How to calculate air consumption for scuba diving - SAC vs RMV
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- เผยแพร่เมื่อ 4 ส.ค. 2024
- There’s a lot of confusion on this topic when you look around on the internet, so hopefully this will clarify how to get the numbers you need for air consumption.
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SAC and RMV are used interchangeably in a lot of the materials that you’ll find online. Here I talk about the different ways they are calculated, what’s the most appropriate use for them, and how to improve your air consumption while scuba diving. If you have further questions about SAC and RMV, leave them in the comments!
0:00 Introduction
0:29 SAC in pressure/minute
2:00 Absolute pressure calculation
2:19 SAC calculation
3:00 SAC in volume/min
4:07 RMV
4:53 Gas consumption run
6:32 RMV calculations
8:34 How tech divers use RMV
9:19 Strategies to improve air consumption
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How do you calculate SAC and RMV? Let's be nice here! 🤭
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Second question, if your SAC rate changes between different sizes of cylinders, why is knowing your SAC rate important?
@@tanabobbi knowing it in L or cu ft/min is helpful. I talk about pressure/min because a lot of recreational divers like to measure it that way.
This video is pretty freaking awesome. Thanks a bunch for sharing your knowledge. I just got certified 60 feet open water with padi last month
That is awesome! Congratulations!
Interesting video Sarah, thank you. 👌I was taught to calc SAC as per the RMV calculation, glad I'm in the UK and we are metric !
Yes, totally good! It’s just about understanding the units right? Metric is always the way to go😂
I'm a giant nerd so I created a dive plan spreadsheet that takes gas use and cylinder size from the last dive to estimate a dive plan for the next dive. It uses rmv because that make the most sense to me(recreational diver guy who is also in aerospace) I have never seen any other recreational diver do that kind of planning though.
We love a nerdy diver! You're the kind of dive buddy we all want :)
Thanks for sharing this information. Safe diving
Thanks for watching!
Thank you for this…..glad to see you are doing well
Thank YOU for watching. Really appreciate you.
Thanks Sarah. As a recreational diver this was very helpful.
Thanks for watching!
Thanks for the fun refresh. I was taught to dive with imperial, but switched to metric after a trip in Asia, and grateful for it when having to make calculations like this 🙂
for real though. we all need to make the switch :)
@@AzulUnlimited Agreed. Metric IS the way forward...
Very cool! I have a computer that computes my SAC rate and RMV once I tell it the tank volume/size. It's fun to see how temps and activity affect my rates between dives. I'm pretty consistent but cold water and a lot of depth changes swimming around terrain negatively affect my rates. On my new computer, it gives me real time SAC rates and it's really neat to see how breathing and/or exertion affects it. If I relax and try to breath more slowly it definitely goes down in a minute or less. It's a great way to work on your personal consumption with realtime feedback on how your behavior affects SAC rate.
That's super cool you have that info available during the dive. I gotta upgrade my computer one day 🤣
Amazing video
Thank you!
Great topic, and so important for a lot of divers to learn about.
Air consumption is sooo depending on things, like workload, stress, air temperature, condition, comfort, trim, and so many others.
Every dive I did I always analyse the dive and gas consumption, I just put some numbers in an Excel sheet,
these numbers are average depth (it's logged on almost all divecomputers) , pressure in, pressure out, dive time, and tank volume(s), that’s all you need to analyse the dive.
The outcome of this formula is average (surface) liters per minute during the dive.
Then I can add for myself things like my workload, water condition, and other things to see either I was relaxed and my consumption was high or low.
I learn from these calculations, and after so many dives I have a clear picture about my average gas consumption, and that number I use doing calculations for my tecnical dives where I need to do decompression, gas-swithes and more.
RMV and SAC can be confusing things If a person is not used to calculate or plan a more complex dive., for me it is like:
I always calculate in liters per minute, I always calculate in metric values, no Imperial for me (gives me a headache)
Calculating with liters always works, and it works on every tank you are using, or a multiple of tanks.
Absolutely. That's an awesome way to track yourself!
Great summary. I was taught and have always used SAC defined in litres/min (Go Metric! ...sums are so easy). But I only use it at the planning stage and for post dive analysis. My Suunto AI computers also do it this way and give you a useful graph when you look at it on the PC. That means you don't need to do specific SAC test dives; you can just look at the graph and see instantly what your average SAC was when stressed/chilled/battling current/safety stop etc. The other useful number I like to have is air consumption rate in Bar/min at depth. Working from my personal SAC data I've made up a simple table for my dive folder so I can instantly see that if I'm going to be at 20m in a drysuit on backmount twin 12s I'll expect to use 2.5Bar/min, or at 10m chillin' on a tropical reef with an Ali S63 it'll be 3Bar/min. That's the number I need when I'm deciding whether to have another 10 minutes or it's time to start back.
