Computer help

[4sak3n]

It is very quick and easy to work out a MOD from first principles. This is one of the reasons why a Nitrox course is so recommended: because you learn the theory behind it.

You basically want to work out the depth at which your mix has the maximum partial pressure of oxygen (ppO2) you are willing to tolerate. This is almost universally 1.4 atm for non-resting portions of the dive. You can do this in 2 distinct steps.

1. First you work out the required ambient pressure to get 1.4 atm by diving (Edit: meant to be dividing. What a Freudian slip!) your mix O2 % into 1.4.

Assuming you are using 28% the calc is 1.4/0.28 = 5.

2.Then you work out what depth gives you that ambient pressure.

In metric you subtract 1 (to account for sea lvl pressure) and multiply by 10 (assuming you are in sea water, not fresh*).

(5-1)*10 = 40metres.

Imperial is just as easy except instead of multiplying by 10 you multiply by 33.

You can combine the two steps and you get the formula which PADI and other agencies make their students learn by rote/off by heart.

Depth = [(1.4/mix%)-1] * 10

Depth = 14/mix% - 10

I find that using first principles is much easier to remember and you know for certain that you understand exactly what it is you are working out.

Edit: that little example also shows why EAN28 is considered generally safe for all recreational depths. Because if you go deep enough to violate the MOD you are violating the 40m/130ft recreational limit.

*Remember: 10 metres of seawater ~ 1 atm/bar of pressure. So 30 metres of seawater exerts ~ 3 atm/bar. That is why you multiply by 10.

Similarly 33 feet of seawater exert ~ 1 atm/14.7 psi which is why you multiply by 33 when using the imperial system.

 

[katdiver]

The oxygen exposure you're worried about, probably isn't going to be a limiting factor in your dives. As you'll learn on your nitrox course this exposure is controlled by depth, dive time and the percentage of your mix (pp O2 and time in effect). You should accumulate no more than 100% exposure in a 24 hour period. The highest I've ever reached is about 45% by choosing the optimum mix for my depths, staying as long as I could and doing a series of 3 dives that day. This weak mix you are on won't cause you to come near your limit as you are diving as if you were on air.

 

[buleetu]

guys i think im going to be in for a very intereseting but difficult nitrox course, does it involve a lot of mathimatics then 4sak3n

at least i know im gonna be safe when it comes to the reading i got from my computer, i was talking to the dive instructor about the reading and he too said it is a perfectly acceptable reading to have

my dive times are a lot longer compared to when i was here in april and december so its perfect for my diving i guess, i have also been coming out of the water with 50 and 60 bars still left in the tank so im not having to hang out of my instructors alternate like i did one dive in april,, it was not a nice experiance i must admit

 

[4sak3n]

No, it doesn't involve a lot of mathematics buleetu. At least, the PADI one doesn't.

The aim of the course is to ensure that you understand the basic theory behind EANx. Why it gives you more NDL, what the dangers are, how to avoid the dangers, when it is appropriate to use it etc etc

There are some calculations involved but your are provided with preset equations which you just plug numbers into.

For example: Depth = 14/mix% - 10

You just plug your mix % into it to get the MOD.

However I would personally only consider it a successful course if you understand the basic theory well enough to understand why you should use that equation. If you do the course properly, you won't need the equations because you can work it out in your head from first principles.

If you have a solid grasp on the theory there isn't a lot of maths involved at all because the calculations become intuitive. If your don't have a rock solid grasp on the theory but know it fairly well then all you need is a calculator, an equation and can go ahead and plug some numbers in.

The course isn't hard at all and you can learn a lot about basic gas physics if you get a good instructor so I think it is a good one to take for general knowledge even if you have no plans to actually use nitrox.

 

[scubadiver888]

I now see why it is not that harmful for a recreational diver to be diving Nitrox 28%. I would still expect the dive operation to have explained this to me rather than just say, "Don't worry about it."

I also agree with you that if you don't understand WHY then you are not successfully educated on the use of Nitrox.

 

[buleetu]

i think that it would be a great thing for me to do a nitrox course because if the diving ive been doing the last few days ie dive times and air used is anything to go by im very impressed with nitrox in general as my other dives on air were all much shorter than these

does nitrox allow much longer dives then yes, like 2 hours kinda thing ?? or is that why people use 2 tanks sometimes,, whats the max MOD of normal air then, when one of the record breaking divers goes to 250 metres or something they use 3 gases right is this to bring the nitrogen down even further

ur right scubadiver888 im not too happy with the dive shop myself,now that i know what i know about this gas,sure its safe but i guess i would have liked a bit more info on it considering the card i handed them when i started to dive the other day there

today one of the guys was doing his actual open water nitrox dive and the instructor had a slate with him, it was like a table or something what would they have been writing down, the student was doing his deep dive and it was to 42 metres just a few minutes below a decompression dive,he said they went to 35 m then and waited another few minutes before it became a decompression dive and so on, did i understand him correctly

 

[kadleck]

today one of the guys was doing his actual open water nitrox dive and the instructor had a slate with him, it was like a table or something what would they have been writing down, the student was doing his deep dive and it was to 42 metres just a few minutes below a decompression dive,he said they went to 35 m then and waited another few minutes before it became a decompression dive and so on, did i understand him correctly

What he was doing was multi-level diving. The plan would have been worked out using 'the wheel', which sort of 'gives back' time as you plan each (shallower) level of the dive. The table you will have used assumes ALL the dive is at the deepest level, so you will have planned dives on that basis.

