From: kehler@dcs.glasgow.ac.uk (Carsten Kehler Holst)
Newsgroups: rec.scuba
Subject: Re: depth in the US of A
Date: 3 Sep 92 14:02:00 GMT
Organization: Glasgow University Computing Science Dept.

spindler@rcf.rsmas.miami.edu writes:

>In article <pbp8i4s@fido.asd.sgi.com>, leej@sgi.com (Lee Jones) writes:

>>Brings up a good question.  I have a question on my OWI test
>>that asks something like "If you're 66 ft. deep in Monterey
>>Bay, what is the ambient pressure?" Multiple guess answers are various
>>numbers in PSI's.  Bottom line, my student has to remember that it's 
>>14.7 psi/ATM, has to allow for the one at the surface, and has to be
>>able to multiply 3 * 14.7.  
>>
>>Do you think this is a useful question to ask?  If not, why not, and how
>>would you change the question to make it more useful?  Remember, this
>>is a NAUI Open Water I test.  These people are not divemaster candidates
>>(nor are they quickie resort course students :-).

>It is, IMHO, a good idea to try to give the student an appreciation of the
>magnitudes of the pressure ranges a diver experiences in the course of a dive.

I agree

>The physical principles (Boyle's Law, Charles's Law, etc) are, of course,
>important to understand, but a grasp of the actual numbers is also central to
>getting a feel for the underwater environment.  The key is to relate those
>pressures in terms that correspond with a student's physical intuition.  And
>since we in the US are still saddled with the English system of measurement,
>the conversion of pressure from ATA to PSI is an annoying but necessary step.

Hmm! Living in Scotland I can assure you that we use Bar or ATA when 
measuring pressure in bottles you will sometimes see the atmospheric 
pressure measured in mmHg. But our pressure gauges are in Bar. I find 
it difficult to believe that anybody has any intuition about how much
xxx PSI is you might know how many PSI there is in your car tires, but 
does that give you any intuition?

I could claim that working in Bar would give you a much better intuition
--- when you descend 10 m the pressure goes up 1 Bar so at 40 m the pressure
is five times what it was on the surface. But does that give me any intuition?
I can't really feel the pressure at depth so I'm not getting any intuition
about how much that actually means. on the other hand if I take an empty
plastic bottle (say a coke bottle :-)) close it at the surface and watch
what happens to it on the way dow! Or fill it at depth and watch in when
ascending (I'm not sure that is such a good idea now that I think about 
it, but try with a ballon)

More important you want to be able to work from Bar to PSI to ATA to
mmHg to whatever. Because manufactures use these "stupid" units (I'm
afraid I've grown up with SI units - though I've grown atached to
Imperial meassures while I've been in Scotland - stone, gill, yard,
furlong,... - but how people can work with them in a lab beats me).
And you want to make sure that you know how much air you got in your
bottle - that you dont fit a low pressure hose to a high pressure outlet...

So in answer to the original question. Yes, I think it is valuable to
know how to convert, and to know the pressure at depth. But I would be
careful about intuition. I don't think people have much intuition about
pressures - weight and volumes, yes, pressures, no.


>TSpindler@rsmas.miami.edu                 RSMAS/MPO  University of Miami

Carsten Kehler Holst
-- 
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N. Carsten Kehler Holst * kehler@dcs.glasgow.ac.uk * Dep. of Comp Sci.
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