Question Definitions for SAC and RMV?

[tursiops]

Curious - with the high dive count/experience you have, what is your RMV on a relaxed, warm water dive?

About 0.4 or a bit less, if relaxed and warm and just moseying around. Jumps up to 0.5-0.6 if I'm working hard.
I've never seen my Teric drop below 12 psi/min with an AL 80, thus 0.31 (like just watching a jawfish for a couple of minutes).

Added: I'm 5 ft 5" and weigh 160 lbs...not svelt! I'm also old.

 

[Belzelbub]

My range is all over the place. Just looking at Subsurface, it goes from 0.42 cu ft / min to a max of around 1.35 cu ft / min. Problem is that I don’t do a lot of leisurely dives. I’m always doing something else. Either spearfishing, or cleaning animal habitats at the aquarium. Not a lot of dives where I don’t have something in my hands.

That 1.35 for example was a short, hard working dive. About 10 minute dive time to a depth of 85’. The pressure line shows greens during descent, and mostly red during ascent. Greens indicate lower air consumption during the dive, reds indicate higher. Makes sense. Descent was fairly easy. Just following a line we had placed on a previous dive. Ascent was hard work. Line was attached to an anchor I wanted to retrieve. So the ascent involved lugging a 22 lb anchor and 25’ of chain up the anchor line. Definitely not typical.

So, a leisurely dive is often in the 0.4 range, while working ratchets it up a bit. Spearfishing is more around 0.6-0.8 or so.

 

[scubadada]

My range is all over the place. Just looking at Subsurface, it goes from 0.42 cu ft / min to a max of around 1.35 cu ft / min. Problem is that I don’t do a lot of leisurely dives. I’m always doing something else. Either spearfishing, or cleaning animal habitats at the aquarium. Not a lot of dives where I don’t have something in my hands.

That 1.35 for example was a short, hard working dive. About 10 minute dive time to a depth of 85’. The pressure line shows greens during descent, and mostly red during ascent. Greens indicate lower air consumption during the dive, reds indicate higher. Makes sense. Descent was fairly easy. Just following a line we had placed on a previous dive. Ascent was hard work. Line was attached to an anchor I wanted to retrieve. So the ascent involved lugging a 22 lb anchor and 25’ of chain up the anchor line. Definitely not typical.

So, a leisurely dive is often in the 0.4 range, while working ratchets it up a bit. Spearfishing is more around 0.6-0.8 or so.

This is a good example of knowing your RMV. @Belzelbub knows his leisure, hunting, and working RMV and can use that for gas planning.

 

[NW Dive Dawg]

OK...... so this may add to the confusion.......(which is not my intention).

Below in italics is a cut and paste direct from the E-Learning portion of the SDI Solo Course that I just took and passed over this last weekend.......

I took out the metric examples and just left in the imperial ones....

What I can say is that regardless of whether PSI or volume is used for the FINAL calculation of SAC or RMV......that if a person uses PSI, they won't pass the SDI course quizzes or final exam.

For the record, I personally prefer to use, and have always used PSI for basic NDL dive planning. But that won't work for the current Solo SDI course. Just say'n.

Calculating SAC & RMV​

The calculations needed to work out how much gas will be used during a particular dive begin with plugging some sort of constant into the gas management plan. That constant is the diver’s SAC rate. SAC stands for surface air consumption and is a measurement in litres or cubic feet (never in units of pressure) that describes how much air a diver breathes every minute while on the surface at rest. The SAC rate for an experienced diver, such as one enrolled in an SDI Specialty, will have leveled out over time and be about the same from season to season. Some things, such as cardio fitness, body mass, and age will affect a diver’s SAC over time, but for all intents and purposes, we can look at it as a constant: it remains the same from day-to-day, week-to-week, month-to-month.

Although SAC rates vary from diver to diver, and depend on things like size, fitness and sex, a great number of divers have SAC rates that are reasonably close to each other. A commonly used default SAC is about 14 litres or half a cubic foot per minute, and we’ll be using this figure in the examples contained later in this chapter.

