Nitrox course. What's the point?

[doctormike]

I see where you are going wrong with this. Here is my original equation: MOD = ((ppO2 / fO2) - 1) x DPA. DPA is depth per atmosphere which is derived from the salinity factor you mention. The ppO2 value is not a measured or calculated value, it is an entered value by the user. Typically, for recreational divers its value is 1.4 atm. This value is entered by the diver before the dive. The fraction of oxygen, fO2, is also an entered value as well as the salinity factor. Before the dive begins the MOD is already known. This value gets stored and is compared to the depth (d) which is calculated from the actual measured pressure. If the pressure (p) that is measured from the sensor is in atmospheres then the formula becomes: d = p / DPA. An alarm is given if d > MOD.

The MOD formula is what I use in my spreadsheet and probably what other dive programs use as well. For dive computers there is no need to calculate depth. The measured pressure can be used to calculate a ppO2 which is then compared to the value entered by the user (for recreational divers it is typically 1.4 atm) and if it exceeds this value an alarm is generated.

Nope.

PPO2 is absolutely calculated, continually, during the dive. How else would your alarm work?

PPO2 = FiO2 (entered by the diver) x ATA (measured by the DC).

What you are thinking of is the PPO2 alert value, that is what the diver enters into the DC.

If you use an equation that involves DPA, that only has meaning if you know your depth. So unless you include a salinity factor, you can't know your depth with a DC only.

I mean, we are basically saying similar things, but the reason I brought this up (see upthread) is that an error in entering the salinity factor won't cause you to mistakenly descend to an unsafe PPO2. Because the only purpose of the salinity factor is to correct the depth display.

Yes, you can also use your DPA formula, but that requires adjusting the DPA based on what kind of water you are diving in to get your MOD, which will give you a MOD in feet or meters (for that specific salinity). And then using the SAME SALINITY FACTOR to convert your DC's measured ATA to a depth in feet or meters, so that you can compare the two numbers and see if an alert sounds. That's what I mean by putting it in and taking it out.

OR, you can just take the ATA value that your DC measures directly, multiply it by the FiO2 that you know from analyzing your gas, and if that number is more than 1.4, you ascend. And that simple process does not require a salinity factor.

 

[EFX]

@EFX

NOTE
Depth and the salinity conversion factor would only be important if you where using a measuring 'stick' or rope to control your depth. i.e. descended down the 'line' to 33.75m, as marked on the rope. Now, if the fluid you where immersed in increased its density (sea water), you would certainly experience a PO2 greater than your specified 1.4.

The conversion from pressure to depth is always needed for the user. How do you know that dive computer's use only the pressure and not calculated depth for MOD. Do you have access to a dive computer's proprietary source code? If so, give me a link.

 

[Angelo Farina]

The conversion from pressure to depth is always needed. How do you know this? Do you have access to a dive computer's proprietary source code?

It is needed for displaying the depth in meters, not for checking if the PRESSURE at which you are determines an excessive oxygen partial PRESSURE in your lungs...
You do not need a computer for evaluating the oxygen pressure. You could use an analog pressure gauge, giving you the current pressure in bars. Then you multiply it by 0.32 (for 32% NITROX) and you have the ppO2, in bars. Check it does not exceed 1.4 bars and you are safe...
The depth in meters is not needed to be known...

 

[doctormike]

I was trained with FIPSAS method in 1975, here in Italy. The course was 6 months long. It did include 1 months "naked", 1 month free diving with fin, mask and snorkel, 2 months using the ARO (CC rebreather in pure oxygen) and finally 2 months using OC with air.
The training included deco stops using US Navy tables, with the "recreational" modifications I explained, for making diving simpler and safe (but paying this with more deco time).
This was the first course. One year later I was allowed to make the second course, after at least 10 dives in between. And another year later, after further 20 dives, the third course, giving me a 3-stars CMAS certification, for recreational dives down to 50m, in air, with deco stop done on "back gas" and with a buddy.
Later on I stepped on the instructor pathway, and in the end, in 1984, I became a full 3-stars CMAS instructor. In 1985 I started woking as a professional instructor in diving resorts, did it for five years, ending my pro career at the end of 1989.
Here in Europe deco diving with back gas is considered the standard way of recreational diving, and planning a dive with deco stops is considered safer than planning an NDL dive, with the risk of going in deco without being prepared and equipped for it.
In my knowledge this is very similar to dive training in UK done by BSAC.
When I was working in the resorts of Club Vacanze (a leading Italian tour operator) most dives were planned with deco, and everything was properly planned for it, including an additional "tender" diver on the boat ready to intervene in case of accidents, carrying additional air to the divers.§
In or resort at Alimatha (Maldives) we did even have a two-volumes hyperbaric chamber for emergency recompression in case of DCS, and an hyperbaric doctor on duty.
I understand that "our" recreational approach is now considered "tech" in US and other countries, were the basic rec training was never done with our standards...

