Isomeric Counterdiffusion

[BRW]

UP,

Yes, such is risky. The risk increases as
ingradient for He increases, and outgradient
for N2 decreases. This can happen late in the
deco strategy I summarized above. In the field
such might happen when stage gas is used up, and
diver needs to hit bottom gas. Or drown.

It's complicated overall, but the above is
true, plus the generic below.

The greater the nitrogen in stage gas, and
greater concentration of helium in bottom mix,
the greater the risk when switching back to
bottom gas. And especially under heavy tissue
gas and bubble loading.

Have had both occular and vestibular hits in
operations here -- nothing serious though.

Again, this all goes back to isobaric
saturation (re-saturation here) when
heavy-to-light switches are made.

Cheers,

 

[Uncle Pug]

BRW once bubbled...
In the field
such might happen when stage gas is used up, and
diver needs to hit bottom gas. Or drown.

What about using back gas on the last stop before switching to a new deco gas or taking back gas breaks when on 100% O2 at the 20' stop? Do either of these pose a problem?

 

[BRW]

UP,

All of the above still hold in any case. The
risks depend on ingassing gradients for He and
outgassing gradients for N2 AT ANY TIME you
switch heavy-to-light.

If your switch to back gas (high helium) is for
a long deco stop and you breathe EAN at deeper
stop and for a long time, the risk is there.
For all the reasons above. Short deco stops
with switches are in noise. But long BTs on He,
with switches to EAN for many minutes, then
switches back to high He back gas are bad
no matter where you do it, or sequence it.
If I were carrying O2, I would rather hit it
before hitting bottom gas (maybe alternate).

The switch to pure O2 will certainly help
deco, outgassing, and bubble contraction for
He and N2. That is, provided isobaric saturation
is sub-symptomatic.

The scenarios you describe above in both emails
are situations we AVOID like the plague here

Regards,

 

[MikeFerrara]

BRW,

We often have in excess of 40 minutes on O2. We alternate between O2 and bottom gas. Would you recommend breaking to a lower ppo2 eanx as apposed to a mix that contains He?

Is the risk reduced by the fact that we are only on the heavier gas for a short time with a high fo2 (the intermediate stop 70 ft to 20 ft)?

How do you avoid the scenario?

 

[BRW]

Mike,

**See my comments**

We often have in excess of 40 minutes on O2. We alternate between O2 and bottom gas. Would you recommend breaking to a lower ppo2 eanx as apposed to a mix that contains He?

**Yes, once you have jumped off helium bottom gas onto EANx
stage gas, don't go back to helium bottom gas. The longer
you have been on helium bottom gas, and then stage
EANx, the worse a switch back to helium bottom gas**


Is the risk reduced by the fact that we are only on the heavier gas for a short time with a high fo2 (the intermediate stop 70 ft to 20 ft)?

**If you are on stage EANx for a short period of time, the risk
drops on switching back to helium bottom gas certainly.
Switching back and forth between helium and nitrogen
for short periods of time, of course, maximizes deco time
and eventaully catches up to you with no isobaric advantage.
The higher the O2 in EANx the less the isobaric risk in
switching back to helium bottom gas. Bottom line, avoid
switches to helium rich mixes after breathing stage EANx,
the longer the EANx time, the greater the switch risk to
helium rich bottom gas**


How do you avoid the scenario?


**We are 5 four man dive teams, with full surface support.
We ride helium rich bottom mixes as far as possible
to pure O2 at 20 fsw, using RGBM deco schedules. We dive
RBs and can switch diluents, or OC with EANx switches never
below EAN50. We carry no stage tanks. Ever**

Regards,

 

[MikeFerrara]

Thanks BRW