You will calculate your personal SAC rate during this course. There are several ways to calculate it. Many divers take an average from past dives and rework the figures showing consumption at a depth during a dive to extrapolate what their consumption would have been at one atmosphere on the surface. However, this method has a basic flaw: it actually gives the Surface Respiratory Minute Volume (SRMV) of the dive not the SAC. Without clogging our minds with acronyms let’s just say that the figure one arrives at using this method is not a SAC rate because it has built into it some allowance for things like workload and “mental” stress that are part of an actual dive. These outside factors need to be subtracted to arrive at a true SAC figure. Why? Simply put, if one uses the volume based on a SRMV calculated using consumption on easy dives where everything went well and the diver was in familiar surroundings, the volume calculations for a stressful, hardworking dive will be unrealistically light. Conversely, if the SRMV figure used is based on hard dives where the diver was stressed by things like poor visibility and cold, and swimming against a heavy current, the volume calculations for an easy dive will be unrealistically heavy.

If you have calculated a SAC rate based on gas volumes derived from actual dives without factoring out workload, stressors such as temperature, visibility, current, and related issues, we suggest multiplying it by 60 percent to arrive at a more realistic figure.

A better, more accurate method of calculating SAC is to actually measure how much gas is breathed while on the surface; let’s say sitting watching TV or reading, and using that value as a baseline. Hook up a tank of known volume and take a reading of its starting pressure. Breathe from it for 10 or 20 minutes and measure the difference between the starting and ending pressures. Calculate how many litres or how many cubic feet this pressure drop represents and divide that figure by the number of minutes to find the average consumption per minute, which will be your SAC. Make a note of that and use it in all future gas calculations.

Here are examples using metric and imperial units to help explain the process.

Diver Vickee uses an aluminum 80 cubic foot cylinder for her SAC finding mission. She puts on her mask (for accuracy), pops a regulator into her mouth and watches a show about diving on cable TV. When the show begins, her SPG reads 2800 psi. After 22 minutes, she has breathed the pressure down to 2500 psi. The pressure drop is 300 psi. The conversion of pressure to volume is a little more complex for Vickee than it was for Jack. The formula to convert psi to cubic feet is rV/WP = ft3 per psi (rV = tank’s rated volume; WP = tank working pressure). So in this example 80/3000 = 0.02666 cubic feet per psi (this is called the one psi baseline for the tank). Vickee used 300 psi so to find how many cubic feet that is we now multiply our one psi baseline by the total psi to find the volume of gas consumed. That’s 0.02666 * 300 = 7.99 cubic feet: let’s call that 8 cubic feet. Finally we divide the total volume of gas used by number of minutes to find SAC (8 / 22 = 0.36 cubic feet per minute). Vickee’s SAC is slightly below average.

To calculate how much gas we need on a real dive, we take our SAC rate and work some simple arithmetic with it. The first step is to multiply our SAC by our target depth in atmospheres. (To convert depth to atmospheres divide by 33 and add 1 (imperial)). Once that’s completed, we have to factor in environmental stressors such as temperature, visibility, surface conditions, how well-rested the diver is, etc., and take into consideration the expected workload on the planned dive, such as swimming against a current. To make calculations somewhat simpler, we can collectively refer to the factors for environment, stressors and for workload as the dive factor. A typical dive factor for an easy wetsuit dive in tropical water with good visibility and minimal current is 1.5. A dive in cold water wearing a dry suit, carrying a camera, working with a current and being a little stressed because there are hammerheads in the water, could have a dive factor of 3 or more. Finally, we multiply this figure by our bottom time, which for this exercise is the total elapsed time from leaving the surface to getting back to our safety stop somewhere between six and three metres or 20 and ten feet.

Here is an imperial example:

Solo diver B plans an air dive to a local kelp bed at 80 feet for 20 minutes to take still photographs of sea-life. His SAC is 0.5 cubic feet per minute. His target depth is (80 / 33) + 1 or 3.42 atmospheres. Multiply that by his SAC and we arrive at 1.71 cubic feet per minute. This is going to be a moderately challenging dive because of currents so diver B uses a dive factor of 2 and gets a per minute volume of 3.42 cubic feet. Since he intends to stay for 20 minutes, he multiplies 3.42 by 20 which means his consumption on this dive equals 68.4 cubic feet. Diver B dives with a large volume, low-pressure steel cylinder that has a rated volume of 104 cubic feet. He considers his gas needs and his starting volume compatible.

