SAC And RMV Made Simple | Deep Dive

SAC And RMV Made Simple | Deep Dive
Now online, depending on where you look some people work out their values in different ways and refer to SAC and RMV differently. This can be confusing because you’ll read one thing on a website and then something similar, but different on another. The important thing is that you understand your breathing rates and how to use them for what you need. I do it the Shearwater way because that’s how my dive computer does it, and I trust Shearwater.
So, if you’ve never heard of SAC Rates or your RMV, or you’ve googled them to find out more, but have just gotten yourself more confused, let's dive a little deeper and talk about SAC and RMV.
Timestamp
00:00 - Hello
01:20 - Sac & RMV
02:54 - Calculating SAC
05:26 - Calculating RMV
06:41 - Using SAC & RMV
10:00 - Factors Affecting SAC & RMV
10:54 - K-Bye
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#deepdive #SACrate #RMVrate #SAC #RMV #scubadiving #scuba #diving #scubaadvice #simplyscuba

[@simplyscuba]

This is how Mark calculates SAC and RMV but others do it differently, if you do, let us know how in the comments below so we can have a full database and how everybody calculates their breathing rates in the comments below.

[@timgosling6189]

Knowing your consumption in Bar/min at your instantaneous depth during a dive is very useful, and I always have a planning figure in the back of my mind as a check. But SAC is widely defined in litres/min (or cuft/min) to make it easily compared between different kit configurations and situations without having to factor in cylinder capacity. So to adjust Bar/min for depth and call it SAC is not universally accepted. Also RMV is actually a medical quantity used on land, particuarly to measure lung efficiency, that has been adopted by some in the scuba community. To factor it for depth is OK but what you are actually doing is calculating SAC. I'd leave RMV out of any scuba discussion completely.

[@tobiashartung856]

Hi Mark, I'm sorry but I think this is an incredibly confusing way of using SAC rates if they are cylinder dependent. In my opinion, SAC is RMV adjusted for surface pressure. So SAC is the "surface air consumption" in the sense of "the air you consume at if you are at surface pressure". If you wish to do this in bar per minute (or psi/min), fine but confusing as you need to remember the tank size as well. I use liter per minute although cuft/min are fine too. RMV is the volume of gas going through your respiratory system per minute. At the surface, this is equivalent to your SAC. At depth, this differs. So, a SAC of 20L/min at 30m is an RMV of 80L/min. In dive planning, you compute your RMV based of your SAC for each level of your dive, multiply by the time you spend at this level, and sum them up to obtain your total gas required for the dive. Then build in your rule of thirds, fourths, sixths, whatever floats your boat for the planned dive.

[@thomastse6228]

Mark, excellent presentation. Thanks

[@Frankiewizard]

Thanks Mark, life , the universe and everything seem much clearer.

[@nyax129]

I love that you guys are doing more tec stuff

[@nyax129]

Btw. I know shaun was groaning in the podcast but this might be your best video for us junkies that still suck with sac rates. Got to love my terric......... But I wish it could tell me my resting and working rates not the average.

[@ivoryjohnson4662]

Thank you Mark for helping us get our geek on. Trying to learn the numbers as I want to get into technical diving .

[@GilMarr]

Very nice, Mark! Being a US-based diver and using the archaic Imperial measurement system, the correlation between 14.7psi being equal to the fluid volume of the cylinder isn't easily explained. We see that cylinder as being 80 cubic feet at 3000 PSI. All the math and relationships gets easier in the Metric system. It's always nice when I'm teaching a Deep Diving class and the students realize the need to understand RMV when planning dives. Many of them have never used anything other than the standard "Aluminum 80".

[@LINKRETROGAMING]

Wow, just calculated my SAC, yesterday I dived and the instructor could not believe, he just entered reserve when I was at half, It seems I have a SAC of 8.7l/min, I don't know but maybe I have a very natural low consumption since I could free dive up to 2:30 min with no training!

[@seymourpro6097]

I remember finning up and down a pool with a novice diver. When they had emptied a full cylinder I starting at 50bar was down to 25bar. Air use varies hugely with all sorts of figures including probably mostly confidence.

[@johnbspringer]

Hi and thanks for your video. I am just learning SAC. When you refer to determining SAC rate by swimming at a constant depth (3:30), how does any constant-depth translate to a "Surface" depth. I was under the impression that the word Surface in "Surface Air Consumption" literally means "at the surface" - i.e. head in the water swimming on the surface.

