Need some advice please.

I live in Southern California and am looking forward to diving Wreck Alley sometime in the near future after completing my AOW & Nitrox certs.

I understand that on EAN36 I could extend my bottom time to near 50 minutes on a 90fsw dive. That beats the heck out of having to turn my dive on air at 20-25 minutes.

What kind of gas supply would you recommend?

Thanks guys

P.S. I have no ambition deco diving but would like to make the most of recreational limits. Also, I have no desire for wreck penetration, looky louing outside is good enough for me.

More than likely your gas supply will limit your dive more than your NDL at that depth. Let's say you got a steel 130 (which is a big butte tank) and you have a SAC rate of .6 (which is pretty good for an average guy) going to 90' (for ease in math call it 99'). So (99'/33')+1+=4ATA so with a SAC of .6 x 4 ATA you breath 2.4cft a minute. Steel 130cft/2.4cftm=54 minutes until that tank is dry. If you do the math to get back to the surface with 500psi it's more like 45 minutes total.

Really unless you are doing multiple dives in a day Nitrox doesn't do a whole lot for you unless your going technical. Just MHO.

By Wreck Alley I assume San Diego, and with the depht that would be the Yukon. Just dove there yesterday, max depht about 100 to the sand cutouts. You can really do the whole dive on a 80 with air and circle the whole wreck. If you are worried about having a longer bottom time just dive the upper deck which is only 60-70 FSW. Most of the good stuff is along upper deck and midships anyway. Other wrecks there are shallower than Yukon, plenty of time in a Al 80.

sea2summit:

More than likely your gas supply will limit your dive more than your NDL at that depth. Let's say you got a steel 130 (which is a big butte tank) and you have a SAC rate of .6 (which is pretty good for an average guy) going to 90' (for ease in math call it 99'). So (99'/33')+1+=4ATA so with a SAC of .6 x 4 ATA you breath 2.4cft a minute. Steel 130cft/2.4cftm=54 minutes until that tank is dry. If you do the math to get back to the surface with 500psi it's more like 45 minutes total.

Really unless you are doing multiple dives in a day Nitrox doesn't do a whole lot for you unless your going technical. Just MHO.

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Sea2summit, looks like i would have to carry a substantial amount of gas to stay at depth for 50 minutes. As far as nitrox goes, Wreck Alley is usually a 2-3 tank dive day so that was my premise.

THANK YOU, for stopping by to comment, it is much appreciated!!!

E Szostak:

By Wreck Alley I assume San Diego, and with the depht that would be the Yukon. Just dove there yesterday, max depht about 100 to the sand cutouts. You can really do the whole dive on a 80 with air and circle the whole wreck. If you are worried about having a longer bottom time just dive the upper deck which is only 60-70 FSW. Most of the good stuff is along upper deck and midships anyway. Other wrecks there are shallower than Yukon, plenty of time in a Al 80.

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Szostak, that sounds like a good dive plan for the first trip. I dive a steel 100 so that will give me a little more time as well.

THANK YOU!

A steel 100 would work well for you, but to be honest you don't NEED to dive nitrox to lengthen your dive time, you just need to become more comfortable on the wreck and slow your breathing. I along with most divers on that wreck dive AL80's and have sufficient bottom time. Especially if you aren't penetrating the wreck then an 80 will do.

Keep in mind, you have to surface when your buddy does anyway. So unless you dive the same size tank and have matching SAC rates then it isn't worth it to blow the cash on a HP120.

My friends and I dive double 130's on must wrecks deeper than 4 ATA or hp 100's on wrecks 3-4ATA. The redundant back gas could potentially save your life when doing a penetration dive. We also do almost every dive on Nitrox blended to match our MOD. On a charter that is not charging you a tech dive price you potentially will get about 60% more bottom time than the standard AL80 without going into deco. Sounds like a good investment to me...

watchmanjc:

What kind of gas supply would you recommend?

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As other posters have alluded to, your initial question breaks down into two sub-questions.....

1. What sort of gas volume will I need?

The volume of gas that you carry can determine your dive duration in respect of your air consumption. To answer this question you must;

A. Calculate your Surface Air Consumption. This is a figure respresented in Liters or PSI per minute. Firstly you must record you air consumption at a set depth, for a set time. You can then use this ONLINE SAC CALCULATOR. There is also a useful Scubaboard thread HERE.

