Thanks, Rob!
You know, it's pretty amazing that this graph that has been reprinted and re-used in one form or the other for years by many different authors can have these issues. That's why I'm so happy that @Shearwater is willing to work with us to optimize it.
Again, I may be missing something, but I think that it would make a lot more sense to label the X axis "Partial pressure of inert gas in breathing mix", the Y axis as partial pressure of inert gas in controlling tissue compartment" and to rename the "ambient pressure line" as the "saturation line".
Now one problem with that (apart from the wordiness) is that the graph is supposed to help you visualize the process that happens during ascent, which is why the x-axis is usually labeled some variant of "depth or ambient pressure". Sure, gPPN2 is proportional to depth if you don't switch gasses, but that's not intuitively apparent to a new diver. With traditional labeling, it makes sense that as you move towards the surface, the ambient pressure drops, and decompression happens.
There are no units on the traditional graph, but you could make a graph with units and with the X and Y axes scaled differently. That way you could keep the slope of 1.0 for any given mix. The fact that it is generally depicted without units means that people assume that pressure is measured in the same units. But if that's the case, and if you are plotting tPPN2 vs gPPN2, you only would get a slope of 1.0 with 100% N2. You either have to change the slope of the "saturation line", or change the scale of one of the axes.
You know, it's pretty amazing that this graph that has been reprinted and re-used in one form or the other for years by many different authors can have these issues. That's why I'm so happy that @Shearwater is willing to work with us to optimize it.
Again, I may be missing something, but I think that it would make a lot more sense to label the X axis "Partial pressure of inert gas in breathing mix", the Y axis as partial pressure of inert gas in controlling tissue compartment" and to rename the "ambient pressure line" as the "saturation line".
Now one problem with that (apart from the wordiness) is that the graph is supposed to help you visualize the process that happens during ascent, which is why the x-axis is usually labeled some variant of "depth or ambient pressure". Sure, gPPN2 is proportional to depth if you don't switch gasses, but that's not intuitively apparent to a new diver. With traditional labeling, it makes sense that as you move towards the surface, the ambient pressure drops, and decompression happens.
There are no units on the traditional graph, but you could make a graph with units and with the X and Y axes scaled differently. That way you could keep the slope of 1.0 for any given mix. The fact that it is generally depicted without units means that people assume that pressure is measured in the same units. But if that's the case, and if you are plotting tPPN2 vs gPPN2, you only would get a slope of 1.0 with 100% N2. You either have to change the slope of the "saturation line", or change the scale of one of the axes.