Quiz - Physics - Water and Heat

[shurite7]

In various manuals there are different answers: PADI is 20, NAUI is 25, and the US Navy is 30 times faster. What I find interesting is none of the three inform the reader how each of the entities came up with the number. Myth Busters, on the other hand, came up 28 times and demonstrated how it came up with the number; if only I could find the episode.

Nevertheless, don't underestimate water temperature (and water movement).

 

[yle]

This one should go down the Rabbit hole fairly quickly once the professors wake up.

<<yawwwwwwwwwwwwn>> all right, I'm here.

The heat capacity of water is around 4200 J/kg-K

The heat capacity of air, at standard pressure and temp (which is realistic for most of the diving we do... standard pressure is at sea level) is around 1000 J/kg-K

So of course 4200 is not "thousands" of times greater than 1000.

Claiming that water conducts heat better than air because it has greater heat capacity is like claiming that trucks can accelerate faster than cars because they're bigger.

Water conducts heat better than air because its heat capacity is 4 times greater, for a given mass, but the mass of a given volume of water is much greater because water is 800 times denser than air.

I'll be honest, this is the first time I've really paid attention to the wording of any of these PADI physics questions. I never read the related sections of the OW (or DM) manual, I've just taught my OW students the relevant topics correctly from basic physics principles. I guess when I did the DM and IDC exams, I must have just answered the questions thinking "yeah, not exactly... but I know what you want."

 

[Angelo Farina]

Again a question based on a wrong physical explanation. The thermal conductivity of water is larger than air, but this is not BECAUSE the heat capacity of water is approximately 4 times larger than that of air. 4 times, not thousands times !!!
Heat capacity and thermal conductivity are two entirely independent concepts.
Let's compare with the capacity of a bottle, and the size of the orifice. The capacity is the volume of liquid you can store in the bottle, the diameter of the orifice changes the flow rate coming out when you pour water from the bottle.
Albeit some large bottles have also a large orifice, in can also well happen that the orifice remains the same, or even becomes smaller in a large bottle.
The same can happen with heat capacity and thermal conductivity. Going form air to water, both are increasing, but there is no direct cause-effect relationship.

 

[StefinSB]

These tests are a sad state of affairs.

 

[TMHeimer]

In various manuals there are different answers: PADI is 20, NAUI is 25, and the US Navy is 30 times faster. What I find interesting is none of the three inform the reader how each of the entities came up with the number. Myth Busters, on the other hand, came up 28 times and demonstrated how it came up with the number; if only I could find the episode.

Nevertheless, don't underestimate water temperature (and water movement).

Yeah I was wondering about the exactness of 20. I put that because that's what I recall from a course. Since it's a PADI test, 20 is correct. Fair enough. If you read 20 in a PADI manual the question is clear as can be.

I suppose if you wanted to nit pick (as we like to do here), "more than 20" could be like 3,476,290.

 

[BlueTrin]

...

This one should go down the Rabbit hole fairly quickly once the professors wake up.

 

[TMHeimer]

Yeah! How about:
e. more than 20 but less than 24

 

[BlueTrin]

Yeah! How about:
e. more than 20 but less than 24

Are you talking in base 10 ?

 

[TMHeimer]

T

Are you talking in base 10 ?

Think I used to know what base 10 means.

 

[saxman242]

I know what PADI wants for the answer here, but let's actually calculate it...


Thermal conductivity of sea water is temperature and salinity dependent, but if we assume 20c water with a salinity of 25 g/kg, the thermal conductivity is .6 W/m-K
For air at 26c, thermal conductivity is .026 W/m-K. Given those conditions, the thermal conductivity of water is 23 times higher.