Also, I remember seeing a picture of a guy in the early days of diving (1940's or so) where he went to around 70 feet on an O2 rebreather and lived.
I really like the way you described this by saying on an oxygen rebreather at 70FSW
(
And not on oxygen at 70FSW as there is a huge difference I will detail later)
Back in my youth and diver training we did what is described as an oxygen tolerance test it was standard procedure 60 FSW in the chamber breathing oxygen from the built in breathing masks (BIBS) for 30 mins
The other test was a 165 FSW dry dive in the chamber on air. If you passed that and the medical and could count to ten and remember your name you passed and proceeded with the 12 week divers course.
Laughing your head off when narked off your head when the instructor told us all not to fart when arriving at 165 foot was apparently not reason enough to kick us all off the course. That occurred six weeks into the course after a week end break from the 7 days a week training and before we started on the underwater explosives part. At which time six out of the twelve men on the course were NRB (not required back)
Now the Royal Navy has a multi purpose semi closed circuit rebreather called the Clearance Divers Breathing Apparatus CDBA that you configured 4 ways
1. Pure oxygen in a closed circuit mode shallow
2. 60/40% mix in a semi closed mode
3. 40/60% mix
4. 32.5% mix for 180 FSW
Prior to that our oxygen fully closed circuit rebreathers could be operated for special operations deeper by
not performing a procedure know as going on the bag.
In normal operations you would fill the bag with pure oxygen twice each time breathing down the bag content and breathing out through your nose. This ensured that little or no residual nitrogen was in your lungs and little to no air was left in the bag. You're now on pure oxygen and metabolically each breath would consume around 3% oxygen of your total tidal lung volume and you would produce the same 3% of Co2 by volume. The Co2 would be absorbed by the chemical soda lime and the bag volume would decrease by the loss of the O2 metabolically together with the absorption into the chemical of the produced Co2 When the bag collapsed or the inherent buoyancy changed you could re inflate the bag manually or allow the flow frit a ruby jewelled disc with a small hole in it to bleed into the bag and compensate for the metabolic consumption known as a jewelled jet. Same today with the current Draeger LAR systems and others.
Now for special operations you knew your residual tidal volume lung capacity and the bag volume so could calculate a fag packet sketch outline of what in effect was an oxygen content for say 50/50 and by not ventilating out the air your lung volume you had an easy made reduced oxygen concentration with the inert N2 loading being the only thing to be aware of was if you allowed the bag to start to collapse as the 02 loading decreased over time/exertion while the N2 volume loading in the bag remained the same while consideration that an pure oxygen gas flush may spike your PPo2 at depth to a dangerous level if you lost the nitrogen loading you had carefully manufactured topside.
Like wise on accent it was not a required standard procedure to vent the bag and flush for fear of a O2 hit a depth.
Put it shorter your original 1940's diver wasn't on pure oxygen at 70FSW but had a good dose of nitrogen in the bag for inserting protection. A special operations procedure not to be recommended or practiced and it was not until years later that a PP02 PPCo2 detector was perfected for oxygen diving and while it was shown here on scuba board some years back it was removed together with the photos I submitted for reasons best left unstated.
If I find them again I will give it another go. I had a post removed a couple of days ago about Italian navy divers and a current commercial salvage attempt. Can't for the life of me think why. Payback can be a tad troublesome. ROFL
