SAC Rates

[Rick Murchison]

I am surprised that it is normally first taught as a psi concept. ...teach it as a volume-consumption concept... simplifies things slightly.

The reason is that it's not really simpler. Remember the audience here. Most OW students are going to begin their diving adventure with the same type and size tank they used in training, and teaching the concept in PSI obviates the necessity of the double conversion to and from CFM to yield something useful in the water - units displayed on the SPG.
As you say, "many students glaze over once you get into the math," so keeping the math to a minimum is often the best way to get them to do any gas consumption calculations.
By the time a diver is ready to use tanks of varying sizes they're ready for the "new" concept of CFM and will do it because now it is simpler..
Rick

 

[AevnsGrandpa]

I am!

I understand the over all formula for SAC and RVM. My question would be what is considered a good SAC? I know it will vary for diver/water temp/vis/depth/etc, but there must be some range that is realitively good to shoot for.

Looking over some of my dives, which are mainly in colder (<75 degree) water and lower vis (<20'), I have a range of around .45 to .67. How's is that for a diver of 2 years and less than 50 dives???

Jeff

 

[MXGratefulDiver]

I am surprised that it is normally first taught as a psi concept. That makes it tank-specific, and more confusing, in my view. I prefer to teach it as a volume-consumption concept, and adjust for pressure at depth. That takes out the issue of the tank size and simplifies things slightly.

And how do you relate that to what your pressure gauge is telling you during the dive? An SPG knows nothing about volume.

It's important to know both ... and how they relate to each other. The math is fairly simple, and as for physics ... well, if Boyle's Law stumps you then perhaps you should find a different recreational activity. It's a central concept to everything you do as a scuba diver.

OTOH - I have yet to meet a student who wasn't able to grasp the concepts ... when I do, I'll consider that I've not done my job properly.

... Bob (Grateful Diver)

 

[MXGratefulDiver]

I am!

Looking over some of my dives, which are mainly in colder (<75 degree) water and lower vis (<20'), I have a range of around .45 to .67. How's is that for a diver of 2 years and less than 50 dives???

Jeff

Actually ... pretty good for those conditions and experience level.

... Bob (Grateful Diver)

 

[triton94949]

I am!

I understand the over all formula for SAC and RVM. My question would be what is considered a good SAC? I know it will vary for diver/water temp/vis/depth/etc, but there must be some range that is realitively good to shoot for.

Looking over some of my dives, which are mainly in colder (<75 degree) water and lower vis (<20'), I have a range of around .45 to .67. How's is that for a diver of 2 years and less than 50 dives???

Jeff

New male divers in cold water tend to start out around 1.0 cu ft per min. You could probably find teaching literature on that.

As you become more efficient, 0.5 to 0.75 is really quite good.

If you graduate to a scooter, then you can get it even lower, in the range of 0.4.

With a good dive computer that gives you average depth per dive, you can recalculate your rate for each dive. Then you would see how it varies, depending on the activity level you did during your particular dive.

The thinner shorter people can sometimes do extremely well due in no small part to their physiology.

 

[Lexy]

Calculating SAC is a lot of work. I'm glad that one of the folks on this post mentioned - http://www.spearfishing.org/bruces_tips/java/sac.html

My dive computer has sampled data every 2 seconds giving me 1000+ data points to run an average depth on..... (Yozers)

So, now how do I know if I'm doing really well on SAC or not? Does someone have a link to a rough chart on where to gauge yourself?

For the record ~ I've logged 27 dives, average temp = 59F, SAC = 0.682.

I am such a number cruncher....

 

[triton94949]

Calculating SAC is a lot of work. I'm glad that one of the folks on this post mentioned - http://www.spearfishing.org/bruces_tips/java/sac.html

My dive computer has sampled data every 2 seconds giving me 1000+ data points to run an average depth on..... (Yozers)

So, now how do I know if I'm doing really well on SAC or not? Does someone have a link to a rough chart on where to gauge yourself?

For the record ~ I've logged 27 dives, average temp = 59F, SAC = 0.682.

I am such a number cruncher....

It all depends on the size of your chest.

Basically anything in the range of 0.5 to 0.75 cu ft per min is great.

 

[Hemlon]

Did anyone else have trouble the the Spearfishing site's SAC calculator? It's not working for me! (or maybe I'm inputting the wrong data) It keeps defaulting to 45 ft for 40 mins...even though that's not what I'm entering!

 

[Hemlon]

Duh...I just figured it out!

 

[JustinW]

I have revived this thread, because its a good one, and parrothead needs answers dang it!!

My take on it is that perhaps the water temperature is affecting how much air you are breathing. I seem to remember that night diving has physiological effects that cause you to have an elevated heartrate and breathe more, darker colder waters should do the same.