Love it! You sound like the kind of buddy we all wish to have!!
@@AzulUnlimited I also bring choclolate brownies 😂
@@timgosling6189 yep, we're diving together one day 🤣
@@AzulUnlimited Now that would be something to look forward to 🙂
Haha!!! If I have an extra $10,000 laying around, I’m going to wherever you are and diving!!!
When and where?? hahaha
hahaha done deal!
Thanks 😊
You’re very welcome!
If you are really obsessed about your SAC rate, just buy air integrated dive computer and you'll get SAC calculated automatically for every dive. There's even a graph showing the SAC rate variations during the dive.
Very true!! Some of us don’t have the funds for that just yet😉
Innerspace Explorers has somefairly decent videos on calculating "minimum gas" which works through these calculations and shows you what they can be used for in a very simple way. Definitely worth checking out.
Too many people just plan to dive until they run out of gas and that's pretty bad if something goes wrong.
100% I'll check out their videos. Thank you!
Awesome video... While I love me some Metric calcs over Imperial, it never sits well when a diver says they were breathing 16.2l per minute on a 12l cylinder for 15 minutes. Mental gymnastics to say the least.
Thank you! Totally agree. Metric is the best
Can you make a video showing how to plan a dive around your RMV?
Thanks for your interest and I appreciate you watching the video! I’ll be honest, I’m really only taking video requests from Patreon members since I have a lot of topics in my queue. I’ll keep it in mind for the future, but it may not get to the top of the list🙏
Biggest thing you missed, don't chase sac rate. It just leads to a build up of co2.
If you want to improve your sac rate get in shape and get your trim right.
But 100% on the air you use is the air you use.
Yes, great point. Thank you so much for watching🤙
Hi Sarah,
I've learned to calculate a SAC rate in LITERS not BAR per minute so in that case it applies for all tank sizes. I mean, if I use e.g. 12l tank 200 bar (2400 liter) still can compare air consumption with my buddy who has a 10l tank also with the same 200 bar (2000 liter). And yes, it is only usefull for afterwards evaluation and not air consumption calculations during the dive (my computer does all the math and I trust it).
Anyway, another great vid BTW (as usual 🙂)
greetz from Holland
Thanks so much! Computers are wonderful things :)
@@AzulUnlimited hmmmm....do I feel a scent of sarcasm...🙂
@@dre64 actually no! computers are truly wonderful things 😆
@@AzulUnlimited yes they are, indeed (unfortunately also tend to crash at always unfortunate possible moment...😞)
BTW. Speaking of SAC/RMV or anything connected to breath control. This was (still is actually) my nightmare. I'm a bit air junky underwater and try to decrease consumption by getting more and more experience. Unfortunately in cold Holland's waters there is only possible (for me of course as I'm only a recreational diver without a dry suit) to dive maybe a half of a year and then on vacation so not that often as I'd like to do. I reduced my SAC/RMV to about 16 l/min but still find it too much. Would like to spend at least an hour under water and get out with enough spare air. I don't give up and work on it!!!
And your videos are sooooo relaxing (and professional the same time) that actually only by listening to your voice my SAC rate drops 🙂(unfortunately cannot play it back under water...)
greetz
@@dre64 Aw thank you! You could just get a bigger tank... just saying ;)
I think we are lucky in that Diving Ireland (CMAS) always taught us in and emphasised volume. Though I can see how perhaps with transmitters it might be easy to fall into the bar per minute thinking.
Totally. It just depends on what you need it for
When you look to a RPG of a computer with transmitters its both the tankpressure you see, you know the size of your tanks, and you know your average gas consumption, its easy to calculate during your dive, and when you made a proper planning before your dive then you also know your turn pressure,
just keep everything on the safe side of your calculations and planning.
When you perhaps use a Shearwater computer, its also calculating tour TTS and your GTR for you
Rmv is the core of gas management in technical diving from total gas needed for a given dive, turn around pressure all the way to contingencies such as loss of deco gas etc.
Absolutely. Very important stuff.
RMV is a fairly recent change, SAC was used for both cuft and psi based calculations. And you will see a lot of older materials and some people like myself that are obstinate using SAC for both. Like in science class, always provide the units. If you don't provide me units I am just going to blow you off.
Great point! Units are key.
