This is a photo of a 19cu on my back, (yellow tank- octo clipped on my right BC strap, a complete redundant air source. As a newer diver on a trip to Palau it gave me the confidence I needed. I've only used it once but for that trip it was perfect. On a sad note I did find that a lot of boats in HI did not allow me to bring it on with my gear, for various reasons, but mostly they didn't want to "fuss" with it.
actually- I am using the octo to inflate my surface signal- I just clicked on the picture to enlarge it and saw that- another use.