Question calculating NDL is difficult

[L13]

According to this thread, Buhlmann Calculation Help

It looks like you should use them to account for the composition of the inspired gas.

Maybe someone with more knowledge than myself can clarify better?

The water vapor in question is NOT the vapor in inspired air. It is the water vapor in the lungs, which is almost independent from that in the inspired air. It is driven almost exclusively by the lungs adding water to the air in the alveoli.

 

[Wibble]

The water vapor in question is NOT the vapor in inspired air. It is the water vapor in the lungs, which is almost independent from that in the inspired air. It is driven almost exclusively by the lungs adding water to the air in the alveoli.

Nothing to do with comparing OC gas to CCR? Moist warm gas from a rebreather, sore throat from OC's dry gas.

 

[L13]

Nothing to do with comparing OC gas to CCR? Moist warm gas from a rebreather, sore throat from OC's dry gas.

Not really, humidity in the inspired air determines the rate of dehydration via the lungs, not the humidity in the lungs.

If all other conditions are the same, the humidity of the expired air (and the air in the lungs) is essentially constant until you become dehydrated.

EDIT: I need to correct myself. For CCR, humidity in inspired air will be significant, particularly in warmish water. Since it displaces N2 in the other (not O2) gasses in the inspired gas, it will affect the partial pressures of N2 and He. In a well insulated loop, or in warm water the inspired humidity will be close to lung humidity. If the water is cold, or the loop poorly insulated, most of the humidity will condense out and the inspired air will be drier.

For OC, the variation in inspired humidity is insignificant. lung effects dominate.

 

[tursiops]

Yes. Well I set my computer for meter, and just that (depth /10 + 1) is bar isn't it?
And it was set for salt water.

I wonder if you are really trying to get all the details "exact" or just trying to play with some of them. For example, you say you are set for salt water; but maybe not really; you are set (either) for the EU approximation to actual salt water, EN13319, or for SALT, which is slightly saltier than EN13319, but neither is probably what you are actually diving in. If you use EN13319, then indeed 1 bar = depth(m)/10 + 1. I'm sure you understand that these choices are irrelevant for the calculation except for displaying depth; the computer measures and works with pressure, not depth, and displays depth only for your convenience.

 

[BlueTrin]

The water vapor in question is NOT the vapor in inspired air. It is the water vapor in the lungs, which is almost independent from that in the inspired air. It is driven almost exclusively by the lungs adding water to the air in the alveoli.

Do you have an article about how this should be taken in account in formulas?

 

[inquis]

neither is probably what you are actually diving in

But it is important to match if OP is comparing to another implementation as verification of their own code.

OP, I recommend comparing freshwater results first, as all implementations should agree pretty well on that density, depth, and pressure.

 

[inquis]

Do you have an article about how this should be taken in account in formulas?

Wikipedia mentions it. More details are at Morrison's walkthrough.

 

[L13]

Do you have an article about how this should be taken in account in formulas?

See the link @inquisit gave in post#57. That covers OC. For CCR, you will have to come up with a way to estimate inspired humidity, maybe based on how much condensate accumulates in your loop?

OC: lung mix = known mix diluted with lung water vapor.
CCR: lung mix = (O2 + residual vapor + dil inert mix) diluted by (lung vapor - residual vapor)

 

[dmaziuk]

Do you have an article about how this should be taken in account in formulas?

Which part? The one that's accounted for in Schreiner's equation is folded into respiratory quotient RQ and is a constant whose value you get to choose in your implementation. It doesn't really matter that much: the absolute value ends up in the 3rd digit after the decimal point or thereabouts.

 

[dmaziuk]

But a change rate of 'ndl' about depth is too big. I have no idea why it is at all.

(Without looking at the attachments)
In terms of coding, it is easier to just run Buhlman's formula in a loop adding a minute of exposure on each iteration until you get a ceiling. Keep It Simple Stupid.