Quiz - Physics - Fill a Container at 40 metres/132 feet
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Some confusion about type of container, whether it was empty at the surface (or at depth), had something other than a vacuum in it, etc. -- some wording like cylinder--that PB fixed. Don't know why, but I didn't think the container was flexible like a balloon, but in my mind was a tin box that in which you may store chocolates or candy.
Some agreement that this is a bad question.
I disagree. I think we all know what PADI is getting at here--the obvious point that 165' is 5 ata thus the answer is d. 200 litres. It doesn't matter if air or anything else but a vacuum was originally inside the container-- all that matters is how much air has to be pumped down to fill it. Not a bad question for layman divers who study the concept and take the test. There is only one answer that can be deemed correct. It's the ones where more than one could easily be correct that bug me.
I suppose we could argue as well that the question mixes imperial and metric measurements so it's a bad question--if we really want to.....
Some agreement that this is a bad question.
I disagree. I think we all know what PADI is getting at here--the obvious point that 165' is 5 ata thus the answer is d. 200 litres. It doesn't matter if air or anything else but a vacuum was originally inside the container-- all that matters is how much air has to be pumped down to fill it. Not a bad question for layman divers who study the concept and take the test. There is only one answer that can be deemed correct. It's the ones where more than one could easily be correct that bug me.
I suppose we could argue as well that the question mixes imperial and metric measurements so it's a bad question--if we really want to.....
But an incompressible container doesn't have to be filled with anything at whatever depth. It is what it is. It could be a 20 L cylinder with 40 L of air at the surface (1 ATM gauge pressure) and still be a 20 L cylinder with 40 L of air at 40 meters (1 ATM gauge pressure) without adding any air from the surface at all. That's the whole principle behind the S and the C in open circuit scuba. The question must assume a compressible container with a volume of 40 L at the surface for 200 L to be an answer.
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I'm gunna add that OK I can see that someone could ask what's in the 40 litre container to begin with if other than a vacuum? If it had 40 litres of air in it to begin with than maybe one could answer that only 160 had to be pumped down to fill it at 165'. If the question simply asked how many litres is needed to fill it at 165' instead of using the words "pumped down", that would clearify that little bit of confusion. Either way, I answer 200 and don't give it a second thought.
I agree with jcp2. If the container were a balloon and had 40 litres in it at the surface (and didn't explode with that amount), it would have a certain circumference. You'd need 200 litres in it at 165' to achieve that same circumference.
What may have really been confusing would've been if one of the possible answers was like 199 or 201 litres. Now you could get into the salinity of the water--heavier due to more salt the further away from the ice caps, thus nearer the equator you'd need a little more air in it because the weight of the salt water would be a little greater.
I agree with jcp2. If the container were a balloon and had 40 litres in it at the surface (and didn't explode with that amount), it would have a certain circumference. You'd need 200 litres in it at 165' to achieve that same circumference.
What may have really been confusing would've been if one of the possible answers was like 199 or 201 litres. Now you could get into the salinity of the water--heavier due to more salt the further away from the ice caps, thus nearer the equator you'd need a little more air in it because the weight of the salt water would be a little greater.
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BlueTrin-- Exactly. The concept is clear to me, as an average diver taking the test.
It would not have been difficult to write this question in such a way that we didn't have to caveat with "well, if we pretend what we think they're asking in this question, the answer is D"
Hell, one could even make it more situational to diving:
An empty lift bag with 40l capacity is at 40 meters. Approximately how much air must be pumped down from the surface to fill the bag?
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