Question Determining SAC Rate

[chrisbraypi]

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.

Thank you, @inquis . 33ft=1ATM, so 35ft I'm technically in the 2nd ATM. Keeping numbers rounded, the calculation is:

P1-P2=750
750/10=75
75/2=37.5 psi/min SAC Rate

At that rate, I should get 80min out of an 80cf, 3,000psi tank. Now that's what I normally experience it off a tank.

Question 2 of the SAC test is to use my SAC Rate to determine gas requirements for a planned dive. My planned dive is 45 min at an average depth of 45 ft. What is that equation? (Have I said that I hate math yet? )

SAC x Duration = 1,687.5psi

Is the calculation that simple? Is all in trying to achieve is making sure that my dive plan accounts for leaving enough gas in my reserve?

Now, what about my 13cf pony? Shouldn't I do a 10min rest so determine SAC for that size tank?

 

[Alekseolsen]

Thank you, @inquis . 33ft=1ATM, so 35ft I'm technically in the 2nd ATM. Keeping numbers rounded, the calculation is:

P1-P2=750
750/10=75
75/2=37.5 psi/min SAC Rate

At that rate, I should get 80min out of an 80cf, 3,000psi tank. Now that's what I normally experience it off a tank.

Question 2 of the SAC test is to use my SAC Rate to determine gas requirements for a planned dive. My planned dive is 45 min at an average depth of 45 ft. What is that equation? (Have I said that I hate math yet? )

SAC x Duration = 1,687.5psi

Is the calculation that simple? Is all in trying to achieve is making sure that my dive plan accounts for leaving enough gas in my reserve?

Now, what about my 13cf pony? Shouldn't I do a 10min rest so determine SAC for that size tank?

For what I would expect you to utilise the Pony for (air supply of your main tank is not accessible in an emergency) I doubt "at rest" would be the breathing rate you would need for the pony.

 

[gbirch]

Thank you, @inquis . 33ft=1ATM, so 35ft I'm technically in the 2nd ATM. Keeping numbers rounded, the calculation is:

I think that you may have misunderstood this point, it is not that you are "technically in the second atmosphere", at 35ft you have 35/33=1.06 ATM of water pressure PLUS 1 ATM of atmospheric pressure giving you 2.06 ATA (Atmospheres Absolute). A lot of divers forget to add or subtract that pesky atmosphere when doing this sort of calculation.

So, for example, if you dive at 45ft you are at (45/33)+1 = 2.36 ATA, so you will consume 2.36 times as much gas at this depth as you would at the surface so your gas consumption at this depth would be 2.36xSAC.

To answer your pony question, we need to first talk terminology. Unfortunately, there is no standard terminology used across the industry, so I will use 2 terms:
SAC = (Surface Air Consumption) the amount of gas that you breathe in a minute on the surface measured in psi/min
RMV = (Respiratory Minute Volume) the amount of gas that you breathe in a minute on the surface measured in cuft/min

It may sound like splitting hairs, but the difference is important because your SAC rate will vary depending on the size of the cylinder: you will breathe down your 13cuft pony (psi/min) way faster than you breathe down your 80cuft cylinder (psi/min) even though you are breathing at the same rate from each (cuft/min).
This is why, once you have your SAC in psi/min, you convert it to RMV by multiplying by the cylinder capacity and dividing by the working pressure of the cylinder that you used for your test swim. RMV is portable across all cylinders, SAC is specific to a cylinder. Using this, you can now answer you 2 questions:

Use my SAC Rate to determine gas requirements for a planned dive. My planned dive is 45 min at an average depth of 45 ft
Let's assume that your SAC rate for your 80cuft cylinder is 37.5psi/min, then you RMV is:
37.5 * 80 / 3000 = 1 cuft/min

On a 45ft dive ((45/33)+1) = 2.36 ATA) you will consume 1 * 2.36 cuft / min, or 2.36*45 = 106 cuft - OOPS, your 80 cuft cylinder will not be big enough.

For reference, at 45 ft (2.36 ATA), you will breathe the entire contents of your pony bottle in 13/(2.36x1) = 5 min, given that your RMV will jump when under stress (which is the time that you need your pony), you may want to reconsider if 13cuft gives you the safety that you are hoping for.

Last question "Now, what about my 13cf pony? Shouldn't I do a 10min rest so determine SAC for that size tank?"
No need to do this as your RMV is the same regardless of cylinder.

If all this is confusing, I apologize - but at least you will have plenty of questions to ask your instructor in your class! - Good luck with the class.

 

[Pavao]

I think that you may have misunderstood this point, it is not that you are "technically in the second atmosphere", at 35ft you have 35/33=1.06 ATM of water pressure PLUS 1 ATM of atmospheric pressure giving you 2.06 ATA (Atmospheres Absolute). A lot of divers forget to add or subtract that pesky atmosphere when doing this sort of calculation.