We could of course work these calculations backwards from the starting volume to find out what volume of gas we are “allowed” to spend on the dive: using the Rule of Thirds for example, we could consume two-thirds of the starting volume on the dive. Once we have the “allowable useable volume” we can use it to calculate the maximum number of minutes for our dive.

If solo diver B starts out with a partial fill that gives him a starting volume of a little more than 95 cubic feet, his allowable usable volume” will be about 64 cubic feet. Since he is using 3.42 cubic feet of gas every minute, his dive can only last for about 18 minutes before he violates the Rule of Thirds.

​

• SAC is a constant and represents gas used at rest on surface.
• RMV is a variable and represents gas needed for specific dive.

 

[tursiops]

OK...... so this may add to the confusion.......(which is not my intention).

Below in italics is a cut and paste direct from the E-Learning portion of the SDI Solo Course that I just took and passed over this last weekend.......

I took out the metric examples and just left in the imperial ones....

What I can say is that regardless of whether PSI or volume is used for the FINAL calculation of SAC or RMV......that if a person uses PSI, they won't pass the SDI course quizzes or final exam.

For the record, I personally prefer to use, and have always used PSI for basic NDL dive planning. But that won't work for the current Solo SDI course. Just say'n.

Calculating SAC & RMV​

The calculations needed to work out how much gas will be used during a particular dive begin with plugging some sort of constant into the gas management plan. That constant is the diver’s SAC rate. SAC stands for surface air consumption and is a measurement in litres or cubic feet (never in units of pressure) that describes how much air a diver breathes every minute while on the surface at rest. The SAC rate for an experienced diver, such as one enrolled in an SDI Specialty, will have leveled out over time and be about the same from season to season. Some things, such as cardio fitness, body mass, and age will affect a diver’s SAC over time, but for all intents and purposes, we can look at it as a constant: it remains the same from day-to-day, week-to-week, month-to-month.

Although SAC rates vary from diver to diver, and depend on things like size, fitness and sex, a great number of divers have SAC rates that are reasonably close to each other. A commonly used default SAC is about 14 litres or half a cubic foot per minute, and we’ll be using this figure in the examples contained later in this chapter.

You will calculate your personal SAC rate during this course. There are several ways to calculate it. Many divers take an average from past dives and rework the figures showing consumption at a depth during a dive to extrapolate what their consumption would have been at one atmosphere on the surface. However, this method has a basic flaw: it actually gives the Surface Respiratory Minute Volume (SRMV) of the dive not the SAC. Without clogging our minds with acronyms let’s just say that the figure one arrives at using this method is not a SAC rate because it has built into it some allowance for things like workload and “mental” stress that are part of an actual dive. These outside factors need to be subtracted to arrive at a true SAC figure. Why? Simply put, if one uses the volume based on a SRMV calculated using consumption on easy dives where everything went well and the diver was in familiar surroundings, the volume calculations for a stressful, hardworking dive will be unrealistically light. Conversely, if the SRMV figure used is based on hard dives where the diver was stressed by things like poor visibility and cold, and swimming against a heavy current, the volume calculations for an easy dive will be unrealistically heavy.

If you have calculated a SAC rate based on gas volumes derived from actual dives without factoring out workload, stressors such as temperature, visibility, current, and related issues, we suggest multiplying it by 60 percent to arrive at a more realistic figure.

A better, more accurate method of calculating SAC is to actually measure how much gas is breathed while on the surface; let’s say sitting watching TV or reading, and using that value as a baseline. Hook up a tank of known volume and take a reading of its starting pressure. Breathe from it for 10 or 20 minutes and measure the difference between the starting and ending pressures. Calculate how many litres or how many cubic feet this pressure drop represents and divide that figure by the number of minutes to find the average consumption per minute, which will be your SAC. Make a note of that and use it in all future gas calculations.

