How Long Will My Tank Last

How Long Will My Tank Last
We get asked all the time, just how long will that tank of air last you under water? Factoring in our depth, SAC Rate, and for safety a third of our tank volume, we can determine how long a tank of air will last us.
SAC RATE Calculation
Air Used / Time At Depth / Depth In ata
Rule Of Thirds
Working Pressure / 3
SAC RATE Formula Explained
• How Much PSI Do You Br...
RMV RATE Formula Explained
• How Many Cubic Feet Of...
Determining Tank Size
• How Big Of A Tank Do Y...
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[@Mikesroadtrip-ec5kp]

love your video's because your clear and simple to understand. That is a plus for me because it takes longer for me to pickup stuff.

[@LakeHickoryScuba]

Michael Rice Glad to hear. If you ever have a question on a diving related situation feel free to shoot us an email at [e-mail]

[@kevineonbasheamfredericks4025]

Thank you for this mathematical simplicity making what seems complex simple as a diver I appreciate this presentation once again Thank for teaching these principals among other diving videos in your series.

[@LakeHickoryScuba]

Kevin Eon Basheam Fredericks You are very welcome. We want new divers to understand that even though diving is technical in nature, it's not hard understand.

[@monicabouteiller8482]

Great video and easy to understand. Thank you!

[@LakeHickoryScuba]

You're welcome, glad you liked it.

[@aurnbob7209]

another great informative video. My galileo luna gives a breath average measured in cft/min. Is there a formula to tell how long your tank will last from there?

[@LakeHickoryScuba]

Hello AU RNBOB. Most computers that give you your RMV rate, is calculated at 14.7 psi / 1ATA. To determine how long a cylinder will last, you simply take the working cf of the cylinder (80cf for a standard 80 would be 53cf) and divide it by your RMV rate. The 53cf is calculated by the [(size of the cylinder / 3) x 2]. This gives you 2/3 working pressure and 1/3 as a reserve (basic rule of thirds). The results you get will still only be calculated for the surface. You can then take the ATA of the depth you plan to dive to, and simply divide in the same manner. Hope this answers your question.

[@aurnbob7209]

@@LakeHickoryScuba it does. Thanks

[@rayhungria1]

Thank you for making this Video sharing your knowledge

[@LakeHickoryScuba]

You're welcome Ray Hungria, glad you liked our video.

[@charlesclements4350]

I hope that you do not mind me asking. How much does an "80" aluminum tank that has a pressure of 3000 psi actually weigh if it were to be measured on a weight scale?

[@LakeHickoryScuba]

Hello Charles Clements, depending on the manufacture that produces the cylinder the average Aluminum 80 weighs 42 pounds when full. This number can be slightly higher or lower based off the weight of the valve.

[@charlesclements4350]

Thank you.

[@charlesbryant0904]

Hi Brian, First off I have watched a ton of your videos and they are all great. I know this video is old but hopefully you'll see the post and respond as I hav question on this formula. This formula is based on an assumed 80cf tank, and no where in the formula have we made adjustments to set the variable for 80cf. So the question I ask is, how does the formula know that i'm not using an aluminum 100 with the same 3000 psi working pressure? Even cut into thirds to 2000 usable pressure the tank and the formula have remained the same, so where do we input 100cf vs 80cf. I understand the RMV computation would change however the SAC rate doesn't adjust for cubic feet, nor this formula. There is definitely more volume as it's a 100 vs 80, and will last longer at depth, so how would I go about computing it? Also if I was using twin 80's manifolded, how would I compute that, since they still have the same working pressure and usable pressure if cut into thirds? Thanks!

[@LakeHickoryScuba]

The best way to look at SAC and RMV is, SAC rate is simply how much psi a diver breathes per minute, computed back to the surface. The RMV is the calculation we finish with to determine how much cubic feet of gas is need for a particular dive. This too is computed back to the surface and can be calculated to a given depth as needed. The best thing a diver can do is, do the math and go dive without to much worry about their actual air consumption. If we over focus on the gas we are using (not to be confused with the gas we used during a dive - this is a different concept all together) then we will not be in the most relaxed state as possible, and our actual consumption will go up. Lastly, when we focus on SAC rate, this will change by simply changing one variable. Tank size, depth, work load, breathing rate, etc. etc., all these will change a diver's SAC rate. With the RMV rate, this will never change because it is calculated back to the surface, and at the surface, a human only has one lung volume. Thus, they will never be able to consume but a said amount of gas per minute. Our lung capacity will never change. Even in a technical situation, I try not to over think gas calculations in regards to what I am using. I always plan conservatively, and simple enjoy my dive. After re-reading your question, and my answer, I'm not sure I specifically answered what you were asking. You would insert the cylinder size in the RMV formula, not the SAC formula. Hope this helps.

