"Reasonable" Deep Air...

[DA Aquamaster]

I agree that if you're going to dive trimix, you need multiple sets of tanks to ensure you have the right mix for the dive without having to dump gas or dive an unnecessarily low END.

I like standard mixes, especially when you have no idea where the next dive will be, but from a cost perspective for a specific pre-planned dive, I'm inclined to use enough helium to reduce the END to an acceptable level.

What that level is varies. In OW, in good viz, with no strong current and a no penetration dive, I may well go to 150' on 24-25% nitrox. In a cave, I won't exceed and END of 130' and for Marci the max END needs to be about 110' in a cave, so that becomes the limiting factor for the He percentage.

 

[Superlyte27]

Last year I co-taught a full cave class in North Florida. The water temperature was 72 degrees. It was warm and sunny and visibility in the cave was crystal clear. The drill we were teaching this very proficient diver was a cut line drill. At about 55' of water the student's manifold was tied in the line and he cut the line holding the proper side of the line. He then attempted to re-tie the gold line (we took our own in on a reel) by using two back to back bowline knots. The student attempted the knot several times with no success. This was in 55' of water on 30 percent Nitrox. After it became painfully obvious that the student would not be able to re-tie the line, I grabbed the line and tied the two knots and we exited the cave.

When we got to the surface the student said multiple times that he just couldn't get his hands to tie the knot. I had seen this person several times tie this knot on the surface, and it is apparent that this guy has tied bowlines hundreds of times. Why couldn't he do it underwater? Because he was impaired by the narcosis. He insisted that this wasn't the case.

That night we went to the Mexican restaurant in Alachua where it was two for one margaritas. This guy liked Margaritas and I'm not sure if it was 4 margaritas or 6 margaritas, but our student drank more than a couple. We finished dinner and walked outside to get into our vehicles. When we approached this student's car we noticed a piece of rope hanging out of the back of his vehicle. I quickly grabbed it and said, "quick, tie a bowline". The student did it without hesitation on the very first try. It was a flawless motion.

What's this tell us? 55' of water caused more impairment than 4-6 margaritas.



-edit
But to answer your question...
I have no problem with 180' on air in the open clear water.
In an overhead environment that number is a lot shallower.

 

[PfcAJ]

Or it could be that cave line is just harder to tie in the water than on the surface. I think tying knots underwater and in the dark is just flat out harder than on land where you get the assist from gravity.

I'm the king advocate of shallow trimix, you guys all know that, and I still goof on knots underwater from time to time.

 

[Superlyte27]

Ok, going to make him get in a pool to eliminate that possibility.

 

[Trace Malinowski]

I like standard mixes, especially when you have no idea where the next dive will be, but from a cost perspective for a specific pre-planned dive, I'm inclined to use enough helium to reduce the END to an acceptable level.

Speaking of standard mixes, many divers seem to believe that only the gases thought to be DIR have a range of use. That's not true. If you take a trimix blend and place the max operating depth at a PPO2 below 1.4 ATA such as at 1.3 or 1.2, and place your maximum END below the industry-wide recommended maximum of 130 feet, say at 100 feet, you'll find you have a wide range of use. Which is exactly what standard DIR gases do!

However, from a cost perspective, where the DIR standard gases break down is that you can often get three trimix dives out of a set of doubles instead of two trimix dives. I can use 18/45 to 1/2 tanks at 200 feet, top off with Nitrox32 to get close to 25/25 to use as deep as 150 feet, then dive to half tanks and top off again with Nitrox32 for a dive to 120 feet.

While I would use strict DIR gases for cave diving because the END would be 100 feet or less. I'd be willing to use a max ppO2 of 1.4 and an END up to 130 feet for open water trimix dives. Although, I would try to keep the ppO2 maxed at 1.3 ATA and the END no greater than 110 feet most of the time.

Air was the first standard mix! It's also the easiest standard mix for EAD calculations.

