How Many Cubic Feet Of Gas Do You Breath? RMV RATE Explained

[@kevineonbasheamfredericks4025]

Thank you for making this simple and clear most appreciating k.F

[@LakeHickoryScuba]

You are welcome

[@brandonwelch2005]

You said at the end of the video that .22 cf or RMV was what the diver uses at the surface. However in the previous video the scenario was that the diver was at a depth of 33 feet. You calculated his SAC at 33 feet and then used the 8.33 SAC to find his cubic foot use. Why wouldn't this be his rate consumed at 33 feet rather than his surface rate since it was calculated at a depth of 33 feet? I was just open water certified and I am taking the deep water diver class now and I am trying to make sense of it all. Thanks in advance !

[@LakeHickoryScuba]

Hello Brandon, the biggest reason we use a surface calculation instead of a depth calculation is, it is a base line. Even the S-A-C acronym is meant for the surface ((S)-SURFACE, (A)-AIR, (C)-CONSUMPTION). Once we have this value then we can very easily convert it to any depth. The same would apply to the RMV rate. This is a resting value that we need a base line for at the surface. This in return can be converted to any given depth, thus allowing us to plan for the appropriate cylinder(s) for any given dive. Many variables can change underwater (current, medical issues, equipment failures, etc. etc.), thus producing a base line at the surface (hence the calculations being reverted back to the surface since the surface is a constant that never changes), is a more accurate way to plan dives. This too, is why we work off an average SAC rate and not one from a single dive. Hopefully this answers your questions. If not I will try to make a video explaining it in more detail for you.

[@ceriops123]

Not sure if you have made a video like this before, but maybe making a video showing common tools you should have in a field tool kit, i.e. wrenches, allan keys etc.

[@LakeHickoryScuba]

We made a video several years ago on Save A Dive Kits, but we will do an updated one for you showing you what we carry with us. We may call it A Diver's Every Day Carry (EDC). Understand our's will be a biased video based what we carry as professionals but we will make suggestions for the average diver as well.

[@Jacob-pr5cd]

After having the RMV, we can take that value and multiply by the bottom time to get the total ft3 used couldn't we?

[@LakeHickoryScuba]

We can use the RMV for a multitude of different things. Primarily we use it to determine what size cylinder we will need for any dive. And as you stated, we could multiply it by our atmospheric pressure to help determine how many cubic feet we would be breathing at depth as well.

[@johnwilliams2014]

Can you explain why you are using 80 cu ft in the calculation when a "standard aluminum 80 cu ft cylinder" only holds 77.4 cu ft?

[@LakeHickoryScuba]

Hello John Williams, that is an excellent question and one that comes up all the time with technical divers alike. The short and simplified answer is one that tends to cause controversies across the industry. But here it is. For the average sport diver (which make up 95 % of the industry), the rule of thumb is to use the law of averages. The biggest variable as far as a tank's physical capacity is the manufacture. Luxfer's Aluminum 80cf, as you stated only holds 77.4 cf of gas at 3000 psi, however, other manufactures hold between 75 cf - 79 cf (as not all tanks are manufactured equally). So as a whole, divers do what they have always done, round up. We simply round up to 80 cf. We do the same thing with Decompression Theory. If we look at a set of tables, we always round up to the nearest depth. If I wanted to calculate a depth of 37 ft, I would simply round up to 40 ft on the tables. Now from a physics stand point, this is not exact, and we are gambling with our safety, but we have to remember the tables have so much conservatism built into the algorithms, the rule of averages (rounding up) still works and is safe to do. Another great example of this is when we Gas Blend mixtures. If I was to blend a mix of say .324 Nitrox, I would label it as 32%, but if I blended .325, then I would label it as 33%. This 1,000th of a difference makes very little difference at face value, but could cause issues from a liability stand point. So at the end of the day, we use the Law of Averages, and we round up to an whole number to keep the math simple. In short, we use the KISS method. Keep It Simple. Great question and I hope this answers it for you.

[@johnwilliams2014]

@@LakeHickoryScuba Thank you very much, that is an excellent explanation. Is this also why you use "working cylinder pressure" versus the actual pressure that's in the cylinder on a particular dive? Even with a full cylinder, there's also the Charles Law, where one's psi drops when we enter the water in the summer...or would all of this only make decimal point differences?

[@LakeHickoryScuba]

Correct John Williams. It again is based on the law of averages. Now when filling Nitrox via partial pressure blending, using exact psi without rounding up is necessary. This too though is never 100% accurate, as all manufactured gauges will have a + or - 10% discrepancy in accuracy. Digital gauges are a different subject all together. You make a great point about Charles' Law. For every 1 degree temperature change we either gain or lose 5 psi. But from a planning view, we will always plan with the constant temperature from the surface. Plus the constant of 1 ATA of pressure will always be constant at sea level, where the ATA will always change through out your dive. Thus, we plan using ATA at the surface. These are the topics that we discuss in detail in the SSI Science of Diving Course. They always raise a lot of discussions between divers. Here is another one that you may find interesting about the Scuba Industry. We have always used 14.7 psi as the variable for Atmospheric Pressure at Sea Level, and we continue to use that calculation every 33 feet of depth in Seawater, and every 34 feet of depth in Freshwater. Now if we calculate using bar instead of ATA, the actual gauge pressure is calculated with 14.5 psig (gauge) instead of 14.7 psia (absolute). So instead of confusing divers world wide, we simply adapt the 14.7 as the psia as the constant atmospheric change per ATA change. Note: The difference between the Imperial System and the Metric System always tends to confuse people when we talk about calculations for Freshwater and Saltwater. In the Imperial system, we change between 33 and 34 feet for calculations. However in the Metric System, we use the constant of 10 meters (instead of using exacts 33 ft is 10.05 and 34 ft is 10.36). As you can see, the discrepancies show the conservatism built in. And it goes to prove that Decompression Theory is theoretical at best.