OK, so then I guess I don't understand what you mean by this:

The method we were taught allows for significant modifications to the original plan, with no other consequences than some longer deco stops.

When you say that you are not a tech diver and that you never exceed rec limits, that has certain implications to many of the new divers reading this thread.

Saying that you can significantly modify your dive plan up to the point of requiring deco should be qualified by saying that (1) you are in fact trained in what would be considered technical diving today by most people, and that (2) when you plan your recreational dive, you are sure to carry enough back gas to safely complete your unplanned "longer deco stops" with an adequate reserve for gas loss, since you no longer have the option of surfacing through a virtual overhead.

 

[doctormike]

The conversion from pressure to depth is always needed for the user. How do you know that dive computer's use only the pressure and not calculated depth for MOD. Do you have access to a dive computer's proprietary source code? If so, give me a link.

It's needed for the USER, since divers like to see how many feet deep they were. I have no idea what the source code is, but I understand the math enough to know that if they work by using that salinity factor to convert from ATA to feet, then they have to use the SAME factor on the other side of the equation when you are figuring out what your PO2 is at that depth. Which means that you are putting it in and taking it out.

Seriously, PPO2 = FiO2 x ATA. PPO2 is what we care about for ox tox, ATA is what the dive computer measures. Not sure why you think that that calculation needs to be adjusted for salinity.

 

[tursiops]

If the thing you're eating is moving you need the knife.

I'm beginning to think my analogy has not had much impact.

 

[Angelo Farina]

Saying that you can significantly modify your dive plan up to the point of requiring deco should be qualified by saying that (1) you are in fact trained in what would be considered technical diving today by most people, and that (2) when you plan your recreational dive, you are sure to carry enough back gas to safely complete your unplanned "longer deco stops" with an adequate reserve for gas loss, since you no longer have the option of surfacing through a virtual overhead.

The basic plan always already assumes you will be in deco, it is safer than planning to be within NDL... Modifying the plan just changes from a 10 minutes deco (original plan) to a 20 or perhaps 30 minutes deco. Not a big deal, just a bit boring...
Millions of divers are still being trained this way here in Europe, in Italy, France, Spain, UK, Germany, etc...
The boundary line between "tech" and "recreational" is quite arbitrary, and depends on the certification agency. All those adhering to CMAS, in my knowledge, train for deco stops, and predicate that planning for deco is always safer than planning for not-deco dives.
Of course, a dive planned for deco requires more air (no diving center here uses AL80 tanks, the standard tank here is steel, 15 liters, 232 bars, with approximately 3500 liters), and twin tanks are also common and considered recreational: in fact during my first course the standard air tank was a 10+10 liters twin set with double posts for two independent regs.

 

[dmaziuk]

... I understand the math enough to know that if they work by using that salinity factor to convert from ATA to feet, then they have to use the SAME factor on the other side of the equation when you are figuring out what your PO2 is at that depth. Which means that you are putting it in and taking it out.

Not only salinity, temperature as well. And adjust for thermoclines and so on. You could do all that, but that way lieth madness.

 

[doctormike]

Well, millions of divers are still being trained this way here in Europe, in Italy, France, Spain, UK, Germany, etc...
The boundary line between "tech" and "recreational" is quite arbitrary, and depends on the certification agency. All those adhering to CMAS, in my knowledge, train for deco stops, and predicate that planning for deco is always safer than planning for not-deco dives.
Of course, a dive planned for deco requires more air (no diving center here uses AL80 tanks, the standard tank here is steel, 15 liters, 232 bars, with approximately 3500 liters), and twin tanks are also common and considered recreational: in fact during my first course the standard air tank was a 10+10 liters twin set with double posts for two independent regs.

Right. But there is a clear line between a dive that requires staged decompression and one that does not, whether you call it tech or European style rec.

So we all need to be careful in implying that "significant modifications to the original plan [involves] no other consequences than some longer deco stops."

You made that statement in the new divers forum. In my opinion, that is a very dangerous implication.

 

[doctormike]

Not only salinity, temperature as well. And adjust for thermoclines and so on. You could do all that, but that way lieth madness.

But that's exactly the point. All of that stuff is irrelevant for calculating PPO2.

PPO2 = FiO2 x ATA. So simple.

Now of course, all of this discussion has just been about not going over a specific, entered, planned maximum PPO2. The real physiological variability comes when you try to determine WHAT your planned maximum PPO2 should be. There is a LOT of stuff that affects that (genetics, hydration, medication, etc..).