Question Determining SAC Rate

[chrisbraypi]

Hello, All:

Planning on going for my Self Reliant Certification next week in Curaçao and my instructor has sent reading material for review. I understand the mathematics of calculating SAC rate. But I'm confused about the  method of determining my SAC Rate.

As I understand it, establishing my SAC Rate is achieved by using known data and then plugging that data into the SAC Rate equation. That "known data" comes from 1 of 2 places:

(1) Data from a prior dive, using data stored in my dive computer/logbook

(2) A 60ft/10min "swim" in all dive gear.

If we're determining SAC Rate using method #2, my question is about this reference to a "swim" in full gear. Are we talking about a surface swim - or are we talking about swimming at depth?

Are we talking about gearing up, entering the water and kicking on the surface for 10min while breathing through the regulator?

Thanks to all that care to to respond.

 

[sea_ledford]

Flowstate Divers actually made a very good video on this:

I really like their videos, short and sweet without the 15 minute intro. I can't figure out if the fact the narration sounds like the Diary of a Sad Cat guy (Ze Frank) makes it better or worse though.

 

[tursiops]

By "Swim" is meant a normal dive, at depth. You want to know your SAC rate for a normal dive...not a surface swim.

 

[gbirch]

There are probably more than one way to do this, but here is what I do:
1. Descend to a known depth and shoot a DSMB (just for reference to help maintain a constant depth)
2. Record cylinder pressure
3. Swim at constant depth for a know period of time ( e.g. 10 min)
4. Stop and record end cylinder pressure

Now you can calculate your air consumption rate at that depth ((P1-P2)/duration), divide it by the depth ( in ATA) and you convert that to SAC in psi/ min.
Multiply that by the capacity of your cylinder (in cuft) and then divide by the working pressure of your cylinder and you have RMV in cuft/min.

This, of course, assumes that you are using the imperial “system”, if you are using SI then the calculations are way easier

 

[happy-diver]

I really would jot down a dozen specific questions that require only brief answers
and contact - phone your instructor again to see how they expect you to do stuff

 

[clownfishsydney]

Download a previous dive into Subsurface, add in cylinder size and start and end pressures. Gives you the result! Easy.

 

[Jcp2]

Download a previous dive into Subsurface, add in cylinder size and start and end pressures. Gives you the result! Easy.

If you have an air integrated computer then all you need is cylinder size, and you can get SAC rate at any point in the dive.

 

[chrisbraypi]

There are probably more than one way to do this, but here is what I do:
1. Descend to a known depth and shoot a DSMB (just for reference to help maintain a constant depth)
2. Record cylinder pressure
3. Swim at constant depth for a know period of time ( e.g. 10 min)
4. Stop and record end cylinder pressure

Now you can calculate your air consumption rate at that depth ((P1-P2)/duration), divide it by the depth ( in ATA) and you convert that to SAC in psi/ min.
Multiply that by the capacity of your cylinder (in cuft) and then divide by the working pressure of your cylinder and you have RMV in cuft/min.

This, of course, assumes that you are using the imperial “system”, if you are using SI then the calculations are way easier

Thank you @gbirch. So here's my data from my first dive yesterday. Don't laugh. I usually get 60-70 min out of an 80cf tank.

P1: 2500
P2: 1750
Duration: 10 min @ 35ft (1.05 ATM)

(P1)-(P2) = 750
750/ATA = 75
75/1 = 75psi/min

An 80cf tank holding 3000 PSI gives me 40min of air.

 

[inquis]

Duration: 10 min @ 35ft (1.05 ATM)

An error here: it's 2.05 atm at 35 ft (assuming sea level). Your surface consumption rate is 36 psi/min (AL80) or 1.0 cuft/min (any cylinder). I expect a lower rate is more realistic if you were, in fact, towing a DSMB during this test (as suggested in the post you quoted). It's almost a certainty that you will improve with more dives as well.

 

[FishWatcher747]

what computer do you use?