 

[DA Aquamaster]

Speaking of standard mixes, many divers seem to believe that only the gases thought to be DIR have a range of use. That's not true. If you take a trimix blend and place the max operating depth at a PPO2 below 1.4 ATA such as at 1.3 or 1.2, and place your maximum END below the industry-wide recommended maximum of 130 feet, say at 100 feet, you'll find you have a wide range of use. Which is exactly what standard DIR gases do!

However, from a cost perspective, where the DIR standard gases break down is that you can often get three trimix dives out of a set of doubles instead of two trimix dives. I can use 18/45 to 1/2 tanks at 200 feet, top off with Nitrox32 to get close to 25/25 to use as deep as 150 feet, then dive to half tanks and top off again with Nitrox32 for a dive to 120 feet.

Trace, that's exactly what we do in N FL where we do not have more than 2 sets of tanks each. After the third dive, the He content is so low, we no longer consider it in dive planning.

---------- Post added December 17th, 2012 at 08:27 AM ----------

Or it could be that cave line is just harder to tie in the water than on the surface. I think tying knots underwater and in the dark is just flat out harder than on land where you get the assist from gravity.

I'm the king advocate of shallow trimix, you guys all know that, and I still goof on knots underwater from time to time.

It's one reason I don't mess with the bowline, but rather focus on simpler knots where right/left, over/under differences don't really matter since that's generally where knots like the bowline get blown underwater, in the dark, and under stress.

 

[Trace Malinowski]

Trace, that's exactly what we do in N FL where we do not have more than 2 sets of tanks each. After the third dive, the He content is so low, we no longer consider it in dive planning.

Or, for the fun of phoning in a pizza order after a good drag from a reg.

 

[LiteHedded]

Or it could be that cave line is just harder to tie in the water than on the surface. I think tying knots underwater and in the dark is just flat out harder than on land where you get the assist from gravity.

I'm the king advocate of shallow trimix, you guys all know that, and I still goof on knots underwater from time to time.

this.

it's way harder.

 

[Kevrumbo]

At the Blue Lagoon Dive Ops: One 20/20 "Economy" Trimix dive (twin AL80's tanks) with Eanx50 and O2 (in AL30 deco bottles) cost $190 total for He/O2 gas blending . . .[MOD 60m @1.4ata; END 45m]. I did ten of these dives last year November for nearly a $2k add-on gas bill.

This alternative mix is perfectly fine for taking the edge of the narcosis, down here in sunny tropical waters of 29deg C, no current, & good viz. I'm very comfortable traversing inside a wreck at 51 to 54m on 20/20. . .

Blue Lagoon Dive Shop (Truk Lagoon):
Oxygen: $0.06/L or $1.67/cuft
Helium: $0.16/L or $4.50/cuft

btw, the only useful knot I can tie underwater is the double overhand knot into a loop (good for forming loops at the ends of a broken line, so you can bridge them together with a gap/safety reel). . .

 

[Colliam7]

If I could dive air to 185 it would certainly increase my ability to dive more frequently. I am not asking for any guarantees or definitive answers but I am curious how others who have dove air (recently) to 185-ish (1/3 more PPN2), have to say about it.

For warmer (above 72 degrees), low current, good visibility dives, I go to 185-200 on air periodically (6-8 times / year), with no perceptible narcosis (which, for me, is primarily manifested by noticeable paranoia). I do so because of cost, as He is simply too expensive for simpler dives - I either don't do the dive, or I dive it on air. I have never (or, perhaps better put - not yet) had problems understanding u/w communication with my buddy, performing simple tasks, following my dive plan, etc. when diving deep air. But, I will only dive deep air when the conditions are benign, and the dive mission is simple. For dives I consider challenging (cold, dark, deep, low viz, high current), I will go to mix beyond 150' - I don't want the risk of impairment if the fertilizer impacts the ventilator. As one poster noted, diving the Vickery on air is not an issue for me, but I wouldn't dive the Jodrey on anything except mix.