Nitrox mixes that you use - 'best mix' or 'what is available'

[diverrex]

For local California diving where much of our diving is not deep and we usually don't know ahead of time where we are going 32 is the standard mix. I've dived in Jupiter, FL where it was common to use 32 for the first dive and stay around 100 -110 FSW becoming fairly saturated so then switch to 36 for the 2nd and 3rd dives and stay around 80-90 FSW.

 

[billgraham]

Most of my diving these days -- unless I'm teaching an OC program -- is on CCR... so I guess i dive best mix, constantly!!

:blinking:

Yes, but I know you keep the OC skills sharp too. Got to be able to do it all!

 

[RJP]

Since I do my own fills on a partial-pressure TMx station, and have two sets of doubles, I will frequently blend a "best mix" for a given dive. Exactly what this mix is will depend on a few things:

1.) Depth of planned dive
2.) Likelihood of planned dive getting blown out
3.) Depth of NEXT planned dive
4.) What gas I have in my other set of doubles
5.) Likelihood I can get back to the shop to refill for subsequent dives.

So, for example...

If I'm mixing gas on Friday night for a Saturday morning dive to a wreck that's 75fsw to the sand I'll blend up some 43% for that dive if the weather looks clear and we have a high likelihood of the dive happening. However, if the wind is blowing and weather looks iffy I'll take a look at the depth of NEXT WEEK'S dive before filling. If next week's dive is to 130fsw, I will then look at what other gas I have on hand. If my other set of doubles is filled with 28%, I'll go ahead and mix 43% "at risk" for tomorrow's planned dive to 75fsw, since I know I have the right mix for next week's dive on hand if tomorrow's gets blown out. If, on the other hand, my other set of doubles is filled with 36% then I'll plan to dive that for the 75fsw dive and then fill whatever I need next week. If the other set has something like 32% in it, I might blend 43% for tomorrow figuring I can easily bleed off some of the 32% and refill should I get blown out tomorrow and then need 28% for my next dive. Now suppose the "subsequent" dive to 130fsw is SUNDAY, and I won't be able to get back to the shop to refill and/or bleed off the 32% and refill to 28%? Well, then I'd probably dive the 32% on Saturday and blend some 28% for Sunday.

Pretty straightforward, huh?



I also do a lot of work with students in the quarry, with a max depth of 60ft. For this I will fill up both sets of doubles with 50% for an OW weekend, figuring the likelihood of doing multiple CESA runs, ascending to catch runaway students, etc I'd like to keep nitrogen loading as low as possible.

 

[Colliam7]

If I'm mixing gas on Friday night for a Saturday morning dive to a wreck that's 75fsw to the sand I'll blend up some 43% for that dive if the weather looks clear and we have a high likelihood of the dive happening. However, if the wind is blowing and weather looks iffy I'll take a look at the depth of NEXT WEEK'S dive before filling. If next week's dive is to 130fsw, I will then look at what other gas I have on hand. If my other set of doubles is filled with 28%, I'll go ahead and mix 43% "at risk" for tomorrow's planned dive to 75fsw, since I know I have the right mix for next week's dive on hand if tomorrow's gets blown out. If, on the other hand, my other set of doubles is filled with 36% then I'll plan to dive that for the 75fsw dive and then fill whatever I need next week. If the other set has something like 32% in it, I might blend 43% for tomorrow figuring I can easily bleed off some of the 32% and refill should I get blown out tomorrow and then need 28% for my next dive. Now suppose the "subsequent" dive to 130fsw is SUNDAY, and I won't be able to get back to the shop to refill and/or bleed off the 32% and refill to 28%? Well, then I'd probably dive the 32% on Saturday and blend some 28% for Sunday.

Pretty straightforward, huh?

Whew! I feel like I just watched the scene in the movie, Princess Bride, where Wallace Shawn is trying to figure out in which cup Cary Elwes has put the poison.

Actually, it does make sense.

