Standard Mix reasoning

[b1gcountry]

Eventually, an edict was issued that you were only allowed to dive to the limits of your GUE certification regardless of whatever other certifications you held, including instructors.

That sounds interesting. IOW, GUE issued some statement trying to limit the sort of fills that tech 1 students could get? Doesn't sound like it would be too successful...or well received.

Followup question...how many people of you are in the habit of getting a rich fill of 21/35 (like 21/45 or so), and then topping it with air after one dive for logistical/cost reasons?

Tom

 

[Rainer]

I'm looking at my T1 card right now (issued this past July) and it says limit 160'.

I know plenty of T1 divers pushing down to about 170' (with only a few doing 180'). As dives get past 160', bringing a bottle of O2 along starts making a lot of sense.

 

[rainman_02]

That sounds interesting. IOW, GUE issued some statement trying to limit the sort of fills that tech 1 students could get? Doesn't sound like it would be too successful...or well received.

Followup question...how many people of you are in the habit of getting a rich fill of 21/35 (like 21/45 or so), and then topping it with air after one dive for logistical/cost reasons?

Tom

I'll fill my dubs with 21/35...with a "standard" T1 dive of 150' for 25, I'm left with about 80cf of 21/35. Add 500psi He and top with 32%...you get 25/25. Most economical use for left-over 21/35 IMO. You can also brew up 30/30 but it is more complicated.

 

[TSandM]

There aren't any scuba police, and no one is going to stop you from diving to 170 on 21/35 if you want to (and if your team is willing to go there with you). I know for me, the idea of being that deep AND pushing the END of the gas is a no-go.

 

[lamont]

Followup question...how many people of you are in the habit of getting a rich fill of 21/35 (like 21/45 or so), and then topping it with air after one dive for logistical/cost reasons?

There's an old (techdiver?) GI3 article on starting with rich helium mixes when the boat only does air tops...

 

[b1gcountry]

There aren't any scuba police, and no one is going to stop you from diving to 170 on 21/35 if you want to (and if your team is willing to go there with you). I know for me, the idea of being that deep AND pushing the END of the gas is a no-go.

I'm just trying to understand the reasoning behind the recommendations at this point. It does appear that pushing 21/35 10' past the current recommendation is less aggressive than diving 18/45 at the recommended deepest depth. I've found most recommendations from GUE to be well thought out, but after looking at the MOD and 100' equivalent END for 21/35, I just couldn't understand how they came up with that value. I didn't see how they were all spaced 50' apart.

Tom

 

[rainman_02]

MOD @ <1.2
30/30 - 100'
21/35 - 150'
18/45 - 180'
15/55 - 230'
10/70 - 360'

MOD @ <1.4
30/30 - 120'
21/35 - 180'
18/45 - 220'
15/55 - 270'
10/70 - 430'


GUE Limits
30/30 - 100'
21/35 - 150'
18/45 - 200'
15/55 - 250'
10/70 - 400'



On a normal technical dive I will plan to average 1.2 ppO2 or shallower with a MOD based on 1.4 ppO2 as I was taught. If there's something to check out at 170' -- like on the dive we had a year or so ago with the rat fish at Pt Lobos -- I'll drop down with my team. No big deal. Even though it is a tech dive it is not a "working" dive so I feel no need to be limited to 1.2 ppO2. If there were current or other things making it a working dive (elevated breathing etc) you can bet the ppO2 limits will be kept below 1.2.

 

[Vie]

Followup question...how many people of you are in the habit of getting a rich fill of 21/35 (like 21/45 or so), and then topping it with air after one dive for logistical/cost reasons?

I've done this (start with rich fill of backgas) in the past on trips where we were basically diving with an AL80 stage on every dive (and didn't have unlimited access to He). We pretty much only used our backgas to get up from depth to the first gas switch so the backgas was hardly touched (until the last dive) anyway.

 

[rjack321]

Followup question...how many people of you are in the habit of getting a rich fill of 21/35 (like 21/45 or so), and then topping it with air after one dive for logistical/cost reasons?

Basically never.

If I do 1 dive one 21/45 (weird but ok) and have 1/2 left over my air top gets me 21/22.5 That's just about useless. I would bring a stage bottle first and use that + some of my backgas for dive 1 and then the remainder of the backgas for dive 2. If you are doing 21/35 dives Hp130s/lp104s/108s can be used for 2 dives, although they are hard on the bones.

 

[rjack321]

I'm just trying to understand the reasoning behind the recommendations at this point. It does appear that pushing 21/35 10' past the current recommendation is less aggressive than diving 18/45 at the recommended deepest depth. I've found most recommendations from GUE to be well thought out, but after looking at the MOD and 100' equivalent END for 21/35, I just couldn't understand how they came up with that value. I didn't see how they were all spaced 50' apart.

None of these are ideal for any dive. All the standard gases except 30/30 are based on ease of mixing at home or po-dunk facility with no booster. The depth recommendations are just easy round numbers which fit within the general parameters, some being slightly more "aggressive" (in your parlance) than others.

30/30 is not really made for tech dives, it was made for high flow NFL caves.