[@mikelgheredero]

Can you do a explanation of how to calculate rock bottom gas? ( Enough to make an ascension from X depth with your mate under stress Sac, and doing all the ascension-stops) and can u explain also how to calculate these "stops", I'm sure there are more ways of doing these stops and not only 3min at 5 meters ( how PADI taught me) ... Thanks a lot and sorry if I didn't explain myself, my English is not my native language

[@leopoldbloom4835]

„The metric version is much easier.“ Yes, of course. It always is. 😏

[@tobiashartung856]

Actually, since 1cuft is about 28L, 1cuft per minute adjusted for depth is a great estimate for new trainee divers who are still uncomfortable in the water and running through their air.

[@psarmstr]

I really don't understand why anyone would teach US customary. Even in the US, every time I've seen someone insist on using it "because USA", they've always struggled heavily while the rest of the class waits around. IMHO, it's professional misconduct to dive or teach using non-metric units just due to the safety issues involved in miscommunication.

[@echols2010]

What is considered a good SAC rate??

[@brendanmarktoh3314]

Thanks for the update Our dive shops Don't really do the maths Can you please continue For leisure dive time Tropical Asia Warm waters

[@Teampegleg]

No issues with the calculations, big problem with the naming. RMV is a medical term, and one that measured quite different, as it is measured at rest. IMO we shouldn't be using it as I will cause confusion. Instead we should have the expectation that SAC comes with the units. Be it pressure psi/bar per minute per atm or volume cuft/liter per minute per atm.

[@Chogogo717]

Everyone trying to do this math in imperial is hating life right now. I’m literally considering swapping my SPG and switching my computer to metric.

[@Exlr8Films]

Honestly this has just added to the confusion in a bad way. A better way to think about SAC would be to convert away from pressure to uncompressed volume consumed. RMV is litterally just the nr. of free uncompressed litres of gas consumed in a minute. It does not account for tanks sizes or depth. RMV will litterally be Gas consumed in liters divided by nr. of minutes. Say 100b from a 12l tank. 1200 liters. Divide by dive time say 30min. 1200/30 = 40l/min. That means your RMV is 40l/min. Its much easier to always convert to amount of uncompressed gas to calculate both RMV and SAC as it can then be used for all tank sizes. You may use more gas with heavier twins than with a single alu 11,1L, but then just figure out your approx SAC for those types. Dont go and calculate for every bloody type of tank you come across. You should always add conservative factors to your gas planning anyway. SAC is RMV, but accounts for depth. To calculate SAC you have to have three things. Average depth, dive time and gas consumed in uncompressed form. In this example say its a dive with an average depth of 15m, dive time for 40min and gas consumed is 130b from a 12l tank. Calculation goes as follows: 130b * 12l = 1560L. 1560L / 40min = 39L. 39L / 2,5ATA(15m) = 15,6L/min. This would give you a SAC = 15,5L/min. The most useful out of RMV and SAC is SAC. I have never come across anyone who has ever discussed their RMV. It means nothing and gives me no useful information. A high RMV could mean you were simply deep or breathing a lot in the shallows. SAC tells me if the diver generally breathes alot or not. In my own case i notice i have a higher SAC breathing Air or Nitrox on shallow dives compared to Trimix on deeper dives. I know what i generally breathe on different setups. My advice is not to do as Mark suggests here and go and dive a completely flat profile for a specific time and check pressure before and after. It will not give you a realistic number as most people dont dive completely flat profiles. To properly figure out your SAC you should just calculate your SAC from normal dives with ascents and descents and combine them into an average. If you dive a flat profile you will not take into account more gas used for ascents and descents as well as more gas used while swimming vs. stopped. Just dive 5 normal dives and calculate an average SAC from those 5 dives. This nr. will continue to change during your diving career. Measure SAC once in a while and see change over time. Imperial can go deal with their own BS :P Metric is best.

[@wolfgangmuhsal]

That's exactly how I calculate my SAC rate and also find the result most useful. Using the total used bars of the dive, duration, the average depth and the volume of the tank we get the average SAC rate. And doing that for every dive we get a nice and pretty easy history of air consumption performance (always writing into my log details like currents or other stressful factors). So I can track my improvements over time. In your example about 15,6 L/min I usually write down the unit as "L/(bar*min)" -> Liters used each Atmosphere-and-Minute", because we divide by the average depth in bar of the dive so it belongs into the denominator. And that's makes it pretty clear and useful to convert back to certain scenarios. How much air will I consume at average depth 10m (2 bar) each minute? 15,6 L/(bar*min) * 2 bar = 31,2 L/min" Enough for a 55 min dive? 31,2 L/min * 55 min = 1716 L ... fine my 12 L with 150 bar (+ 50 bar safety) will be enough because it holds 12 L * 150 bar = 2400 L surface-level-air-equivalent) of air. Honestly, why should there be any other way to calculate air consumption?