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Discussion Starter · #1 ·
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
 

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

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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 :D) 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.
 

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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.
 

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Discussion Starter · #7 ·
thanks

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
 

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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 :)
 

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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
 

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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.

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 :)

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 ;))
 

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Okay didnt mean to offend i was merely trying to show the formulas in action so the guy could see it visually as sometimes it helps.

Yes I know about the saltwater, freshwater formulas published inside the front cover of the DM exams and IDC Theory Exams and should have explained this more fully, my apologies for this.
 

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Okay didnt mean to offend i was merely trying to show the formulas in action so the guy could see it visually as sometimes it helps.

Yes I know about the saltwater, freshwater formulas published inside the front cover of the DM exams and IDC Theory Exams and should have explained this more fully, my apologies for this.
doesn't offend me, just trying to help :)
 

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your Depth gauge is actually a Pressure Gauge. when you look at it underwater it is convenient to have a depth instead of a pressure. - Its easier for most people to read a gauge that says 10m rather than 2x10E5 N/m

Now unless you recalibrate that gauge between a salt and fresh water dive, a reading on your gauge will be different to a tape measure. yes "10m" in salt water is slightly shallower than "10m" in fresh. HOWEVER we don't actually care about the depth, its the pressure and its effects on our body that we care about.
so if your SPG reads "10m" you're at the same pressure in salt or fresh water and the volume in your collapsable bottole will be the same.

So a question along the lines of "if you fill a collapsable container with 1 L of air at the surface and take it to 10m depth in salt and then fresh water, what is the difference in volume at 10m between salt and fresh water?" will depend on if you're using an depth gauge or a tape measure to measure depth
 

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your SPG is a Sumersible Pressure Gauge. when you look at it underwater it is convenient to have a depth instead of a pressure. - Its easier for most people to read a gauge that says 10m rather than 2x10E5 N/m

Now unless you recalibrate that gauge between a salt and fresh water dive, a reading on your SPG will be different to a tape measure. yes "10m" in salt water is slightly shallower than "10m" in fresh. HOWEVER we don't actually care about the depth, its the pressure and its effects on our body that we care about.
so if your SPG reads "10m" you're at the same pressure in salt or fresh water and the volume in your collapsable bottole will be the same.

So a question along the lines of "if you fill a collapsable container with 1 L of air at the surface and take it to 10m depth in salt and then fresh water, what is the difference in volume at 10m between salt and fresh water?" will depend on if you're using an SPG or a tape measure to measure depth
That may be well & true, but since he specifically asked about DM, which uses the constants given and just says 10m (thus actual depth) :)
 

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Nigel Hewitt
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Sumersible Pressure Gauge to read your depth?

And there was me thinking the SPG was the thing that read your tank pressure.
Well you live and learn....
 

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Sumersible Pressure Gauge to read your depth?

And there was me thinking the SPG was the thing that read your tank pressure.
Well you live and learn....
:embarassed:what the hell was i thinking of :embarassed:
now i've sorted my english i think my physics still stand up :)

infact even a freshwater lake will give different results from a tank of distilled dionised water at STP
 

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14-9-09
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Finally, what's the definition of Coles Law?
Cole's Law states; "At any given pressure or temperature the amount of shredded cabbage/carrot gloop slapped on one's plate is directly proportional to how much space is left next to one's shrivelled steak" :teeth:

oreBk:redface:
 
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