a) he did call OP's instructors incompetent for no apparent reason and b) the thread header has the word "theory" right there in bold. The question was the math problem requiring a math answer. The fact that you'd freedive with the anchor, hook it to the mounting bracket, get back on board and pull, is not relevant.
NAUI Master Diver Course: understanding theory regarding amount of air needed in lift bag
- Thread starter Zef
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a) he did call OP's instructors incompetent for no apparent reason and b) the thread header has the word "theory" right there in bold. The question was the math problem requiring a math answer. The fact that you'd freedive with the anchor, hook it to the mounting bracket, get back on board and pull, is not relevant.
CptTightPants21
Contributor
What thread were you reading? Mine is
All that while the correct answer was "litre is a measure of volume and volume doesn't compress -- what's inside it may."
Look, it is totally unusual for me to ask advice and then argue with a person who offers it, especially in an online forum but I am not sure what has given you the impression that my instructors know little or nothing about salvage or using lift bags. My question was based on increasing my understanding of the theory and question presented in the NAUI Master Diver textbook...which is fraught with errors from spelling mistakes to the calculations they use in the examples they present.
In the lectures our instructors loosely follow the book/instructor guide. In addition to covering the material required they are teaching the class more akin to a masters degree seminar class where the class time has focused discussion where they share their experience and the students, who all have a wealth of experience of their own share theirs.
The students in the class were all invited to participate based on our experience and displayed competence in the water. We have 2 NAUI instructors (one from England who is also a BSAC instructor, and the other from Poland) and a NAUI dive master (from the US) teaching the class...they each dive year round all over the world and bring an abundance of experience to the classroom and the club we are members of.
We have definitely discussed bottom suction and the need to use more air to overcome this as well as the implications in terms of the great amount of buoyancy this will cause once the object breaks free. The point of my post was two fold:
1. Better wrap my brain around the theory
2. To help myself better understand certain theories and calculations, I have built a set of calculators that I and the members of the diving club I am a member of can use in the dive planing process. So far I have created a Rock Bottom calculator, a SAC/RMV calculator, A nitrox calculator that gives best mix, MOD, and equivalent air depth, and now a calculator that gives a basic idea of how much air one would need to lift an object from a given depth and it also converts that air volume into Bar so it is functionally useful to those in the club who choose to use it.
The amount of air that is calculated is a baseline/starting point for the object needing to be salvaged, not the finite amount truly needed and the calculator displays a note stating such.
Unless you are one of the other 4 students in the class with me, then your assessment of my instructors is fairly pompous of you.
Thank you though for what you have contributed to help my understanding of the theory.
-Zef
Rich Keller
Contributor
I wasn't responding to the theory part, regardless of how bold the lettering was. I was responding to the "I also understand that in the real world one would just use a lot of air to get the object moving upwards." part of post #1. Someone with less experience who read post #1 might come away with the idea that just using a lot of air in a lift bag was a good idea that came from a credible source and neither one of those things is true. If this guy gets the question on the test wrong no one will die. The same can not be said if someone follows his advice about using a lift bag. His instructors are inexperienced in this area not incompetent. The fact that the OP, who seems intelligent to me, could sit through the salvage part of the course and then make a statement like that proves my point. You are majoring on the minor, put the calculator down and look at the big picture.
Rich Keller
Contributor
If you are going to just "use a lot of air in a lift bag" for this example the safest way would be to tie about a 12 meter line between the object and the bag and the bag itself may be a meter tall or more depending on the size bag you use. Now when you fill the bag it can only rise a meter or two, breaking the suction without allowing the object to rocket to the surface and requiring less air from your tank.
The safest way would be to not use a lift bag at all for something this small and this shallow. A continues feed rope come along would be the best choice in this example. Maasdam Pow'R Pull 3/4-Ton Rope Puller - 20 ft. rope-A-20 - The Home Depot Secure the line to the object, run it through the come along that is secured to your vessel and start cranking. This requires no air from your tank, you have a safe and controlled lift with the diver on the surface. If you have a davit on your vessel the come along will not only get the object to the surface, it can place the object on the deck for you as well.
The come along option is something you will likely never hear about in a scuba course because dive shops sell lift bags not come alongs. Again they are inexperienced not incompetent.
Again, you are making a bold assumption about my instructors that is patently untrue. They have nothing to do with what I posted. I put the info about "in the real world...." because after reading through other threads about using lift bags here on Scubaboard, I found that there seemed to always be a few folks that would chime in with something to that affect. I did not want to discuss real world situation, I wanted to keep this on the theoretical level so that I could better understand what is behind the calculation. Your contributions have really taken this whole thread askew.
But in reality, it is impossible to account for bottom suction or how stuck the object is in the equation for how much air to put in the lift bag...so the only time the amount of air calculated will actually work is if you labor to free the object from the bottom first. My understanding is that in the real world, one would indeed use more than the calculated amount of air and one person in the dive team would follow the liftbag upwards to monitor and vent it once the object starts ascending.
You could have easily made your point by stating that the "in the real world" part of my discussion was totally offbase...but instead you keep choosing, in every one of your followup responses, to pass judgement on my instructors whom you know absolutely nothing about. The word arshole keeps coming to mind but I dare call you that as it might violate the forum rules.
-Zef
This information is not germane to this thread as it was not asking about how to get something off the bottom, instead it was asking about the theory behind the math on how you calculate the amount of air to put in a lift bag. I also made it clear in my original post that the question you have quoted me on was directly from the NAUI Master Diver text book. I am sure you are a smart guy and a competent diver...but you are killing me in how you have totally ignored all the pertinent details of the original post.
Again thanks for what you have contributed.
-Zef
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A lot has been written here, which I am not going to read, but I think what is confusing you is the fact that people use volume as an amount when it is not. It only gives an amount if the pressure is also stated. This language "simplification" brings with it some inaccuracy.
So, when thinking of the needs to lift an object, what is important is the displaced volume. This is a 3D volume, not an amount of gas. Then when thinking of the gas contained in that volume, its amount will be different depending on pressure and, thus, depth.
So, when thinking of the needs to lift an object, what is important is the displaced volume. This is a 3D volume, not an amount of gas. Then when thinking of the gas contained in that volume, its amount will be different depending on pressure and, thus, depth.
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