DM physics

[two.crows]

Sooooo richarddean ... do you understand Boyle's law now? Can you explain the calculation back to us? I'm curious, it might after all be a niece or nephew of mine who you're DM'ing some time in the future . No excuse of "crappy at physics" once youre down there ...

:1poke:

 

[richarddean]

Thanks for the help om Boyles law guys, I think I have it now so here is one I have had a go at just to make sure I do have it

A diver consumes 5 bar per minute at 28m in seawater with a given cylinder. Using same cylinder what would he consume at 18m

so 28/10 = 2.8ata + 1 ata = 3.8

5/3.8 = 1.31

18/10 = 1.8 ata + 1 ata =2.8 ata

1.3 x 2.8 = 3.64 bar


Is that right

Thanks

 

[Thalassamania]

To aid in understanding: Since the diver is moving shallower the gas will last longer since the consumption rate will decrease. So you know that you need a fraction smaller than one to multiply the consumption rate by. You have two depths of interested, 33m and 25m. Which yields a fraction less than one? ²⁵/₃₃ clearly. But … that needs to be in absolute pressure, not just depth, so make it absolute ^(25+10)/_(33+10) = ³⁵/₄₃=0.81

See, you don’t even have to transform it to atmospheres, since it is the ratio that’s important and, for that matter, meters of sea water absolute is a perfectly good measure of pressure.

So know you know that at 25m the gas will be consumed at 0.81 time the rate that it was at 33m. That rate was 7bar/min … so the answer is 0.81*7 = 5.7bar/min.

Hope that helps.

 

[two.crows]

You got the numbers right, Richard

A diver consumes 5 bar per minute at 28m in seawater with a given cylinder. Using same cylinder what would he consume at 18m

so 28/10 = 2.8ata + 1 ata = 3.8

... 3.8 ata, right. Or 3.8 bar which is the same for this purpose. Better to keep the units consistent. (See below).

5/3.8 = 1.31

Right. Here you are calculating the SAC (Surface Air Consumption). Even righter would be:

(5 bar/min) / 3.8 = 1.31 bar/min

Why just 3.8 and not 3.8 bar? Think about this for a moment.

The reason is that you are not dividing your (consumption at depth) with the (ambient pressure at depth). That would be a meaningless calculation. You are dividing (consumption at depth) by the ratio between (pressure at depth) and (pressure at surface)! If you get that, this is the part where you are truly beginning to understand the calculation. By doing this we are using the Surface as our reference point for Air Consumption. We could use a different reference point, but using 1 bar (the surface) has the nice advantage of being easy to calculate and meaningful to interpret.

18/10 = 1.8 ata + 1 ata =2.8 ata

Right: here you are calculating ambient pressure at 18m depth.

1.3 x 2.8 = 3.64 bar

Right numbers: but wrong units
1.3 bar/min x 2.8 = 3.6 bar/min

Don't underestimate the importance of keeping tracks of units. They are not here to make life difficult, but they actually tie the numbers to their real-world meaning. And they allow you to do some powerful sanity checks! If the units don't turn out right, you must have had an error in your calculation. For example if your units turn out to be bar² where you were expecting a dimensionless ratio, you must have multiplied where you were supposed to divide. This is why I personally would express everything in bar, rather than mixing bar and ata.

All clear?