Here are examples using metric and imperial units to help explain the process.

Diver Vickee uses an aluminum 80 cubic foot cylinder for her SAC finding mission. She puts on her mask (for accuracy), pops a regulator into her mouth and watches a show about diving on cable TV. When the show begins, her SPG reads 2800 psi. After 22 minutes, she has breathed the pressure down to 2500 psi. The pressure drop is 300 psi. The conversion of pressure to volume is a little more complex for Vickee than it was for Jack. The formula to convert psi to cubic feet is rV/WP = ft3 per psi (rV = tank’s rated volume; WP = tank working pressure). So in this example 80/3000 = 0.02666 cubic feet per psi (this is called the one psi baseline for the tank). Vickee used 300 psi so to find how many cubic feet that is we now multiply our one psi baseline by the total psi to find the volume of gas consumed. That’s 0.02666 * 300 = 7.99 cubic feet: let’s call that 8 cubic feet. Finally we divide the total volume of gas used by number of minutes to find SAC (8 / 22 = 0.36 cubic feet per minute). Vickee’s SAC is slightly below average.

To calculate how much gas we need on a real dive, we take our SAC rate and work some simple arithmetic with it. The first step is to multiply our SAC by our target depth in atmospheres. (To convert depth to atmospheres divide by 33 and add 1 (imperial)). Once that’s completed, we have to factor in environmental stressors such as temperature, visibility, surface conditions, how well-rested the diver is, etc., and take into consideration the expected workload on the planned dive, such as swimming against a current. To make calculations somewhat simpler, we can collectively refer to the factors for environment, stressors and for workload as the dive factor. A typical dive factor for an easy wetsuit dive in tropical water with good visibility and minimal current is 1.5. A dive in cold water wearing a dry suit, carrying a camera, working with a current and being a little stressed because there are hammerheads in the water, could have a dive factor of 3 or more. Finally, we multiply this figure by our bottom time, which for this exercise is the total elapsed time from leaving the surface to getting back to our safety stop somewhere between six and three metres or 20 and ten feet.

Here is an imperial example:

Solo diver B plans an air dive to a local kelp bed at 80 feet for 20 minutes to take still photographs of sea-life. His SAC is 0.5 cubic feet per minute. His target depth is (80 / 33) + 1 or 3.42 atmospheres. Multiply that by his SAC and we arrive at 1.71 cubic feet per minute. This is going to be a moderately challenging dive because of currents so diver B uses a dive factor of 2 and gets a per minute volume of 3.42 cubic feet. Since he intends to stay for 20 minutes, he multiplies 3.42 by 20 which means his consumption on this dive equals 68.4 cubic feet. Diver B dives with a large volume, low-pressure steel cylinder that has a rated volume of 104 cubic feet. He considers his gas needs and his starting volume compatible.

We could of course work these calculations backwards from the starting volume to find out what volume of gas we are “allowed” to spend on the dive: using the Rule of Thirds for example, we could consume two-thirds of the starting volume on the dive. Once we have the “allowable useable volume” we can use it to calculate the maximum number of minutes for our dive.

If solo diver B starts out with a partial fill that gives him a starting volume of a little more than 95 cubic feet, his allowable usable volume” will be about 64 cubic feet. Since he is using 3.42 cubic feet of gas every minute, his dive can only last for about 18 minutes before he violates the Rule of Thirds.

​

• SAC is a constant and represents gas used at rest on surface.
• RMV is a variable and represents gas needed for specific dive.

Prior to Steve Lewis (now with RAID) writing the SDI Solo Manual, the manual for the SDI Solo class was "Solo Diving," by Robert von Maier, published in 2002. SAC was clearly defined as psi/minute. No "dive actors" and no volume considerations. If you used a different sized tank, you were told to recalculate your SAC. The HUGE advantage of psi/minute is because your SPG reads in psi, so there is no math, no complication, no confusion in seeing how many minutes you have left. You don't even have to know what size tank you have on your back, so long as the SAC you are using is for that size tank.

