Description of the Inspiration Closed Circuit Rebreather Course:

The Inspiration Closed Circuit Rebreather is the first CE approved closed circuit breathing system. The Inspiration Rebreather gives all the benefits of Nitrox but without the pre-mixing and without the depth limitations.

Differences - The first difference is the way oxygen is added to the breathing loop. Whereas semi-closed rebreathers inject oxygen along with other gases, closed-circuit rebreathers generally consist of at least two independent gas supplies.
One of these contains pure oxygen, which is injected into the breathing loop to make up for the oxygen that is consumed by the diver.
The other gas supply is called the diluent. The diluent usually consists of either compressed air or a special gas mixture such as Nitrox, Heliox, Neox or Trimix.

The diluent gas mixture usually contains enough oxygen so that it can be breathed directly from the cylinder via an open-circuit system at the operating depth of the dive. This supply is used to maintain system volume during excursions to depths where the volume of gas in the breathing loop is compressed.

In some rebreathers the diluent is also used for the emergency open-circuit bailout gas supply in the event of a total system failure of the rebreather apparatus.

The second major difference between closed-circuit rebreathers and semi-closed rebreathers is how the two systems maintain the concentration of oxygen in the breathing loop.

Whereas most semi-closed rebreathers maintain a (more or less) constant fraction of oxygen (FO2) throughout the course of the dive, closed-circuit rebreathers maintain a relatively constant partial pressure of oxygen (PO2) in the breathing loop.

In most cases, closed-circuit rebreathers also incorporate an electronic O2 control system, which automatically adds oxygen when the PO2 drops below a certain level (this level is called the PO2 set-point).

This level of discipline requires a great deal of discipline and training. Thus rebreather divers must have a higher dedication to equipment maintenance and operation than is generally required for open-circuit divers.

Furthermore, rebreathers are generally more complex devices than open-circuit scuba gear, which also accounts for why they require more training time.