Owner`s manual

Dive Rite

Ch. 1 Pg. 6

a demand valve with fresh oxygen rich gas. Since there is venting of used or excess gas while

at a constant depth, the descriptive name of “semi-closed-circuit” is appropriate. The amount

and quantity of exhaled gas will depend on the flow rate of the replacement gas source, whether

its fixed or tied to the diver’s respiration rate.

The main limitation of SCRs is that the system usually has to be setup for a specific target depth,

using a specific gas and predefined flow rate prior to the dive. The current configuration limits

the dive to the gas's physiological limits. Current designs have become more user friendly, but

the basic breathing loop remains unchanged from the earliest units.

Closed-Circuit Systems

The closed-circuit system operates on the same concept as a

"semi", but does not inject gas at a fixed rate nor does it lose any

gas to ambient except during ascents. There are many variations of

closed-circuit design that have evolved over the years. There are

rebreathers that are being sold as “closed” systems that continually

inject a very small amount of oxygen. These systems in reality are

a hybrid system because while the diver is breathing off the loop

they normally act the same as what is traditionally called a closed

circuit system but when the diver is off the loop they will

continually vent gas.

The first CCR systems used a single gas supply that was usually

100% Oxygen and were generally called Oxygen Rebreathers.

These are the simplest form of all rebreathers and have a very

limited maximum working depth due to the inability to vary the breathing mix. The limited depth

and dangers associated with the use of oxygen underwater have restricted the use of oxygen

rebreathers to military and scientific applications.

The oxygen rebreather is special in that it is the only rebreather type that can be truly bubble-less.

As the diver ascends, the gas in the breathing loop expands and normally requires excess gas to

be vented, but on an Oxygen rebreather, gas addition can be prevented and the loop can be

reduced by metabolizing the oxygen prior to ascent. Oxygen rebreathers are still used by most

militaries and find considerable use in non diving life support systems (fire, mine rescue, space

ect.).

With the advent of electronic control and monitoring of the breathing loop, two types of

electronic CCRs developed: constant percentage of Oxygen (fO ) and constant dosage of Oxygen

(PO ). Current designs have standardized on constant PO loop.