 

[tursiops]

Prior to Steve Lewis (now with RAID) writing the SDI Solo Manual, the manual for the SDI Solo class was "Solo Diving," by Robert von Maier, published in 2002. SAC was clearly defined as psi/minute. No "dive actors" and no volume considerations. If you used a different sized tank, you were told to recalculate your SAC. The HUGE advantage of psi/minute is because your SPG reads in psi, so there is no math, no complication, no confusion in seeing how many minutes you have left. You don't even have to now wht size tank you have on your back, so long as the SAC youa re using is for that size tank.

By the way, at Air Management and the Importance of Learning Surface Air Consumption Rate (SAC) - International Training - SDI | TDI | ERDI | PFI TDI/SDI defines SAC as pressure/minute.....

"The SAC formula helps a diver understand, on an average basis, what their air consumption will be at a given depth. This allows the diver to better plan how long their dive can be, giving two points for dive turn times: no decompression times and available air. The SAC formula is: SAC = (PSI at depth used/time) x 33/(depth+33)). To perform this in metric units, use bar instead of PSI and 10 instead of 33."​

 

[tursiops]

DAN uses SAC for pressure/min and RMV for volume/minute.

Estimating Your Air Consumption

Do you have enough breathing gas to complete the next dive? Here's how to find out.

dan.org

 

[NW Dive Dawg]

Prior to Steve Lewis (now with RAID) writing the SDI Solo Manual, the manual for the SDI Solo class was "Solo Diving," by Robert von Maier, published in 2002. SAC was clearly defined as psi/minute. No "dive actors" and no volume considerations. If you used a different sized tank, you were told to recalculate your SAC. The HUGE advantage of psi/minute is because your SPG reads in psi, so there is no math, no complication, no confusion in seeing how many minutes you have left. You don't even have to know what size tank you have on your back, so long as the SAC you are using is for that size tank.

By the way, at Air Management and the Importance of Learning Surface Air Consumption Rate (SAC) - International Training - SDI | TDI | ERDI | PFI TDI/SDI defines SAC as pressure/minute.....

"The SAC formula helps a diver understand, on an average basis, what their air consumption will be at a given depth. This allows the diver to better plan how long their dive can be, giving two points for dive turn times: no decompression times and available air. The SAC formula is: SAC = (PSI at depth used/time) x 33/(depth+33)). To perform this in metric units, use bar instead of PSI and 10 instead of 33."​

Dude......I am totally in agreement with you on this. Just say'n......that if you want to pass the current SDI / TDI Solo course, your calcs need to be in volume or you will not pass the course. I didn't write the course or make the requirements.

Maybe there is an active SDI / TDI instructor here who currently teaches the SDI Solo course that can comment on the validity of the cut and paste info I provided above.

 

[tursiops]

Dude......I am totally in agreement with you on this. Just say'n......that if you want to pass the current SDI / TDI Solo course, your calcs need to be in volume or you will not pass the course. I didn't write the course or make the requirements.

Maybe there is an active SDI / TDI instructor here who currently teaches the SDI Solo course that can comment on the validity of the cut and paste info I provided above.

See Info - Published Standards for SSI, SDI, and PADI "solo" courses. there is nothing in the SDI standards that requires you to use any particular definition of SAC or RMV. What you claim is a requirement is not. Yes, you DO need to be able to plan your dive based on your air consumption.

 

[NW Dive Dawg]

See Info - Published Standards for SSI, SDI, and PADI "solo" courses. there is nothing in the SDI standards that requires you to use any particular definition of SAC or RMV. What you claim is a requirement is not. Yes, you DO need to be able to plan your dive based on your air consumption.

Whatever........ Agree with it or not, the simple fact is that the SDI Solo Diver course that I just took teaches volume (not pressure) for gas consumption calcs.....

On the course chapter tests and final exam, answers based on volume are required if you want to correctly answer the questions related to gas calculations for this course.

Here's an example right from the SDI course materials. ( I bolded the important part)

SAC stands for surface air consumption and is a measurement in litres or cubic feet (never in units of pressure) that describes how much air a diver breathes every minute while on the surface at rest.