BSAC published new gas density recommendations

[NothingClever]

In my neck of the Red Sea woods, people will relate to a BSAC article.

If I show them some DIR article from that long ago, they’ll just disregard me as some street corner preacher. Unless, of course, they’re some Dutch cats from that bizarre George Irvine hero worship club and then they’ll think I’m some kind of theologian steeped in the ancient texts.

 

[Edward3c]

Ahh, so you took a study funded by offshore dollars (US$) and compiled some tables, and feel justified in charging for them?

I must say I'm not as happy as some of you with these tables. IMHO increasing the He will increase the price of the mixes, but also is an important driver of IEDCS because of counterdifussion. Now the gas switches will have a bigger PN2 differential, and thats an important factor. The tables estate, i.e. that for a dive to 65msw the He must be 54%. Thats a lot of He when switching to a typical Nx mix. Lets see what happens.

So the cost of your diving is more important that having a safe mix.

 

[atdotde]

Is there a way to get to the tables without joining BSAC? I would guess that BSAC didn't entirely fund the research, it seems unreasonable to put them behind a paywall.

Maybe not the tables, but Subsurface has been computing gas densities for a while (implemented after watching on of Mitchell's videos on youtube). You can see them displayed in the info box.

 

[KenGordon]

I'm certainly glad not everyone feels that way about numerous topics that affect our safety while diving.

The various hyperbaric academic societies charge a subscription to get their publications. There is a cost in providing a service even if the origin of the paper is funded already. BSAC has costs and Is funded by subscription. If it gave away the product would not be offering value to those who fund it.

You can read the papers and do the maths for yourself, or you can spend a few quid (about 75USD) to have access to the various BSAC resources.

BSAC have been instrumental in raising the profile of this issue, they certainly put 600 divers in a room with Simon Mitchell to hear about it.

As to OC vs CC, I will guess they said “Gavin, what about OC?” and did something based on the answer.

 

[taimen]

Is there a way to get to the tables without joining BSAC?

Gas blender toolkit also available as a standalone android app can be used for calculating gas densities and ENDs.

 

[kensuf]

ICD isn't a problem for diving within normoxic dive depths. Counterdiffusion may be an issue diving at hypoxic ranges, but generally isn't an issue diving at depths shallower than 80m.

Staying on helium mixes until you're at about 70' will help minimize the risk of ICD.

Candidly, a bigger concern beyond ICD is narcosis when making gas switches to non-helium mixtures. Back when I did my very first hypoxic dive, we were told "you need to get off the helium as quick as possible" and our protocol involved switching back to air at 190-200'. The problem is this abrupt switch is extremely more powerful of a narcotic hit than if you were to descend from the surface to 200' on air.

While I can assure that switching from trimix to air at 200' was like being kicked in the head by a mule, I can also tell you that the switch to nitrox or air even as shallow as 110' can produce extreme narcosis as well.

My recommendation is stay on helium mixes until you're at 70'/21m before switching to nitrox. If you're on a CCR, just stay on helium DIL.

 

[Deep_Down]

the talk of ICD and inner ear DCS I thin is a bit misunderstood. Having lower gas densities does not mean that the gas switches are as abrupt. If the gas density is maintained, the ratio of pn2 increase to he decrease should not be any worse than it is with the WKPP standard gases which have a similar limit of 30m/100ft for END which they use based off of 32% instead of air, but it's close enough. This isn't going to cause any wild switches from 12/70 straight over to air as a deco gas which would probably cause some problems. It's literally a monkey see monkey do since the WKPP has had this recommendation out for almost 30 years...

You are totally right, but look at the tables.
They recommend an He mix of 54% for a dive to 60msw. If done as it is usually done, with one switch to EAN50, the PN2 differential is much bigger than with a 18/35, and there is where I see a problem. Another gas is then needed or just to use the back gas until reaching 02. I personally prefer for such a shallow dives to Deco on a EAN50, and therefore will keep using my loved 18/35.

 

[tbone1004]

You are totally right, but look at the tables.
They recommend an He mix of 54% for a dive to 60msw. If done as it is usually done, with one switch to EAN50, the PN2 differential is much bigger than with a 18/35, and there is where I see a problem. Another gas is then needed or just to use the back gas until reaching 02. I personally prefer for such a shallow dives to Deco on a EAN50, and therefore will keep using my loved 18/35.

I can't see the tables, but I suspect the differences will be in standard gases. The "next" WKPP gas is 15/55 and for that you'd have 35/25 as a deco gas so it wouldn't be a switch straight to 50%.

 

[Deep_Down]

ICD isn't a problem for diving within normoxic dive depths. Counterdiffusion may be an issue diving at hypoxic ranges, but generally isn't an issue diving at depths shallower than 80m.

My recommendation is stay on helium mixes until you're at 70'/21m before switching to nitrox. If you're on a CCR, just stay on helium DIL.

Exactly. And here is where I dont really agree with the tables. They recommend too much He in the normoxic range, where usually only one switch is done and therefore the pn2 differential is too big, in my opinion.
I totally agree with your point of view.

 

[Deep_Down]

I can't see the tables, but I suspect the differences will be in standard gases. The "next" WKPP gas is 15/55 and for that you'd have 35/25 as a deco gas so it wouldn't be a switch straight to 50%.

They recommend to 64msw 18/54, instead of the 18/35. If you then switch to EAN50... the pn2 differential is big enough to not like it (at least myself).