 

[waterpirate]

I am glad that I do not have to suffer in silence anymore. I thought I was the only one plagued by the described proccess. I will add one more dimmension to the confusion, re mixing trimix left over from a deep dive into something usefull in the 75 to 100 fsw range, still containing a meaningfull amount of HE. Thank the Lord for mixing software!
Eric

Since I do my own fills on a partial-pressure TMx station, and have two sets of doubles, I will frequently blend a "best mix" for a given dive. Exactly what this mix is will depend on a few things:

1.) Depth of planned dive
2.) Likelihood of planned dive getting blown out
3.) Depth of NEXT planned dive
4.) What gas I have in my other set of doubles
5.) Likelihood I can get back to the shop to refill for subsequent dives.

So, for example...

If I'm mixing gas on Friday night for a Saturday morning dive to a wreck that's 75fsw to the sand I'll blend up some 43% for that dive if the weather looks clear and we have a high likelihood of the dive happening. However, if the wind is blowing and weather looks iffy I'll take a look at the depth of NEXT WEEK'S dive before filling. If next week's dive is to 130fsw, I will then look at what other gas I have on hand. If my other set of doubles is filled with 28%, I'll go ahead and mix 43% "at risk" for tomorrow's planned dive to 75fsw, since I know I have the right mix for next week's dive on hand if tomorrow's gets blown out. If, on the other hand, my other set of doubles is filled with 36% then I'll plan to dive that for the 75fsw dive and then fill whatever I need next week. If the other set has something like 32% in it, I might blend 43% for tomorrow figuring I can easily bleed off some of the 32% and refill should I get blown out tomorrow and then need 28% for my next dive. Now suppose the "subsequent" dive to 130fsw is SUNDAY, and I won't be able to get back to the shop to refill and/or bleed off the 32% and refill to 28%? Well, then I'd probably dive the 32% on Saturday and blend some 28% for Sunday.

Pretty straightforward, huh?



I also do a lot of work with students in the quarry, with a max depth of 60ft. For this I will fill up both sets of doubles with 50% for an OW weekend, figuring the likelihood of doing multiple CESA runs, ascending to catch runaway students, etc I'd like to keep nitrogen loading as low as possible.

 

[Colliam7]

My question: how frequently do experienced divers tweak their mixes, beyond banked mixes, for recreational diving? And, where you do 'tweak' the oxygen concentration - between 21 and 40% - why? Or, perhaps, the better question is, on what quantitative basis do you do so?

First, THANK YOU to all who have commented, for a set of consistently thoughtful and informative posts! Second, I have a few follow-up comments and questions, for clarification.

1. If there is a general practice, it appears to favor the use of standardized, ‘available’ mixes. But, there are also a number of people with the ability to easily blend to a ‘best mix’, who regularly do so, and others who can get custom blends without particular difficulty or expense (or don’t have access to banked nitrox).

For me, 28% becomes about as universal as air

28% is dove frequently in the mid atlantic region.

2. The idea of a 28% mix makes a lot of sense, and provides a PO2 of ~1.4 at the recreational limit of 130ft. I am curious – do any shops actually bank a 28% mix? In my case, the easiest approach is to dilute our banked 30% with air. But, it would be interesting to know if there are ops who take it a step further and bank it.

If it's frickin' freezing, or I will be working hard, I'll drop it to 1.3.

I think that depending on the specifics of the dive ( work load ) the push is to lean it up amoung frequent divers, below what the book says for NDL dives.

3. Several people appear to use a ‘fudge factor’ and lighten the mixture a bit below a P02 of 1.4 if conditions encourage it (high workload, cold, etc.). Or, they blend for a depth a bit greater than the max planned depth, which accomplishes a similar result. Is there a particular reason for picking a specific PO2 of, say 1.3 (or 1.2, or any other value), other than it is just a bit more conservative in terms of the O2 content? I am not disagreeing with the practice at all, just curious about it.

Doing CESA training with students - I'll breathe 40%.)

