at the safety stops is usually 17 so higher than the dive average.

Why is my SAC rate better at depth then shallow? I have 2 thoughts but I won't bias replies.

For short 3-5min shallow safety stops at 6msw, my volume SAC here in cold 10 to 14°C Southern California homewaters stays the same at 17 liters/min with single tank Aluminium 13L, but can be as high as 22 liters/min working against a current or in rough seas.

In US Imperial Units, with a 13L tank (AL100) this comes out to a pressure SAC of 19 and 24 psi/min respectively.

In tropical 26°C waters of Cozumel and Palau on drift dives going with the current, I've had SAC rates as low as 9 to 11 liters/min at an average depth of 20msw with an Alu 11L cylinder.

In US Imperial Units, with a 11L tank (AL80), this is 12 to 15 psi/min.

 

[tomfcrist]

Air consumption will rise with stress and exertion. My consumption is garbage when I jump off the boat, and usually stays that way until I get down the anchor line and start the bottom portion of the dive.. this is mostly a product of battling wave action (pitching decks), rapid temp change, and current. Once I’m on the bottom I can go into chill mode.
The ascent has the possibility of being just as stressful... thus causing an increase in air consumption.

 

[wjeiii]

Hi all,

I just finished my second dive trip with a Perdix AI. It is constantly calculating my SAC rate.

My last two trips were in Cozumel and the dive profile is usually deep (80-90ft) and then slowly working shallower or just shallower (50-60 feet).

My average SAC is usually 15. However, I noticed that my SAC rate when deep is significantly better around 12 but on one deeper dive down to 110-115 I was below 10. As the dives get shallow, my SAC rises to 16-17. My SAC at the safety stops is usually 17 so higher than the dive average.

Why is my SAC rate better at depth then shallow? I have 2 thoughts but I won't bias replies.

I would guess you are over weighted. More expansion and compression of gas in BC with slight depth change means more gas pulled from tank computer reads higher SAC

 

[Kharon]

... In US Imperial Units, with a 13L tank (AL100) this comes out to a pressure SAC of 19 and 24 psi/min respectively.

In US Imperial Units, with a 11L tank (AL80), this is 12 to 15 psi/min.

Just to be clear - SAC can not be measured in psi. The psi depends absolutely on the volume of the tank and it's working pressure. SAC can only be measured in gas volume.

(Tank) Rated Vol / Working Pressure = cf/psi

cf/psi x psi used = cf used

 

[Charred]

I would guess you are over weighted. More expansion and compression of gas in BC with slight depth change means more gas pulled from tank computer reads higher SAC

Not sure I am following this. I add air to my wing at the beginning the dive and usually never again. I am usually purging a bit of air but rarely add. And definitely not as I go shallower.

 

[hammet]

Just to be clear - SAC can not be measured in psi. The psi depends absolutely on the volume of the tank and it's working pressure. SAC can only be measured in gas volume.

(Tank) Rated Vol / Working Pressure = cf/psi

cf/psi x psi used = cf used

SAC is psi/min
rmv is cf/min

 

[northernone]

Surface air consumption rate to be meaningful need be expressed in gas volume/ time. Let's not forget what the acronym stands for when we try to define it.

I'm in a hammock at the moment, what is my surface air consumption rate in psi/minute? I can calculate it in L/min or CF/min because that's what surface air consumption is.

Two dive scenarios:

1. I say my SAC rate is .34 cubic feet per minute. My buddy says theirs is .21 cubic feet per minute. This is usable data.

2. I say my SAC rate is 15psi per minute, my buddy says his is 300psi per minute. This is half the equation and alone tells us nothing of our surface air consumption rate without more math. If I have a tank with 145 cubic feet of gas and he has 19 cubic feet tank and we trade cylinders my SAC rate doesn't change but my psi/minute calculation will dramatically.

I'll add another possibility to get back on topic, some people when experiencing gas narcosis report it is relaxing. Perhaps this state of calm perhaps the relaxed body needs less oxygen reducing the need to breath.

Unlikely at 100ft by another option I've experienced is with horrible reg. It simply can not provide the amount of air needed at depth due to poor design engineering, the increased gas density results in less gas flow. In this situation the reg is "overbreathed" and it is an uncomfortable experience which forces reduced SAC rates at depth... Theoretically that could be a factor, with really poor equipment. I've only seen it in modern toy grade scuba gear (60$ retail reg set circa 2015).

Cameron