Nitrox course. What's the point?

[Celt]

Yes, you should dive your plan, but you also have to be prepared to deal with things that don't go according to plan. That idea should be part of your planning. You cannot go into a dive thinking that "I am so damn good that nothing can go wrong, so I don't have to plan for it."

Sure that’s the reason for the plan in the first place, there’s no one doing any diving thinking nothing can go wrong. There’s no one thinking “im so damn good nothing can go wrong” you’re the only one saying that. Are you making this up as you go along.

 

[boulderjohn]

Sure that’s the reason for the plan in the first place, there’s no one doing any diving thinking nothing can go wrong. There’s no one thinking “im so damn good nothing can go wrong” you’re the only one saying that. Are you making this up as you go along.

I am just responding in sheer disbelief to the things some people are writing.

 

[lostsheep]

I am just responding in sheer disbelief to the things some people are writing.

Welcome to the internet already in progress

 

[EFX]

Perhaps I didn't explain clearly.

The only way to for a MOD alert to use "depth" would be if it actually MEASURED your depth. Which it doesn't. The only way you can measure depth directly would be to use something like a marked line and use that for your calculations during the dive.

Your computer doesn't measure depth. It measures ambient pressure, which it then converts to a depth using that constant so that you can have it displayed on your screen, so you can know how deep you are. But that derived number is not what is used to determine your current PPO2, which is what you actually care about for ox tox purposes. PPO2=ATA x FiO2. No feet or meters in that formula.

So, theoretically, yes, the computer could work with a variable called depth that it would calculate from ambient pressure by taking into account the salinity constant. But after that, to see if you were over your PPO2 alert value, it would have to adjust the result by taking into account that same constant again to calculate the MOD with your formula ("putting it in and taking it out").

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.

 

[Gareth J]

@EFX

I think @doctormike 's point is that PO2 is about pressure, not depth. As divers, we want pressure presented to us as a depth reading, generally in meters or feet. So we think interm's of depth.


We don't really care what depth we are at, it's about the pressure we are at. We don't care what medium we are in, Air, Fresh Water or Sea Water (oil). We are interested in current ambient pressure.

So if we enter a PO2 of 1.4 as our maximum, and Nitrox 32 as our breathing gas. So that gives us an ambient (gauge) pressure of 3.375 bar. or 33.75m in fresh water.
The computer is reading pressure from a sensing transducer, it calculates the PO2 based on the pressure data. It converts the pressure to a depth reading that it displays.
So we don't really care if the displayed depth is accurate (in terms of fresh water, or different salinity), that is not relevant.

With decompression tables it's a similar story. Decompression calculations are based on pressure, the maximum pressure you experienced for how long, the safe drop in pressure you can experience before you cross the M value.

Because we are divers, we have every thing converted to depth (meters or feet). If we where caisson's workers we would be working in pressure units. (Remember decompression theory comes from the work done to make caisson workers safe. It was originally known as caissons disease, not decompression illness).

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.

 

[dmaziuk]

@EFX

I think @doctormike 's point is that PO2 is about pressure, not depth. As divers, we want pressure presented to us as a depth reading, generally in meters or feet. So we think interm's of depth.

Right, "depth" doesn't (shouldn't) exist in a computer, whether it's MOD or gas loading calculations: it's only shown on the screen for our convenience. Your ppO2 alarm is triggered by current pressure sensor reading times pre-set fraction of O2.

 

[EFX]

You know, someday they will invent a fork, a thing to help you eat. I'll bet it will be a whole lot safer and easier than spearing everything with your knife.

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

 

[dmaziuk]

 

[Gareth J]

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

You could use a trident, and that just a big fork!

 

[Angelo Farina]

I'm not sure what you mean by deco stops if you aren't trained as a tech diver. What method are you referring to?

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...