I also do a lot of work with students in the quarry, with a max depth of 60ft. For this I will fill up both sets of doubles with 50% for an OW weekend,

4. I had a bit of a ‘duh-huh’ moment (people in the South will know what I mean ) thinking about the idea of using a richer mix when teaching OW. I use air for OW classes, because the dive depths are generally shallow, the dives are usually short (~40 minutes each), and we only do two in a day (OK, and the gas is free). But, thinking about it – the CESA comment really brought it out – OW class is a time when using a rich mix (40-50%) would make sense, particularly with a relatively shallow ‘hard deck’ beneath me in the quarry, and quite a few ups and downs. Of course, there are a few obvious reasons not to use nitrox: 1) the instructor should be using the same gas as the students (who are breathing air), and 2) I may be the donor in the OOA / AAS use skills. But, maybe the reason I feel so 'beat up' after an OW weekend is not just the workload.

 

[Doppler]

Yes, but I know you keep the OC skills sharp too. Got to be able to do it all!

Right, Bill... and when I dive OC (and usually for bailout) I tend to "make do" with what's banked.

 

[exposure]

3. Several people appear to use a ‘fudge factor’ and lighten the mixture a bit below a P02 of 1.4 if conditions encourage it (high workload, cold, etc.). Or, they blend for a depth a bit greater than the max planned depth, which accomplishes a similar result. Is there a particular reason for picking a specific PO2 of, say 1.3 (or 1.2, or any other value), other than it is just a bit more conservative in terms of the O2 content? I am not disagreeing with the practice at all, just curious about it.

I lighten the mixture a bit below a PPO2 of 1.4 mainly when all the dive will be at the bottom part. When I plan a Tec dive I use to back the dive up with a longer bottom time, say 3 more minutes and a deeper depth, say 3m/10ft. In these two new plans, CNS, OTU and consumption should be fine. Lightening the PPO2 on a Rec dive tend to give the same result that we can call conservatism. It is a short-cut or proxy in the calculation or in the no-calculation.

 

[Tessunderwater]

I tend to stay at a PPO2 of 1.4 just to be conservative. For the diving here along the Gulf of Mexico, people tend to stay between 28% for the deeper rigs and wrecks (100-130ft) and 32% for the shallower ones (60-90ft). For most of the jetty dives here, the usual trend is air since these dives are less than 70ft deep. I'm planning on taking an advanced nitrox/intro deco procedures class this year to get introduced to the technical side of diving. I'm really eager to learn more about fine tuning these mixes for specific dive scenarios.

 

[billgraham]

Since I do my own fills on a partial-pressure TMx station, and have two sets of doubles, I will frequently blend a "best mix" for a given dive. Exactly what this mix is will depend on a few things:

1.) Depth of planned dive
2.) Likelihood of planned dive getting blown out
3.) Depth of NEXT planned dive
4.) What gas I have in my other set of doubles
5.) Likelihood I can get back to the shop to refill for subsequent dives.

So, for example...

If I'm mixing gas on Friday night for a Saturday morning dive to a wreck that's 75fsw to the sand I'll blend up some 43% for that dive if the weather looks clear and we have a high likelihood of the dive happening. However, if the wind is blowing and weather looks iffy I'll take a look at the depth of NEXT WEEK'S dive before filling. If next week's dive is to 130fsw, I will then look at what other gas I have on hand. If my other set of doubles is filled with 28%, I'll go ahead and mix 43% "at risk" for tomorrow's planned dive to 75fsw, since I know I have the right mix for next week's dive on hand if tomorrow's gets blown out. If, on the other hand, my other set of doubles is filled with 36% then I'll plan to dive that for the 75fsw dive and then fill whatever I need next week. If the other set has something like 32% in it, I might blend 43% for tomorrow figuring I can easily bleed off some of the 32% and refill should I get blown out tomorrow and then need 28% for my next dive. Now suppose the "subsequent" dive to 130fsw is SUNDAY, and I won't be able to get back to the shop to refill and/or bleed off the 32% and refill to 28%? Well, then I'd probably dive the 32% on Saturday and blend some 28% for Sunday.

Pretty straightforward, huh?



I also do a lot of work with students in the quarry, with a max depth of 60ft. For this I will fill up both sets of doubles with 50% for an OW weekend, figuring the likelihood of doing multiple CESA runs, ascending to catch runaway students, etc I'd like to keep nitrogen loading as low as possible.

Yeah, that's why I'd go with the 28% and the o-2, makes life easier. The CCR eliminates all that jackassing around, as Steve said!