Quiz - Physics - Minimum Displacement

[HKGuns]

The scary thing about this question is that they are implying an OW certified diver should even toy with the idea of trying to raise an object that weighs more than a half ton with a lift bag.

I don't think these are coming from the OW exam.............Lift bags were not covered in my OW exam.

 

[CT-Rich]

But I see lift bags with lbs/kilogram ratings, not volume.

I believe these questions are for a DM course. When it comes to recreational S&R, the training is just for 25 lb objects.

One could argue that one should figure out how much gas one will need to actually lift an item, but my guess is most would just bring more than enough gas to do the job.

One liter of air displaces one Kilogram of fresh water. A bag rated for 25 kg is displacing 25 liter water.

 

[CT-Rich]

I don't think these are coming from the OW exam.............Lift bags were not covered in my OW exam.

I haven’t taken a class in 30 years, I have no idea....

 

[-JD-]

please explain. Not looking to argue, but if you have something heavy to lift, you are not going to measure its dimensions and Google the density of whatever it is (assuming it is homogenous material). You will simply try to figure out how much it weighs on dry land and then use enough lift to bring it to the surface. Lift bags have buoyancy ratings.

Now if lift bags were rated by the amount of water being displaced, then we'd have to do some math. But that's ridiculous.

Again, I don't see the practicality of the question.

What I'm getting at is information retention and interference theory. While I know how to perform such calculations from my physics/engineering background, if I were from a liberal arts background for example, I'd probably have to learn it.

I don't like learning things just for the sake of passing an exam and never using it afterwards.

Well, in this case (I am assuming here that the stated physical properties are reasonable) if you have to go out to borrow/rent/buy lift bags and hardware (line, shackles, etc.) to complete the task and base it solely on the dry-weight, you will have to expend around twice the resources that are really required to complete the job.

In the end it is the same principals that lead to an understanding of proper weighting and required BCD lift.

They are rated by the water they displace, but if you use the imperial system the math is a PIA. A 100 liter lift bag would displace 100 kg in fresh water or about 220 lbs. when you see them rated by weight, someone else has already done the math for you. Understanding the underlying principles saves you from doing the really dumb stuff because you have an intuitive understanding of the principle. This is only a theory, I have seen plenty of people who should know better do some pretty dumb stuff.

The scary thing about this question is that they are implying an OW certified diver should even toy with the idea of trying to raise an object that weighs more than a half ton with a lift bag.

6" off the bottom, to reposition in calm water, I'd give it a methodical whirl, it is not rocket science. (Redundancy is your friend and keep your fingers and toes clear, kiddies). Of course, I have moved my 2-ton milling machine between locations twice, on and off trailers. You learn to plan and improvise.

 

[saxman242]

I don't think these are coming from the OW exam

I think a lot are coming from the DM test

 

[tursiops]

When it comes to recreational S&R, the training is just for 25 lb objects.

@wetb4igetinthewater won't see this response (I think he has me on ignore because I keep correcting him), but the weight limit for the S&R course in PADI is 100 pounds. The 25 pound limit is only for Dive 1, i.e. the AOW dive.

 

[BlueTrin]

I have never lifted anything from water but I have been told that you shouldn’t use way too much lift or the object will be thrown to the surface with a lot of speed and that may be dangerous: it could hit the boat on the surface , it could fall back if a lift bag detach or flip ... etc

 

[tursiops]

I don't think these are coming from the OW exam.............Lift bags were not covered in my OW exam.

As Pedro says, these questions are from the Dive Theory exam, which is part of the DM/Instructor course.

 

[wetb4igetinthewater]

I have never lifted anything from water but I have been told that you shouldn’t use way too much lift or the object will be thrown to the surface with a lot of speed and that may be dangerous: it could hit the boat on the surface , it could fall back if a lift bag detach or flip ... etc

Well yes, you don't fully inflate your lift bag, just enough to get it to inflate. Then you dump gas as you ascend to counter the lower pressure/expansion of gas.

 

[Storker]

I have never lifted anything from water but I have been told that you shouldn’t use way too much lift or the object will be thrown to the surface with a lot of speed and that may be dangerous: it could hit the boat on the surface , it could fall back if a lift bag detach or flip ... etc

Yep, lifting heavy stuff off the bottom is no joke. A lot of bad things can happen if you don't know what you're doing (and still can, even if you know what you're doing).

If I were you, I'd get competent instruction before I tried anything more advanced than clipping a dSMB to a catch bag full of scallops and send it up by itself. Heck, I'd do that if I were me as well!