B. Utilise your known SAC rate to predict your gas requirements for a future dive at a given depth for a given time. You need to calculate your gas consumption rate at your planned depth. The formula is SAC Rate x (Depth + 33)/33. You can then multipy this number by the time (in minutes) that you plan to dive for to provide a total gas requirement. Alternatives, you can determine the volume of gas available and deduct the consumption rate from this (allowing for a reserve) to provide a predicted gas duration. An 'appropriate reserve' should follow the guidelines stipulated by your training agency (usually 500psi/50bar), but you may wish to adopt the Rule of Thirds and/or Rock Bottom gas management principles.

C. If you wish to dive to a certain depth, for a certain time, then you will need to multiply your gas consumption rate (at that depth) by the dive time, then add a suitable reserve, to determine your minimum gas requirements. This can then be compared against the gas volumes provided by differing cylinder options.

2. What sort of gas mixture will I need?

The mixture of gas that you utilize can determine your dive duration in respect of your no decompression limit (NDL). The use of nitrox mixtures allows extended bottom times.

To determine your optimum gas mixture you must:

A. Find the richest nitrox mixture that can be utilised at the deepest depth you will dive to. The optimum Eanx is calculated by dividing the max PPo2 (1.4) by the ata. There is a (metric) nitrox 'best mix' calculator HERE

B. Utilise the Equivalent Air Depth (EAD) formula so that you can obtain your No Decompression Limit (NDL) and plan your dive on air tables. Alternatively, you can plan directly on suitable Nitrox tables or utilise a nitrox-capable Dive Computer. There is a (metric) EAD calculator HERE

Here is the EAD Formula:

FO2 = Fraction of Oxygen (i.e. the nitrox %)
FSW = Actual Depth in Feet Sea Water



THE PERFECT SOLUTION

Having found your answers to both of these questions, you must then compare your results to provide the best workable solution. At this stage, you must also consider any dive duration limitations that may be imposed by your buddy or team.

To achieve your overall solution, you need to compare the maximum dive durations allowed by both your Gas Volume and your Gas Mixture. You should aim for both factors to provide you with approximately equal Dive Durations.

There is little point going to the expense and trouble of obtaining a nitrox mixture allowing you a 60 minute NDL, when your gas supply only provides 35 minutes of gas supply.

Likewise, there is little point in obtaining a large capacity cylinder/s that provide 60 minutes of gas supply, when your NDL will be limited to 40 minutes.

And, of course, there is no point going to any of this trouble if your buddy is, regardlessly going to dive with on Air with an AL80 and suck it dry in 20 minutes.... or if the dive charter limits the dives to 30 minutes...

There is little point going to the expense and trouble of obtaining a nitrox mixture allowing you a 60 minute NDL, when your gas supply only provides 35 minutes of gas supply.

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I'll gently disagree with this. For most of us, "Nitrox" has a price, and it's unrelated to the O2 percentage. Using a richer mix than needed (always assuming it's a safe mix for the depth!) simply results in decreased decompression stress. It also results in some increased cumulative O2 toxicity, but as, in recreational diving, cumulative O2 toxicity is rarely, if ever, a real concern, I'd say that there ARE reasons to dive a richer mix than necessary, particularly if you are doing multiple dives in a day.

To the OP: PLEASE read NwGratefulDiver's article on gas management (link above). One of the biggest issues with deeper diving isn't decompression -- it's gas supply, and it isn't the gas just to do YOUR dive. Having an understanding of gas consumption and gas reserves will make your deeper diving much safer.

ScubaInChicago:

My friends and I dive double 130's on must wrecks deeper than 4 ATA or hp 100's on wrecks 3-4ATA. The redundant back gas could potentially save your life when doing a penetration dive. We also do almost every dive on Nitrox blended to match our MOD. On a charter that is not charging you a tech dive price you potentially will get about 60% more bottom time than the standard AL80 without going into deco. Sounds like a good investment to me...

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Great thing to do but don't try it without experience in the equipment and - preferably - some advanced training!

This thread is getting scary - seems like it is turning into a nitrox class. If you go take the Enriched Air course it will answer all of your questions.