skewer
New
It is utterly baffling as to why the U.S. persists with imperial measurements.
ppO2 nitrox class question
- Thread starter unwantedsn
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sea_otter
Contributor
@unwantedsn, I'm not sure you actually got an answer here.
The second table lists actual depth and PPO2 in the cell. For example, looking at the 32% column, and finding a depth of 109 ft, your PPO2 is 1.39. You already know how to determine this.
On the far left, the table lists equivalent air depth. That's calculated by multiplying actual depth (in atmospheres) by the ratio of nitrogen % in the gas you're breathing to nitrogen % in air.
First let's convert actual depth to atmospheres. 109 ft / 33 ft/atm = 3.3 atm. Add 1 atm for the planet = 4.3 atm
Now let's get the ratio of nitrogen. Gas in the tank is 32% O2, so the remaining 68% (100 - 32) must be N2. Air is 21% O2, so the remaining 79% must be N2. That makes the ratio 68/79 = 0.86.
Depth * this ratio = 4.3 atm * 0.86 = 3.7 atmospheres equivalent air depth.
Let's convert this back to feet. 3.7 atm - 1 atm for the planet = 2.7 atm. Multiply by 33 ft/atm = 89.1 ft. As a result, we're looking up NDLs as though we were diving air to 90 ft. That's where we get the 90 on the far left.
There is a lot of arithmetic here, so it's pretty common that tables will be printed for a specific gas, like 32% nitrox. Your computer will also crunch the numbers, but it is important to understand the reasoning behind it.
Also, as an aside, I find it curious that the lower left part of that table isn't cropped. By listing the numbers, it seemingly sets the expectation that a 141 ft dive using 26% O2 is appropriate within recreational depth limits. I would expect these cells to be blocked out or gray in recreational textbook.
The second table lists actual depth and PPO2 in the cell. For example, looking at the 32% column, and finding a depth of 109 ft, your PPO2 is 1.39. You already know how to determine this.
On the far left, the table lists equivalent air depth. That's calculated by multiplying actual depth (in atmospheres) by the ratio of nitrogen % in the gas you're breathing to nitrogen % in air.
First let's convert actual depth to atmospheres. 109 ft / 33 ft/atm = 3.3 atm. Add 1 atm for the planet = 4.3 atm
Now let's get the ratio of nitrogen. Gas in the tank is 32% O2, so the remaining 68% (100 - 32) must be N2. Air is 21% O2, so the remaining 79% must be N2. That makes the ratio 68/79 = 0.86.
Depth * this ratio = 4.3 atm * 0.86 = 3.7 atmospheres equivalent air depth.
Let's convert this back to feet. 3.7 atm - 1 atm for the planet = 2.7 atm. Multiply by 33 ft/atm = 89.1 ft. As a result, we're looking up NDLs as though we were diving air to 90 ft. That's where we get the 90 on the far left.
There is a lot of arithmetic here, so it's pretty common that tables will be printed for a specific gas, like 32% nitrox. Your computer will also crunch the numbers, but it is important to understand the reasoning behind it.
Also, as an aside, I find it curious that the lower left part of that table isn't cropped. By listing the numbers, it seemingly sets the expectation that a 141 ft dive using 26% O2 is appropriate within recreational depth limits. I would expect these cells to be blocked out or gray in recreational textbook.
Just a comment on some of the calculations appearing to be off by a penny (i.e. 1.57 versus 1.58): it actually takes 32.9 feet of saltwater (assuming the water has a density of 1.03 g/cc, which varies a tiny bit also) to create one atmosphere of pressure, and not 33 feet. Of course it's easier to round that to 33 feet and then accept that the error will accumulate... so calculations might be off by a fraction of a percent.
They're not "wrong". The simple matter is that the precision of your calculated results can only be as precise as the numbers used to calculate the results. If you round your data to two sig figs, your result will only be accurate to two sig figs (i.e. 1.57 and 1.58 are the same number... to two sig figs.)
Most important, you just need to know when rounding has an practical relevance. That is, will the difference between 1.57 and 1.58 cause any problems with your dive? Doubtful. Please, enjoy the dive.
They're not "wrong". The simple matter is that the precision of your calculated results can only be as precise as the numbers used to calculate the results. If you round your data to two sig figs, your result will only be accurate to two sig figs (i.e. 1.57 and 1.58 are the same number... to two sig figs.)
Most important, you just need to know when rounding has an practical relevance. That is, will the difference between 1.57 and 1.58 cause any problems with your dive? Doubtful. Please, enjoy the dive.
Purely economics. Same reason a business might resist upgrading equipment. When there's a clear economic advantage (i.e. the practical advantage to the metric system is obvious; but there has to be an economic gain to the general population to force the change), we'll come around.
To be fair, the metric system has been sneaking its way into American society for the past few decades. Our drinks come in 1/2 liter, liter or 2 liter bottles and many of our highway signs are in km as well as miles. Give it another 100 years or so and we'll be mostly metric.
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