Kjel to algebra teacher: this is stupid, when am I ever going to use this? Well you won't, but one of the smart kids might" he replied cheerfully
baaaahahahaa the sass
My Shearwater gives me SAC in PSI/Min. I take the average SAC from ten dives with the same tank size and convert the PSI/Min to CuFt/Min. Maybe a little more work but it seems to give me a fairly consistent number.
Awesome! Shearwater is the best
Sarah, Good video. But....
Im an instructor 37+ years and with 3 agency's. While I think that at advanced levels (Dive Master and above) I think knowing how to calculate the SAC rate and the RMV is important. However, for the basic student and the just recreational diver it is a waste of time. Because the average diver necessarily will never use it. If they understand that the deeper they dive the more air they will be using which means less time in the water, is more important.
What is your thoughts?
thanks
ray
I think people are interested in learning things at all levels, so why not clear up confusion they may have from listening to fellow divers?
Got a few questions. First, when calculating SAC, I understand the equation, but what if you are at multiple depths? Do you take your deepest depth and you overall dive time?
Always use the average depth your computer reports.
I would do a gas consumption run where you do the test at one depth(and take the average since you're bound to go a little bit up and down during the test). You can take the average depth of a dive to get a rough estimate of your SAC but it won't be as accurate.
So you've been reading ScubaBoard huh? ;)
I posted this video in those SAC or RMV topics too.
oh no! The trolls are coming for me 😅
Hey, I'm a new diver, and I need a reminder what number do SPGs go up by? for example does it go up by 100s, or 200s? Im not sure and I need a reminder.
I'm guessing you use PSI. The dial should be clearly marked in 1000s with sub-divisions for 500s and smaller ones for 100s. From 700 to zero they are usually marked in red. If you are diving with a new group, eg on holiday, be sure to check with the dive lead how to signal pressure. They may ask you during the dive and there are several different ways of signing out there. Safe diving!
Thanks for jumping in Tim!
Welcome! Tim got you covered 🤙🏼
How did you actually *calculate* the ATA in your metric - Fresh Water example? You simply stated the ATAfw at 15 meters is 2.46.
15/10.3 then add 1 for atmospheric pressure
Third question, is it just the volume of the tank that is important or does material matter?
For the calculation, just the internal volume of the cylinder(s).
Mike got you there :)
Thanks for jumping in!
I would disagree on cylinder sizes as 84.755 cubic feet, or 2.4 cubic meter, air is what it is no matter cylinder size, and as you explained on how long it will last it depends 😂
👌
will be doing my Tec training in the next few weeks so thank for this quick and simple
Best of luck!
I am just getting into tech diving and that was very useful. Thanks
Glad it was helpful!
Why would you make a difference between fresh and salt water? Are you changing your computer every time? Are you taking depth by putting knots into an SMB line? If not then it makes no sense to take a fresh water value as your computer/depth gauge measures depth by measuring the pressure, so your depth is literally whatever density is programmed into the device and the pressure. Changing the density after the fact is falsifying your results.
Never considered that. Good point.
Many computers have a saltwater vs freshwater option in their settings. So if your computer takes it into consideration when calculating depth, this makes sense.
@@zuchtfisch Yes, but your computer does not have a salinity sensor, so whatever your computer is set to, that's what it uses for the density of the water to compute depth from pressure. Hence, my question whether she changes her computer every time? Unless you consciously switch from fresh to salt water and vice versa in your computer settings, your computer won't know whether you are diving in fresh or salt water. This means that your depth reading is usually wrong as well, but who cares? The only reason for us to care about depth is so that we (or the computer) knows the pressure which is what the computer measures in the first place. BTW, your computer does not take depth into account when calculating anything other than safety/deco stop depths. For everything else it uses pressure. It measures pressure and it uses pressure to compute things. The conversion for depth is just for the user to have a more relatable reading.
@@zuchtfisch Besides, salinity changes depending on the body of water you are in. Salinity in the Pacific is not the same as it is in the Med or Red Sea. Again, your computer does not take this into account unless you explicitly keep setting the salinity to whatever value it is where you are. If you don't, your computer won't know the difference.
@Tobias Hartung that‘s right. I did a lot of my dives in Aqaba and they have a condiderably higher salinity as the rest of the red sea which in turn has already a higher one than most other seas…
But in theory also temperature influences water density and thus the precision of the depth reading. The idea to include factors into the calculation examples might be that usually water pressure is refered to as „depth“. If you calculate with ambient pressure you don’t need any of those, no?
I think your an angle/mermaid if I had the money id buy you a rebreather. lol Jon
Hahaha thanks 🙏