Question Deco gas as diluent for deco

[LFMarm]

Is it a good idea to switch to a deco gas (e.g., 60/0) as off board diluent for the shallower stops on a long deco? On a eCCR it would reduce the need for O2 addition while keeping a high pO2.

In case the answer is yes, what is a reasonable shallow set point? I am used to switch to 0.7 but have never used devo gas as diluent.

 

[Wibble]

You don’t inject diluent during your ascent or whilst decompressing. You will periodically do an oxygen flush at 6m/20ft (1.6 ATA) so there’s no diluent in the loop, just your exhaled off gassed nitrogen and helium.

Following a flush at 6m/20ft or shallower, the PPO2 will get to ~1.4ish and decline as you offgas. This flush is basically emptying the loop through your nose and injecting enough oxygen to get to minimum loop volume, i.e. one flush only, not multiple oxygen flushes that you’d do for cell calibration.

Many people turn off the ADV (automatic diluent valve) at the first stop to prevent a flush triggering the ADV to inject diluent which dilutes the oxygen in the loop. It is surprising how much a short hypoxic diluent injection brings down the loop PPO2 and wastes a ton of oxygen as you need to flush the loop… oops, triggered the ADV again…. The ADV shutoff saves a lot of oxygen.

 

[rjack321]

Is it a good idea to switch to a deco gas (e.g., 60/0) as off board diluent for the shallower stops on a long deco? On a eCCR it would reduce the need for O2 addition while keeping a high pO2.

In case the answer is yes, what is a reasonable shallow set point? I am used to switch to 0.7 but have never used devo gas as diluent.

No. Although what is "long"?
1) why are you doing long decos at 0.7?
2) are you CCR and deco trained? You can't even maintain a loop ppo2 of 1.2 at 30ft on 60% - at all - that's not physically possible shallow. You'd need 63% and complete flushes every few breaths, might as well just use it as OC BO.

If I am using an offboard O2 source to "save" my onboard O2 (its running low, I had a leak, etc) then I plug in offboard 100%. And except for air breaks long decos (60-120 mins) are at a ppO2 of 1.1-1.2

 

[LFMarm]

No. Although what is "long"?
1) why are you doing long decos at 0.7?
2) are you CCR and deco trained? You can't even maintain a loop ppo2 of 1.2 at 30ft on 60% - at all - that's not physically possible shallow. You'd need 63% and complete flushes every few breaths, might as well just use it as OC BO.

If I am using an offboard O2 source to "save" my onboard O2 (its running low, I had a leak, etc) then I plug in offboard 100%. And except for air breaks long decos (60-120 mins) are at a ppO2 of 1.1-1.2

Yes I am helitrox trained on CCR, doing Mod 2 in 2 months. I am referring to shallow stops 3-13 m where I usually move from the low set point of 1.2 to 0.7. My question is — does it make sense to also move from the bottom gas (e.g., 21/35) to the deco bail out gas (e.g., 60/0) when you get to the point for the dive. Advantages are (1) saving O2, (2) reducing solenoid injection.

It sounds from both your replies that it doesn’t make a lot of sense.

 

[Wibble]

Yes I am helitrox trained on CCR, doing Mod 2 in 2 months. I am referring to shallow stops 3-13 m where I usually move from the low set point of 1.2 to 0.7. My question is — does it make sense to also move from the bottom gas (e.g., 21/35) to the deco bail out gas (e.g., 60/0) when you get to the point for the dive. Advantages are (1) saving O2, (2) reducing solenoid injection.

It sounds from both your replies that it doesn’t make a lot of sense.

You don't inject diluent at deco**; only oxygen to replace the oxygen you've metabolised.

All helium rich diluent is displaced by the oxygen and your off-gassed exhaled breath.


** Unless you have a massive buoyancy fart and descend so far that the loop collapses, in which case you'll inject your diluent.

 

[LFMarm]

Thank you!

 

[berndo]

Why would you wanna safe oxygen and why switch setpoint to 0.7?
You wanna bump up O2 for deco.
How didn't you get through a helitrox class without knowing this? Did your instructor tell you to lower the setpoint for deco?

 

[Wibble]

You can tell when your deco's pretty much over... you stop offgassing.

At the start of your deco, you will be exhaling a lot of inert gas (N2, He) you've absorbed at depth. This isn't measured by the PPO2-measuring cells, so your loop 'fills'. Occasional nose exhales drops the loop back down to minimum loop, knocking off the slight buoyancy increase.

As you get to the end of your decompression, less inert gas is offgassed, so your buoyancy doesn't change as much.

Occasional oxygen loop flushes -- 10 or 15 mins -- freshens the loop, clearing the inert gas and pushing up the PPO2.


Changing your setpoint to 0.7 needs to be done above 6m/20ft as you cannot sustain PPO2 at 1.3 above 1.6ATA (without wasting a load of oxygen). You should run your PPO2 as high as you can at 3m/10ft which will be around PPO2 1.0ish.

A manual (or hybrid) rebreather with an orifice that constantly injects oxygen makes these stops much easier. An electronic CCR will constantly inject oxygen in relatively large dollops which may affect your buoyancy.

 

[Wibble]

Why would you wanna safe oxygen and why switch setpoint to 0.7?
You wanna bump up O2 for deco.
How didn't you get through a helitrox class without knowing this? Did your instructor tell you to lower the setpoint for deco?

The OP's asked a reasonable question and would appreciate some help in explaining why his ideas aren't actually used in practice.

Experience teaches us these things. Often they're missed in classes, certainly in the MOD1 helitrox as there's so much to learn.

 

[Raphus]

Switching to 0.7 in shallow depths is not a bad idea on a eccr. But you need to drive your unit now manually to a ppo2 higher then 0.7, so you can offgas more efficient and don't get your bouyancy affected by the solenoid.

Diluent should be a breathable gas for that depth, so you can diluent flush in an emergency. But usually there is no need to inject any diluent in deco.

An ccr instructor can tell you more about this in a class. Ask them questions, that's what they get paid for