[@charlesbryant0904]

​@@LakeHickoryScuba Hey Brian, first off thanks for responding, I really enjoy your videos and have heard a lot from them! This is great stuff! I’ve thought about this a bit more. I too dive within my limits, with somewhat decent conservatism, and enjoy it without overthinking it. The question I initially posed was just my curiosity and more from a science and mathematical perspective. I have been diving since 1994 and just recently I started pursuing my DM and then hopefully soon my IDC. So the more I dive deep into the theory of what is happening while we dive, the more i’m intrigued on the formulas. For the most part they all make great sense and in my mind can be useful especially when that new diver questions us why or how. Ive watched all your videos on SAC rate (pressure) and RMV (volume) and understand the two in relation to consumed gas at the surface. It wasn’t till I ran the formula against my twin 80’s that I asked myself “where does the equation change in retrospect to how long the tank(s) will last?”, As I looked at your initial SAC rate video example of the guy that has a SAC rate of 8.33 psi per min. With that as our equation number and this video we have a single Alum 80 cf tank @ 3000 working psi, our 1/3 reserve is 1000 psi, the SAC Rate 8.33 psi at surface, diving to a depth of Depth - 33 ft (2ATA). So our formula would be: 8.33 x 2 = 16.66 psi per min 3000-1000 = 2000 usable psi 2000 /16.66 = 120.04 min. So, based off this formula our 80 cf tank would last us 120.04 min. Then I returned to my twin 80’s setup. In theory this 120 min would double to 240 min because I have double the volume of the initial formula Then I thought of my buddy who has a single alum 100 cf 3000psi working pressure tank. If you ran the same formula as above, the answer would still be 120 because no where in the formula does it have you change the volume from 80 to 100. And no where in that formula are any RMV calculations being performed. I also thought that in theory 100cf is 20% more than a 80 cf so one may say that we’d have 20% more time than 120 min = 142min. The one thing I never accounted for before I asked you the question was that in theory both 100cf and twin 80 cf tank setups the “pressure used” would have been way less than 500 psi when computing the SAC rate initially during that given same 30 min of time. That is where the volume would come into play it seems. Am I correct on this assumption, and is there a way to use computed RMV in a “how long will my tank last scenario? Cheers!

[@rarerecords6405]

Im having trouble with this b/c your example uses an 80cf tank...But if I was using a steel 133...I should get more mins from a use able pressure of 2000psi..So I am missing how the tank size plays into this Cheers RR

[@LakeHickoryScuba]

Hello RARE RECORDS, their always seems to be a confusion between both the SAC rate (psi breathed per minute) and the RMV rate (cubic footage of gas breathed per minute). Your Surface Air Consumption Rate will change simply by changing one variable, such as the size of cylinder, your work load, your buoyancy or trim characteristics, etc. etc. Your Respiratory Minute Volume though, never changes, as this is the physical amount, or volume of gas you breathe, and that your lungs can hold. With this being said, divers will disagree and say that the deeper you go, the higher your RMV rate becomes (which is true but not accurately calculated at the surface). The truth in the matter is that, both the SAC rate and RMV rate is calculated back to the surface to a known constant. At depth, the partial pressure of a gas increases, and so does the ambient pressure of the gas you breathe. This means it takes more air to fill up your lungs per breath, and thus you use it more quickly. At shallower depths this of course is reversed. Hopefully this answers your question.

[@vas4uin]

well explained

[@LakeHickoryScuba]

Thank You

[@Danielstrikezone]

I have a 13.5 pony tank I use as a reserve. At say 100 ft. I get in trouble and I need to make ascent on the pony tank how long will it last before its empty? For what it’s worth I have it filled with 32% Nitrox. I realize panic etc. will also come into play.. lol thanks the tank is filled at 3,000 psi

[@LakeHickoryScuba]

Hello Daniel, the key to knowing how long a tank will last is all based off your own personal SAC and RMV rates. At a hundred feet, the fastest you would want to ascend without exceeding a 30 feet per minute ascent rate would be 200 seconds, 3.33 minutes. Adding in a 3 minute safety stop, now you are looking at a 6.33 minute ascent. Given an average of 1 cubic foot per minute RMV rate, the 13.5 would last you 13.5 minutes. Technically longer given you would be ascending and using less air on your way up. So by theory alone, yes you would have enough air in the 13.5 pony system. Now if your RMV rate is higher than 1 cubic foot per minute, then you may want to consider either a 19 cubic foot, a 30 cubic foot, or a 40 cubic foot cylinder (pony).

[@melvinmoi3302]

Would all the calculations apply to metric units?

[@LakeHickoryScuba]

You would simply change psi to bar and cubic feet to liters. Then, instead of using 33 ft(sw) as a unit of measurement to determine your absolute pressure, you would use 10meters. All that is left, is simply plug in the numbers and you will get your results.

[@melvinmoi3302]

Cool, these calculations are going to be very useful in future diving, thanks a lot!

[@LakeHickoryScuba]

You are very welcome.

[@dios.havanaclub7000]

Please, use metric system as you use imperial. (USA using something "imperial"? Not much sense Haha) Maybe some time everybody in the world will be as inteligent as to use métrica.

[@LakeHickoryScuba]

Hello dios.havanaclub, sorry to hear that you did not like our video. Nevertheless, we appreciate you watching and commenting on it. We will try to add the metric calculations in the future.

[@dios.havanaclub7000]

No, no, no, I did indeed like the video. Sorry if my joke about the "imperial" units transmitted my disliking of it. The only thing is that I would like to see all your content in metric units as well.