Units
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Reinoud
Contributor
radinator:I'll have to look. Perhaps it's poorly written.
I have a 3500 psi HP100 tank (which is 100 cubic feet at 3500 psi).
If I have 1800 psi in it, I have approximately 50 cubic feet.
If I have 1000 psi, I have 2/7th of 100, or 28 cubic feet.
If I have 500, I have 1/7th, of 100, which is 14.
This was done as fast as I can type, no calculator. It's trivial.
I have doubles of 12 liter. With 100 bar I have 2400 liters of gas. (not approximately. No divisions by 7...)
radinator:
What if I want to dive with someone that uses metric?
lamont
Contributor
first you need to get tank manufacturers in the US to sell tanks in L of water volume and bar of fill pressure.
Reinoud:
And what does a liter of gas get you? How much do you use for each breath? (Since you don't want to approximate, please assure me every breath is identical...) If your SPG says 45 bar from your rock bottom, how long do you have at depth?
It's not a difficult calculation because in metric the pressure/volume conversion is easy.
Working in imperial pressure directly is just as easy, and you can compare directly against the SPG. No conversion to tank volume is necessary. That's already taken care of by knowing your SAC rate in psi/min.
My point is, sure, you CAN calculate in volume. I can also calculate in pressure, and it's just as easy.
dherbman:
In that case, the TEAM should decide to use specific units.
What if a metric diver wants to dive with two people who are using imperial? Should he convert or should they?
It's also easier to convert the result of calculations, than to have someone convert every step of the calculations to their 'non-native' units.
I'd be curious what the traveling GUE/DIR instructors have to say about this, since they would probably have to work in one system one week and the other system the next week.
lamont
Contributor
you guys do realize this beats the black-drysuit debate as the most worthless DIR thread ever, right?
No, you just need to convert it once for every tank you buy. Most manufacturers actually have both listed on their websites...
1: SAC rate in metric is in liters/min. Say you have a SAC of 15 liters/min
2: Your tank will last you 2400/10=240+120=360 minutes @ surface.
3: 45bar is roughly 1/5 of 200.
4: 360*0,2=72 minutes. (a bit of a margin of safety is good anyways)
5: 72/ata of your depth (say 40meters=5 ata)=72/10/2=3,6 minutes.
You have 3,6 minutes if there´s 45 bars left until rockbottom on a single 12l @ 40m.
If you´re diving doubles you have 7,2 minutes.
Rows 1 to 3 are easy and "automatic". So is row 5.
The only "real" calculation taking place is row 4 and even that´s hardly "advanced".
Some of the formulas are "harder" than they need to be but I did them in my head, as I was typing, this is just the way I think...
Like someone said, for those that do travel further "than the next town over" this is a real issue...
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