Nitrox course. What's the point?

[Bigbella]

At 1 atm gauge the salt, fresh, and EN13319 depths are: 33, 34, 33.5. At 2 atm: 66, 68, 67.

Not to be that guy, but 33' -- ten meters is two atmospheres, including pressure at sea level; and 66' -- three, not two . . .

 

[EFX]

Right, that's exactly what I think. From an educational point of view, it doesn't make sense to try to get people to think about diving to "atmospheres".

I guess the basic issue is that the salt/fresh setting on your DC is a fudge factor that will make - for example - your MOD alerts accurate in terms of actual depth under water, even though we have no way of detecting our linear depth under water, so that's really irrelevant. All we can do is detect ambient pressure. If we set the setting wrong, the linear depth will be wrong, but that wouldn't change the MOD or N2 loading calculations.

If you don't set your DC correctly for water type it will give you a MOD alarm at the wrong depth. The MOD calculation is dependent on the depth of 1 atm (gauge) of water. Here is the formula for calculating MOD:

MOD = ((ppO2 / fO2) - 1) x dpa ...........;where dpa (depth per atmosphere) is 33 ft for salt and 34 ft for fresh water.

For a ppO2 of 1.4 atm, a fO2 of 36%, the MOD for salt and fresh water is 95 and 98 ft respectively. If you set your DC for fresh water instead of salt your DC will alarm at 98+ ft where you actually have exceeded your ppO2 limit of 1.4 atm. Setting your computer correctly for the actual dive you're going to do is important!

For N2 loading it depends on how the DC is calculating the pressures. The tissue compartment pressures are expressed in units of pressure which then are used in calculations for NDL and deco stops. It's possible to use the pressure reading directly from the sensor after applying the appropriate conversion factor. However, I'm more familiar with calculations based on the conversion from a depth to units of pressure. This is how Erik Baker does it and how I do it in my spreadsheet program. Using the latter method, a wrong setting of water type will lead to a wrong pressure since the depth will be wrong. Here is an excerpt from the help page in my spreadsheet for deriving the pressure from depth:

//////////////
UNITS OF PRESSURE
All pressure values on the ss are displayed as absolute pressures in feet or meters of salt or fresh water rather than the typical units of psi(imperial) or Kpa (metric). This seems odd at first because a distance unit (feet or meters) is used to define a pressure unit. What seems even stranger is that the gauge pressure is equal to the depth in feet or meters. To understand why this is true consider this relationship: 33 fsw/33 ft (or 10 msw/10m). We can describe it this way: there is a pressure of 33 fsw which is equivalent to a pressure exerted by 33 ft (depth) of water. The spreadsheet converts depth to a pressure in order to calculate insP, the inspired inert gas pressure. For example, to convert 80 ft of depth to its equivalent gauge pressure (P) in fsw we could write: P = 80 ft x 33 fsw/33 ft. As a sanity check on the math the ft divide out leaving fsw which is what we want. The really interesting part is that the 33 divides out giving us an answer that is actually the depth but in pressure units of fsw. This is convenient because it eliminates the need to use cumbersome unit conversions in the formulas. Absolute pressures are obtained by adding the pressure of the atmosphere at the surface to the pressure exerted by the water (which is gauge pressure).
///////////////

 

[EFX]

At 1 atm gauge the salt, fresh, and EN13319 depths are: 33, 34, 33.5. At 2 atm: 66, 68, 67.

Not to be that guy, but 33' -- ten meters is two atmospheres, including pressure at sea level, in terms of absolute pressure; and 66' -- three, not two . . .

This is why I said gauge pressure. Gauge pressure is only the pressure of the water column. You are referring to absolute pressure where atmospheric pressure at the surface is added to the gauge pressure.

 

[boulderjohn]

delete

 

[Bigbella]

This is why I said gauge pressure. Gauge pressure is only the pressure of the water column. You are referring to absolute pressure where atmospheric pressure at the surface is added to the gauge pressure.

Yes, but in terms of calculations, you'd be surprised how often our lonely single atmosphere, and absolute pressure, is forgotten, among many divers . . .

 

[EFX]

You aren't wrong to mention absolute pressure. Baker, myself, and probably others who are using Buhlmann's formulas need to calculate in absolute pressure units to incorporate dives above sea level. Buhlmann, from Switzerland, derived his formulas and tables for dives in the high mountain lakes above sea level. His work built upon that of Workman, a US Navy researcher, whose formulas were used for dives starting at sea level. So, Workman's base of zero pressure was assumed to be sea level. That pressure is zero gauge or 14.7 psi absolute. In order to extend to pressures at the surface less than sea level an absolute pressure is needed whose base is 0 psi.

 

[tursiops]

I said half a joke, not 100%. Now, if you want to erect yourself as the Grand Master, bring it on. I believe that a dry suit does not give as much as freedom of movement than a wet suit. And it even have some additional training and safety constraints. You do question that? Fine. Then, I am sure that you will explain to us how a dry suit is much more convenient and has less safety requirement than a wet suit. I am waiting.

Hey, don't get upset! It was a joke!

 

[Belzelbub]

If you don't set your DC correctly for water type it will give you a MOD alarm at the wrong depth.

That’s not true. The computer doesn’t really care what type of water it is in. It only cares about the pressure. In MOD, that’s all that really matters as well. The formulas for calculating MOD are translated to distance so that they will be more meaningful to us. Pressure isn’t really a measure we can visualize well.

The only thing that would be different if you set the wrong salinity is that the depth displayed will be different compared to the actual. But, then again, the depth displayed is just an estimation assuming constant atmospheric (and water) pressure.

 

[EFX]

Actually, we're both right. Since we enter the ppO2 which is a pressure, the computer can compare that value to the actual pressure from the sensor without the conversion to depth. Assuming that is how the computer determines MOD, the depth of the alarm won't matter for water type. What will matter is what gets displayed to the diver. If you're diving in salt water and set your computer to fresh water the computer will display 98 ft as opposed to the actual depth of 95 ft and at that point the computer will alarm for a ppO2 of 1.4 atm.

 

[Dody]

Hey, don't get upset! It was a joke!

I like that one .