Prebreathing Survey

[doctormike]

And one thing that I might respectfully mention in response to the observation that many divers do exceed the manufacturer's recommendations for scrubber time.

In another thread on hypercapnea, I referred to the most important sentence in medical school: "There is a bell curve for everything". When you imply that pushing the scrubber is OK, you can't know the details of every reader's metabolism and CO2 production. While the recommendations might be conservative, they are by design. It's normalization of deviance if you promote the idea that "5 hours on a 3 hour scrubber is OK in X circumstances", because eventually that becomes a norm and people forget the "X circumstances" part of the recommendation.

It's sort of like safe PO2 levels. Yeah, you don't immediately seize if your PO2 goes to 1.61 on deco. But people can and have had ox tox events at "safe" levels. The reason why manufacturers generate standards is that they result in acceptable levels of safety for anyone using that particular stack.

I do understand that might mean wasting more sorb in warmer water. And I know that a thinking, experienced diver can consider multiple factors in making the decision about how long to use a scrubber. I would just be careful about implying that 5 is the new 3.

 

[taimen]

The practice I quoted has not formed by normalization of deviance and it is not done to avoid wasting sorb. AFAIK, the 3 hour duration for JJ scrubber comes from CE certification requirements which include high CO2 production levels in addition to cold temperature. When planning 4 hour deco dives in cold water people do many decisions like that.
If people began starting dives with scrubbers that already have 180 minutes runtime, that would be serious normalization of deviance.

 

[doctormike]

Of course it is normalization of deviance. Now that may be appropriate or safe in this particular situation, but that’s what the term means.

The manufacturer (JJ) publishes guidelines that says the scrubber should not be used for more than three hours. Full stop, no qualifying factors to that. If you say that many people exceed that manufacturer’s recommendation, that’s simply the definition of normalization of deviance.

Again, please realize that I am not criticizing your practices, simply making a larger point.

A new rebreather diver reading this thread my take home the idea that four hours on the JJ scrubber is OK. And they might not remember your distinction that it’s OK sometimes but it’s not OK if you start a dive with three hours on it already (which is not something that has not been formally tested, in any case).

If you want to do a longer dive on a JJ, you can always get the radial scrubber...

 

[taimen]

In the US where you don't have to care for CE requirements you could have your own standards. In this particular case small rebreather manufacturers are tied to CE certification requirements and unable to give real recommendations of their own.
US4 first stage is also rated by Apeks only for tempratures above 10 C. JJ gives their CE certified units a temperature range down to 4 C. Who should we believe in this case?
Anyway, sorb is cheap.

 

[cathal]

US4 first stage is also rated by Apeks only for tempratures above 10 C. JJ gives their CE certified units a temperature range down to 4 C. Who should we believe in this case?.

Logic would be to believe the people who designed and manufactured the Reg and I would be bringing the recommendations of the Reg manufacturer to JJ for them to explain. Then you would get clarification on this contradiction.

 

[taimen]

Logic would be to believe the people who designed and manufactured the Reg and I would be bringing the recommendations of the Reg manufacturer to JJ for them to explain. Then you would get clarification on this contradiction.

There is nothing unclear. With the tiny gas flows in CCR use the US4 performs perfectly in cold conditions. This was just an example that sometimes manufacturer recommendations are not applicable to real world use (as they are written only according to certification requirements).

 

[doctormike]

JJ sells both a CE and a Non-CE version of their rebreather (I have the latter). So they are certainly not tied to CE. If they wanted to release a non-CE version of the scrubber, I suppose they could. But they didn’t. I take that to mean something.

Hey, dive tables are conservative. Many people dive beyond them and don’t get bent. Doesn’t change anything.

You can push your scrubber as long as you feel comfortable with it, no scuba police! I’m just saying what I do.

 

[taimen]

JJ sells both a CE and a Non-CE version of their rebreather (I have the latter). So they are certainly not tied to CE. If they wanted to release a non-CE version of the scrubber, I suppose they could. But they didn’t. I take that to mean something.

They do, the extended range scrubber, available only outside EU. Actually the optional radial scrubber is labeled "for international edition only" in the manual, so they may have lost CE for that too. I think it was CE tested with the analog-JJ previously.
The fact that diving 4 hours (single dive with fresh scrubber, time extended with deco) happens without issues is just a proof that CE runtimes have a good safety margin. Someone doing that kind of dives is probably aware of that and are able to make their decision.

 

[rjack321]

The manufacturer (JJ) publishes guidelines that says the scrubber should not be used for more than three hours. Full stop, no qualifying factors to that.

I don't have a JJ manual but for every manufacture I have actually seen, this isnt the case. They'll say their scrubber is good for X minutes at Y depth in Z temp. Often 40m on air at 4C with 1.6lpm CO2 production i.e. the CE test parameters. This is not diving (there's no ascent, nobody works that hard recreationally, 4C is rare). You are required to interpolate that to your diving. Interpolating to your own diving is not deviance - it might be art I'm not sure. In fact ISC publishes CE ratings and then even does the interpolation for you by giving example diving in Scapa Flow.

Megaldon Scrubber Options - Innerspace Systems Corp

• Gas mix: Air, Depth: 132fsw/40msw, Water Temp 39°F/4°C, 40 RMV/20bpm/2L TV. CO2 production: 1.6lpm. Moderately extreme level of swimming exertion. Time 160 min. CE standard.
• Gas mix: Air, Depth: 132fsw/40msw variable depths, Water Temp 50°F/10°C, CO2 production: Variable. Kick and glide recreational level of swimming exertion. Time 5 hours. The example of this type of diving would be diving in Scapa Flow Scotland doing two 75 min decompression dives a day for two days and then throw the granular material away.

On their 5.5lb radial 160 CE mins turned into 5 hours in the real world right there on the manufacturer's website.

Then there are some units which aren't CE tested and have no formal duration testing at all like the Kiss sidekick and sidewinder.

 

[rjack321]

JJ CE test data is here: https://jj-ccr.com/wp-content/uploads/2018/06/QQ-14-01561-JJ-CCR-June-2014-V1.pdf
202 to 223 mins for standard CE conditions.
I see their 180min recommendation here:
https://jj-ccr.com/wp-content/uploads/2017/09/TECH-SPECS.pdf
Which is also not based on a dive anyone would actually do and is even less based on science and more CYA than the CE data.