Open Circuit, Closed Circuit

[Gerbs]

In the early days they didn't have modern first stage set-ups
You had a round "first stage" about the size of a small dinner plate and two hoses going into/out of it.
This was essentially a semi-closed circuit regulator.
Air would be breathed and then sent back to the first stage which would eventually release excess air. I don't remember if there was any major scrubbing going on.

Eventually the first single hose regulator (AKA open circuit) was made and this is where the distinction came from.

As for where the terminology came from, I don't know the exact history timeline for it.

Ermm... BZZZZZZZT.

Twin-Hose regulators (both single stage and twin stage) are open circuit as well. No air is recycled. And yes, just because it's a big box on the tank valve does not mean that it doesn't have 1st and 2nd stages. (Earlier designs regulated tank pressure to ambient in a single stage, later on, similar to two stage single hose regulators, a 1st stage regulated to intermediate pressure, and a 2nd stage regulated intermediate to ambient, although both were included in the same box).

Semi-closed is a variation of closed circuit, where instead of just adding O2 as it is depleted, part of the breathing loop is replaced with a fresh gas mix, either constant flow or breathing controlled.

AFAIK the open circuit / closed circuit terminology is as old as sports diving (Cousteau with open circuit, Hass with closed circuit)

Gerbs

 

[Akimbo]

To the best of my understanding, at the time Cousteau/Gagnan developed the demand regulator for breathing air, the only available open circuit SCUBA (Self Contained Underwater Breathing Apparatus) was the Fernez/Le Prieur gear from the mid 1920s. It was little more than an HP cylinder, manually operated valve, full face mask, and a side-slung harness. All the other SCUBA and SCBA systems available for divers, mine safety, and firefighters were pure oxygen closed circuit rebreathers.

Practical mixed gas semi-closed and electronic closed circuit rebreathers started to show up in volume in the 1960s. Desco is credited with developing the first closed circuit mixed gas SCUBA rebreather in 1937, but it was not used in military or commercial work.

There was some motivation to support military special forces, but the cost and logistics of conserving Helium for salvage, rescue, and submarine based espionage/spying were probably bigger motivators. SEALs almost exclusively use pure oxygen CCRs today since the depth is rarely a significant operational limitation and the simplicity, ruggedness, and reliability are so much greater.

 

[Walter]

Walter, there was absolutely nothing about rebreathers or the history of SCUBA taught in my OW course.

Thanks for the feedback.