ppO2 nitrox class question

[unwantedsn]

Hello forum,
I have a question about finding how to calculate partial pressure of oxygen.
The formula is ppO2 = [ ( depth / 33 ) + 1 ] × FO2
If I use the formula and go by the chart that is in the reading i get the correct results.

My issue is when I got an insert for my log book the ppO2 values are different, and I can not figure out how the values were obtained. When I asked some of the instructors I got a generic answer of "this is why we use computers, let them do the work for you" etc.
Does anyone know why the values would be different, or how they were obtained.

Thank you for your time.

 

[tomfcrist]

The SSI chart is wrong...the math doesn't add up. For what it's worth your top chart is wrong too

 

[RyanT]

The formula is correct. I ran the formula on a few random cells in the top table and all my calculations matched the cell's within a decimal of rounding.

 

[CuzzA]

The SSI chart is wrong...the math doesn't add up. For what it's worth your top chart is wrong too

Wow, they are. That's a little concerning. 32% at 60 ft. is wrong on both charts.

Even the example in the SSI chart is correct, but wrong in the chart if you go to 100 ft. at 32%.

 

[RyanT]

32% at 60fsw is 0.90. So the top charts are correct. Likewise, look at the actual depth for the SSI chart. Let's take one example. On the SSI chart, go to 98' using 32%. (98/33)+1 = 3.97 then 3.97* 0.32 = 1.27, which is the value the SSI chart gives. If you look at the left-hand column, 98' on 32% gives you an equivalent air depth of 80' in terms on your NDLs. What am I missing here?

 

[RyanT]

And to the OP, your instructor's "explanation" is a little disconcerting. Indeed, computers can be a really helpful tool, but it's important to understand what the computer is giving you. I'd be really concerned if my instructor was unable to work through the math with me.

 

[tursiops]

I don't see a problem, except for people apparently not knowing how to read the second chart. On that chart, the "depth" numbers on the left are NOT the same as the depth numbers on the first chart. Rather, they are Equivalent Air Depths. to use the 2nd chart, go (for example) to the 32% column, then down to 109 ft. that is the actual depth, and the PPO2 of 1.39 is shown below the 109 number. What the "depth" on the left shows is that 109 actual depth, using 32% is like 90 ft on air, in terms of nitrogen uptake. there are wound-off errors in the final digits between the two charts too.

 

[tursiops]

32% at 60fsw is 0.90. So the top charts are correct. Likewise, look at the actual depth for the SSI chart. Let's take one example. On the SSI chart, go to 98' using 32%. (98/33)+1 = 3.97 then 3.97* 0.32 = 1.27, which is the value the SSI chart gives. If you look at the left-hand column, 98' on 32% gives you an equivalent air depth of 80' in terms on your NDLs. What am I missing here?

We cross-posted. We agree. There are no problems with the two charts.

 

[CuzzA]

Equivalent Air Depths.

AHH, I see. I didn't even look at the title of the chart.

 

[tomfcrist]

I was looking at the 32% 130' numbers...top chart is flat out wrong...ssi chart is good, my eyes wandered to the 33% column though. Too early in the morning.