Help understanding OTU calculation?

[dewdropsonrosa]

I'm working on the TDI Advanced Nitrox manual in Chapter 3, Pulmonary Oxygen Exposure. I'm having some trouble understanding what a portion of the equation is doing in the calculation.

Note: I am not having trouble plugging variables into the equation to calculate an answer. I want to understand what function those numbers are serving.

The formula is:

Tx is the exposure time in whole minutes, PO2 is the depth in ATM. What purpose does the highlighted section have?

 

[runsongas]

It is the adjustment factor for when your PO2 is higher or lower than 1.0

It isn't linear because of the model curve for pulmonary oxygen toxicity

PO2 also is pressure in atm x fO2. It only is directly your depth pressure in atm if you are on 100% oxygen.

 

[rsingler]

Pulmonary Ox Tox calculations start from an early assumption based on limited data that the problem will not occur (nor will pulmonary ox tox accumulate) as a result of any inspired oxygen concentration above 0.21 and below 0.5 atm.
I think that might be what is going on in this section of the formula.

 

[rjack321]

The equation and the curve that it's derived from is described here equation 1 is expressing figure 1.
https://www.shearwater.com/wp-content/uploads/2012/08/Oxygen_Toxicity_Calculations.pdf

 

[tursiops]

It is just a curve fit to empirical data. The 0.5 expresses the assumption that no OTUs are accumulated if the PO2 is less than 0.5.

 

[atdotde]

Subtract .5 as only excess of partial pressure above 0.5 bar is supposed to do harm and then divide by .5 to make what is raised to some power be between 0 and 1 for the partial pressure varying between 0.5 bar and 1 bar. Note that it's probably better to think of the fraction inverted as the exponent is negative anyways. BTW, I recently blogged about applications of this formula (and a bit about its empirical status) on The Theoretical Diver.

 

[kensuf]

p.36 of the TDI Guide to Advanced Nitrox has a chart showing you how many OTU's per minute you will be exposed to for PO2 ranges from 0.6 to 1.6.

 

[BenjaminF]

This may be overboard, but @atdotde (one of the Subsurface developers) posted this recently: A few thoughts on Oxygen Toxicity – The Theoretical Diver

 

[J J]

This may be overboard, but @atdotde (one of the Subsurface developers) posted this recently: A few thoughts on Oxygen Toxicity – The Theoretical Diver

I love the conclusion that he came to.

 

[BenjaminF]

What's not to understand about Bakers equation 2 towards the end of the explanation?