dive physics

[scooby]

help i am sat looking at some diving physics Boyles law ,Daltons law etc
but may as well be reading Swahili.Can anybody simplify the equations or put these things into laymans terms so that i can continue with my dive master exams,any help would be welcomed
Stuart

[Gareth Burrows]

I'll pm you my mobile phone number. Call me sometime tomorrow afternoon and I'll walk you through the physics for the DM exams.

[adamr]

likewise, drop me a pm if garf is busy and i'll try and help you out with it all

[mjgreen]

I honestly can't remember the equations, despite also being a trimix diver - for me it makes more sense to think of it in terms of what it actually is, & the theory behind it and work it out from there (and then having solved it, I could work out the equations from what I've just done, if I was suitably masochistically inclined ) My gf on the other hand studied economics at uni, so equations are second nature to her, but she can't do it in reverse.

I'd offer to help as well, but looks like you're already in much more capable hands.

[simon varley]

Okay, I'll have a go. As simple as I can make it:

Boyle's Law - pressure and volume are proportional. If you double the pressure you halve the volume. Implications for diving are that air spaces are compressed on descent (ear clearing, mask squeeze, buoyancy changes) and expand again on ascent (lung injuries, venting buoyancy, bubbles grow)

Dalton's Law - the partial pressure of a component of a gas is proportional to the fraction of that gas in the whole. example - the fraction of oxygen in air at the surface is 21% the partial pressure of oxygen is 21% of 1 bar = 0.21 bar. Implications for diving are that it is the partial pressure of a gas that causes the physiological effects. You can get 1.6 bar partial pressure of oxygen (ppO2) for example with pure O2 at 6m, 50% O2 at 22m or with air at 66m

Henry's Law - the amount of gas dissolved in a liquid is proportional to the partial pressure of the gas. Implications for diving are that as pressure decreases so does partial pressure. This means that the liquid can hold less dissolved gas and the excess comes out of solution (as bubbles)

Charle's Law - at a constant pressure volume of a gas is proportional to temperature. As divers we can turn this round and say that at a constant volume (in a cylinder) the pressure is proportional to temperature. This explains why cylinder pressure falls as it cools off after a fill.

[David G]

Have a look here:

UKDivers.net - Choosing the right equipment

As an aside, is there any difference between Charles' Law and Gay-Lusac' Law?

Finally, what's the definition of Coles Law?

[scooby]

thanks for the link i think it may clarify some of it when i get time to study the able,what i need now are some examples so i can work out how to get the correct answer
Stuart

I beleive the definition of coles law is chopped cabbage & carrot in vinegarette

[mjgreen]

ok, some random examples...

Boyle's Law:
If a bottle has a volume of 2l at the surface, what will it's volume be at say 17m?

If a bottle has a volume of 3.5l at 32m, what will it's volume be at 22m?

Do each for sea water & fresh water.

Dalton's:
What's the PO2 of air at 20m?
What's the PO2 & PN2 of EANx36 at 17m?

do for both free & sea

Post ur answers, with brief calculations of how u got there (so if it's wrong we can see how u got there) and then I (or maybe someone else) will post answers/solution

[hammerheads]

Not great examples from mjgreen what should have been mentioned is that in an example such as this

If a bottle has a volume of 3.5l at 32m, what will it's volume be at 22m?

The best way to work it out is to surface it first then resink it e.g

3.5l x 3.2 ata ( plus 1 ata for the surface ) so 4.2 = 14.7 litres at the surface, then sink it to 22m

14.7 litres / (divide) 3.2 ata = 4.59 or 4.60 rounded

Partial pressure formulas are a doodle its always depth + 10 divide by 10 x percentage of gas

What's the PO2 of air at 20m?

calculate as follows depth 20m plus 10 divide by 10 mulitply by 02 percentage in this case its 21%

so its 20 + 10 / 10 X 0.21 = 0.63

What's the PO2 & PN2 of EANx36 at 17m

same formula again different percentage of 02 and N2 and different depth

17 + 10 / 10 X 0.36 = 0.97 PO2
17 + 10 / 10 X 0.64 = 1.72 PN2

quote "do for both free & sea"

From my recollection this is not that relevant in these examples unless its using 1.03 ata instead of 1 ata in the volume equation

Hope this helps seeing how you work out

[mjgreen]

Originally Posted by hammerheads

Not great examples from mjgreen what should have been mentioned is that in an example such as this

No I was deliberately posting examples for him to work out, then post the above By answering first, the OP doesn't get the opportunity to first try to answer them himself.

Originally Posted by hammerheads

Partial pressure formulas are a doodle its always depth + 10 divide by 10 x percentage of gas

Not in freshwater it's not. All of your calculations above are fine for saltwater, they're wrong for freshwater.

ps. (depth/10 + 1) x (percentage of gas/100) is simpler

Originally Posted by hammerheads

From my recollection this is not that relevant in these examples unless its using 1.03 ata instead of 1 ata in the volume equation

It is. Start of DM exam booklet sets out the constants for calculations etc - weight of fresh/seawater, pressure changes per 10m in fresh/sea, & pressure changes per unit of fresh/sea are all used, and referred to in the questions (and an eagle eye may have spotted just how relevant the examples are )