The AC2/Ellipse Black Regulator 1st Stage The Cressi AC2 piston model. This is a non-balanced first stage with such high performance that it can be compared to lots of excellent balanced models. The small number of components, the low weight and excellent materials make it a great regulator for travel. The AC2 1st stage is a simple, reliable and ultra-durable performing unit .Very easy to be repaired and maintained The Ellipse Black 2nd Stage features the same compact. efficient elliptical shape as the other members of the Cressi Ellipse line, but also features an elastomer dust camp which makes it a very rugged durable unit