So, for example, if you dive at 45ft you are at (45/33)+1 = 2.36 ATA, so you will consume 2.36 times as much gas at this depth as you would at the surface so your gas consumption at this depth would be 2.36xSAC.

To answer your pony question, we need to first talk terminology. Unfortunately, there is no standard terminology used across the industry, so I will use 2 terms:
SAC = (Surface Air Consumption) the amount of gas that you breathe in a minute on the surface measured in psi/min
RMV = (Respiratory Minute Volume) the amount of gas that you breathe in a minute on the surface measured in cuft/min

It may sound like splitting hairs, but the difference is important because your SAC rate will vary depending on the size of the cylinder: you will breathe down your 13cuft pony (psi/min) way faster than you breathe down your 80cuft cylinder (psi/min) even though you are breathing at the same rate from each (cuft/min).
This is why, once you have your SAC in psi/min, you convert it to RMV by multiplying by the cylinder capacity and dividing by the working pressure of the cylinder that you used for your test swim. RMV is portable across all cylinders, SAC is specific to a cylinder. Using this, you can now answer you 2 questions:

Use my SAC Rate to determine gas requirements for a planned dive. My planned dive is 45 min at an average depth of 45 ft
Let's assume that your SAC rate for your 80cuft cylinder is 37.5psi/min, then you RMV is:
37.5 * 80 / 3000 = 1 cuft/min

On a 45ft dive ((45/33)+1) = 2.36 ATA) you will consume 1 * 2.36 cuft / min, or 2.36*45 = 106 cuft - OOPS, your 80 cuft cylinder will not be big enough.

For reference, at 45 ft (2.36 ATA), you will breathe the entire contents of your pony bottle in 13/(2.36x1) = 5 min, given that your RMV will jump when under stress (which is the time that you need your pony), you may want to reconsider if 13cuft gives you the safety that you are hoping for.

Last question "Now, what about my 13cf pony? Shouldn't I do a 10min rest so determine SAC for that size tank?"
No need to do this as your RMV is the same regardless of cylinder.

If all this is confusing, I apologize - but at least you will have plenty of questions to ask your instructor in your class! - Good luck with the class.

Argh, I don't understand why so many insist in making a distinction between what you define surface consumption and consumption at depth, makes no sense to me to talk psi/min at surface but talk cf3/min at depth.

At 3.00ATA you use cf3/min
At 2.65ATA you use cf3/min
At 2.27ATA you use cf3/min
At 2.00ATA you use cf3/min
At 1.70ATA you use cf3/min
At 1.39ATA you use cf3/min
At 1.21ATA you use cf3/min
At 1.03ATA you use cf3/min
At 1.00ATA you use psi/min? Why? Why? Why?

All you need to know is volume/min, whether imperial or metric, all you need is volume/min at 1ATA, this is your consumption no matter what cylinder you are using, from here, you input the variables.
Once you do the calculations to figure out what your consumption rate is, that is your number, that's the only one you need to remember, as it's volume/min, you won't need to calculate it every time you change cylinders, tank size is just your variable like depth.

Btw, you (OP) should do a consumption rate calculation often, in varying conditions to get a more realistic number, as it varies sometimes from dive to dive.

 

[chrisbraypi]

I think that you may have misunderstood this point, it is not that you are "technically in the second atmosphere", at 35ft you have 35/33=1.06 ATM of water pressure PLUS 1 ATM of atmospheric pressure giving you 2.06 ATA (Atmospheres Absolute). A lot of divers forget to add or subtract that pesky atmosphere when doing this sort of calculation.

So, for example, if you dive at 45ft you are at (45/33)+1 = 2.36 ATA, so you will consume 2.36 times as much gas at this depth as you would at the surface so your gas consumption at this depth would be 2.36xSAC.

To answer your pony question, we need to first talk terminology. Unfortunately, there is no standard terminology used across the industry, so I will use 2 terms:
SAC = (Surface Air Consumption) the amount of gas that you breathe in a minute on the surface measured in psi/min
RMV = (Respiratory Minute Volume) the amount of gas that you breathe in a minute on the surface measured in cuft/min

It may sound like splitting hairs, but the difference is important because your SAC rate will vary depending on the size of the cylinder: you will breathe down your 13cuft pony (psi/min) way faster than you breathe down your 80cuft cylinder (psi/min) even though you are breathing at the same rate from each (cuft/min).
This is why, once you have your SAC in psi/min, you convert it to RMV by multiplying by the cylinder capacity and dividing by the working pressure of the cylinder that you used for your test swim. RMV is portable across all cylinders, SAC is specific to a cylinder. Using this, you can now answer you 2 questions:

Use my SAC Rate to determine gas requirements for a planned dive. My planned dive is 45 min at an average depth of 45 ft
Let's assume that your SAC rate for your 80cuft cylinder is 37.5psi/min, then you RMV is:
37.5 * 80 / 3000 = 1 cuft/min

On a 45ft dive ((45/33)+1) = 2.36 ATA) you will consume 1 * 2.36 cuft / min, or 2.36*45 = 106 cuft - OOPS, your 80 cuft cylinder will not be big enough.