You say "became a decompression dive". That is not the intention of multi-level diving. The intent is to extend no-decompression time.

A computer does the above on the fly, so is even better than the wheel, in that as you ascend, the dive time increases. The wheel enables you calculate dive limits based on step changes in depth. The computer will increase the max dive time as you gradually ascend. Hope that makes a small amount of sense!

Note that multi-level diving does not need to be done on EANx. And yes, there is yet another course which goes into more detail (and explains the dreaded wheel!).

 

[emttim]

i think that it would be a great thing for me to do a nitrox course because if the diving ive been doing the last few days ie dive times and air used is anything to go by im very impressed with nitrox in general as my other dives on air were all much shorter than these

does nitrox allow much longer dives then yes, like 2 hours kinda thing ?? or is that why people use 2 tanks sometimes,, whats the max MOD of normal air then, when one of the record breaking divers goes to 250 metres or something they use 3 gases right is this to bring the nitrogen down even further

ur right scubadiver888 im not too happy with the dive shop myself,now that i know what i know about this gas,sure its safe but i guess i would have liked a bit more info on it considering the card i handed them when i started to dive the other day there

today one of the guys was doing his actual open water nitrox dive and the instructor had a slate with him, it was like a table or something what would they have been writing down, the student was doing his deep dive and it was to 42 metres just a few minutes below a decompression dive,he said they went to 35 m then and waited another few minutes before it became a decompression dive and so on, did i understand him correctly

Nitrox will typically not extend your bottom time by something as drastic as two hours, however, it does increase your bottom time significantly as far as percentage goes. For instance, at 80 ft on air I believe the NDL is 30 min, but on a nitrox blend with 36% O2 it's 55 min if I recall correctly, which is almost a 100% increase.

Divers do not use multiple breathing gases at deep depths, such as in tec diving, to bring down their nitrogen level. Rather, they do it to keep their nitrogen level at an acceptable limit throughout the dive, whether to control narcosis or deco time. Air is not used much below 150 feet or so due nitrogen narcosis. I'm not sure if there is an official MOD for air (there may be in the tec community but I'm not a tec diver) but at least in the rec diving world you should be staying at or below 130 feet (or shallower if you're not AOW and/or Deep Diver trained).

Sounds like that diver who was doing the checkout dive was skating the NDL and he was staying just under the NDL so it wasn't a deco dive. I wouldn't necessarily recommend such a thing since more conservative is better, but on the other hand, if he does a complete 3 minute safety stop then it's likely not going to be a dangerous thing to do.

Honestly, you probably will not have to worry about oxygen toxicity with a 28% blend of nitrox. The maximum operating depth for that is 132 feet. The other concern is your oxygen exposure....with an EANx28 blend, I'm guessing at a resort you're probably making 2-4 dives a day, which if you are, you likely will not exceed your O2 limits unless you're making a lot of very deep and/or very long dives. Regardless, I would make sure your computer is set to the blend you're using and follow what your computer says. I'm disgusted/amazed that they would have you dive nitrox without any training in it and try to not even tell you it's nitrox.

 

[buleetu]

Honestly, you probably will not have to worry about oxygen toxicity with a 28% blend of nitrox. The maximum operating depth for that is 132 feet. The other concern is your oxygen exposure....with an EANx28 blend, I'm guessing at a resort you're probably making 2-4 dives a day, which if you are, you likely will not exceed your O2 limits unless you're making a lot of very deep and/or very long dives. Regardless, I would make sure your computer is set to the blend you're using and follow what your computer says.

they say that the computer should stay at 21% for conservatisim, but then the amounts arent right at all are they because im breathing in more o2 arent i??
im diving mainly 1 dive a day but today 2 and then 1 again tomorrow, how deep is really deep and how long is really long

 

[kadleck]

they say that the computer should stay at 21% for conservatisim, but then the amounts arent right at all are they because im breathing in more o2 arent i??
im diving mainly 1 dive a day but today 2 and then 1 again tomorrow, how deep is really deep and how long is really long

They are right that in saying that leaving your computer on 'air' is more conservative, because your actual nitrogen exposure is less than it would be if you were actually breathing 21%.

Also, as has been said, the MOD for 28% EANx is outside the limits for recreational diving. This is no doubt how SUBEX 'invented' air28. It does all seem like a win-win.

But if it was me, I would set my computer for 28% nitrox, so it is at least basing its calculations on what I'm actually breathing. As has been said, diving at the depths you are going will not put you in an oxygen toxicity situation (as you probably realise, even the oxygen in normal air at 21% becomes toxic very deep - way beyond recreational limits); but I'd still prefer my computer to 'know' what I'm breathing.