For reference, at 45 ft (2.36 ATA), you will breathe the entire contents of your pony bottle in 13/(2.36x1) = 5 min, given that your RMV will jump when under stress (which is the time that you need your pony), you may want to reconsider if 13cuft gives you the safety that you are hoping for.

Last question "Now, what about my 13cf pony? Shouldn't I do a 10min rest so determine SAC for that size tank?"
No need to do this as your RMV is the same regardless of cylinder.

If all this is confusing, I apologize - but at least you will have plenty of questions to ask your instructor in your class! - Good luck with the class.

Not confusing. I follow you. What you're saying is that instead of doing a separate SAC calculation for my pony bottle, convert the SAC to RMV to give me my expected air time for that cylinder.

5min is more than enough time for my diving. Truth be told, my recreational diving consists of shore dives here in the warm waters of the carribean. I do not have interest in deep dives, as I prefer the 30-45ft depths. The light is better for my photography, the sea life is awesome.

Honestly, the Self Reliant choice isn't as much about "solo diving" as it is about "I don't trust my buddy 100%". My wife is not a diver, so every time I dive, its with someone I don't know - usually someone I pair up with on the dive shop's buddy board. I don't know their skill level and I certainly don't trust their ability to react correctly in a stress situation. Hence the desire for this certification. If I have an issue at 45ft, I want to have the confidence to know it's up to me to save myself. I just think it makes be a safer, better diver.

Classes have been going great. Swapping to alternate air, swapping makes, deploying DSMBs, improving navigation skills - have all gone well.

The only hiccup has been the math, which is a challenge for a guy with 1.5 years of General Math in an Oklahoma high school ;-) but I understand the SAC/RMV theory.

Thanks very much for the replies.

 

[inquis]

Now, what about my 13cf pony? Shouldn't I do a 10min rest so determine SAC for that size tank?

You could, but it's unnecessary since you can convert the other rate to a volume rate, which works with any tank.

With all these conversation calculations, you're just trying to cancel units. For your AL80, you calculated that you would have used 36 psi/min on the surface. That tank is characterized by a factor of 80 cf per 3000 psi or (dividing 80 by 3000), 0.027 cf/psi. Those are fractions with the units, and you're trying to get rid of the psi units during the conversion. If you multiply psi/min by cf/psi, the psi cancels, leaving cf/min. In your case, 36 psi/min * 0.027 cf/psi = 1.0 cf/min. This volume-based rate is the one to remember, since it is specific to you.

(As others mentioned, the volume-based number may be called RMV, a term used in the medical industry, but that's definitely NOT universal. Search for that here if you've got time to kill...)

For duration from any tank, just work off it's volume. For instance, a full AL80 holds 77 cf. (Why not 80? It's complicated, so just go with it.) 77 cf divided by your rate of 1.0 cf/min cancels the cf unit and flips the min unit up into the numerator. So that tank lasts 77 minutes on the surface.

At 33 ft depth, though, the pressure is the original/surface pressure of 1 atm plus the water contribution (depth in ft/33) or a total of 2 atm. It takes twice as many molecules to expand your lungs in each breath. That means that tank will only last 77/2 or 38 mins at that breathing rate at depth.

(Why the discrepancy from your typical duration? Depth differences and perhaps mental stress during your test.)

I'll let you do the 13 cf tank, but I will mention if you have to use your pony, you will probably be stressed, with an increase in breathing rate. It's common to assume a 50% increase (or a factor of 1.5x) over your normal rate. Based on your test, that's 1.0 cf/min * 1.5 = 1.5 cf/min at the surface. You'll be ascending, so depth (and pressure and therefore gas consumption rate) changes, so we use the average depth: the average of 45 and 0 is (45+0)/2 or 22.5 ft (a pressure of 1.68 atm (=22.5/33 + 1). The time depends on the ascent speed, but 30 ft/min yields 1.5 min.

It's also common to add in enough gas to get sorted at depth, say 1 minute. Oh, and gas for the safety stop (3 min at 15 ft).

If you work through all this, I think you'll find that 13 cf pony won't cut it for a (stressed) surface consumption rate of 1.5 cf/min. Now, it could be that your stressed rate is closer to 1.0 (i.e., you were already stressed during your test) and your normal is around 0.7 cf/min (since you normally get more than 38 mins from an AL80). But I'm sure you'd agree that it's best to figure that out before an actual incident.

Alternatively, get a bigger pony until you get a better handle on this. From numerous past threads, my impression is a 19 cf pony is the most common choice.

Sorry this has gone so long... Hopefully, you'll find it useful!

 

[chrisbraypi]

You could, but it's unnecessary since you can convert the other rate to a volume rate, which works with any tank.

With all these conversation calculations, you're just trying to cancel units. For your AL80, you calculated that you would have used 36 psi/min on the surface. That tank is characterized by a factor of 80 cf per 3000 psi or (dividing 80 by 3000), 0.027 cf/psi. Those are fractions with the units, and you're trying to get rid of the psi units during the conversion. If you multiply psi/min by cf/psi, the psi cancels, leaving cf/min. In your case, 36 psi/min * 0.027 cf/psi = 1.0 cf/min. This volume-based rate is the one to remember, since it is specific to you.

(As others mentioned, the volume-based number may be called RMV, a term used in the medical industry, but that's definitely NOT universal. Search for that here if you've got time to kill...)

For duration from any tank, just work off it's volume. For instance, a full AL80 holds 77 cf. (Why not 80? It's complicated, so just go with it.) 77 cf divided by your rate of 1.0 cf/min cancels the cf unit and flips the min unit up into the numerator. So that tank lasts 77 minutes on the surface.

At 33 ft depth, though, the pressure is the original/surface pressure of 1 atm plus the water contribution (depth in ft/33) or a total of 2 atm. It takes twice as many molecules to expand your lungs in each breath. That means that tank will only last 77/2 or 38 mins at that breathing rate at depth.

(Why the discrepancy from your typical duration? Depth differences and perhaps mental stress during your test.)

I'll let you do the 13 cf tank, but I will mention if you have to use your pony, you will probably be stressed, with an increase in breathing rate. It's common to assume a 50% increase (or a factor of 1.5x) over your normal rate. Based on your test, that's 1.0 cf/min * 1.5 = 1.5 cf/min at the surface. You'll be ascending, so depth (and pressure and therefore gas consumption rate) changes, so we use the average depth: the average of 45 and 0 is (45+0)/2 or 22.5 ft (a pressure of 1.68 atm (=22.5/33 + 1). The time depends on the ascent speed, but 30 ft/min yields 1.5 min.

It's also common to add in enough gas to get sorted at depth, say 1 minute. Oh, and gas for the safety stop (3 min at 15 ft).

If you work through all this, I think you'll find that 13 cf pony won't cut it for a (stressed) surface consumption rate of 1.5 cf/min. Now, it could be that your stressed rate is closer to 1.0 (i.e., you were already stressed during your test) and your normal is around 0.7 cf/min (since you normally get more than 38 mins from an AL80). But I'm sure you'd agree that it's best to figure that out before an actual incident.

Alternatively, get a bigger pony until you get a better handle on this. From numerous past threads, my impression is a 19 cf pony is the most common choice.

Sorry this has gone so long... Hopefully, you'll find it useful!

Extremely useful. Thank you.

 

[scubadada]

...(As others mentioned, the volume-based number may be called RMV, a term used in the medical industry, but that's definitely NOT universal. Search for that here if you've got time to kill...)...

Hi @chrisbraypi

Question - Definitions for SAC and RMV?

This topic comes up over and over. Let's see what you think. Please vote once for SAC and once for RMV or vote for #5 and elaborate in your post or #6 if you don't know what this is all about.

scubaboard.com

SAC in psi/min/atm X tank factor in cu ft/psi = RMV in cu ft/min/atm
RMV can be used for your gas planning and is cylinder independent

You may also like

Average Gas Consumption

There has been extensive discussion lately regarding average gas consumption. A poll may help give us a better idea of what the normal distribution of RMVs is. 1 cu ft = 28.32 liters 1 liter = 0.035 cu ft 28.3 l/min

scubaboard.com

 

[gbirch]

Argh, I don't understand why so many insist in making a distinction between what you define surface consumption and consumption at depth

Not making that distinction at all.

 

[boulderjohn]

Just a couple of observations.

I have had a couple students begin classes telling me that they knew their SAC rates because they were calculated by their air integrated dive computers. In each case, they reported rates better than I could do. Then we dived, and neither had rates as good as mine. I offer no explanation--just an observation.

I found the most useful way to get an idea of your SAC rate was simply to do some dives, noting the total time and amount of gas used. If you have the average depth on the computer, you can do the rest easily. That will tell you the actual rate you use while diving rather than an artificial done while doing a contrived activity. After a few dives, you should have it down pretty accurately.

Different people use the term SAC and RMV differently. I was taught it differently when I was a student in different classes from different agencies. I don't worry about it. I can tell from the context of the sentence what someone means.