Why can't 1st stages sense ambient on RB's?

[Batman]

I have been told that the 1st stage on a Dolphin does not sense ambient pressure, and accordingly does not boost intermeidate pressure in response to changes in ambient. This also appears to be the case on this Kiss CCR http://www.tmishop.com/default.htm.

Why???

I would have thought that a 1st stage only increases intermediate to match increase in ambient, so that the differential pressure (between ambient and intermediate) would stay the same for all devices attached to intermediate hoses. Why would this not be desireable???

Thanks in advance to the board for contributing to my education.

 

[deepblueh2o]

Part of the math for a manual feed rebreather includes orifice diameter and pressure. An increase in the pressure would increare the supplied gas. This usually causes problems with a climbing oxygen particle pressure. So the rebreathers that are not electronically controlled, (turning the gas on when needed and shutting it off when not) have a constant flow that is disrupted by changes in the pressure.

 

[Batman]

Part of the math for a manual feed rebreather includes orifice diameter and pressure. An increase in the pressure would increare the supplied gas. This usually causes problems with a climbing oxygen particle pressure. So the rebreathers that are not electronically controlled, (turning the gas on when needed and shutting it off when not) have a constant flow that is disrupted by changes in the pressure.

If a drager has an intermediate hose pressure of 225 PSI above ambient at the surface, and then you dive it to 130 FSW with a corresponding increase in ambient pressure of 58 PSI (130/33 X 14.7), you now have an intermediate pressure that is only 167 PSI (225-58) above ambient. Would this not slow the flow down below your intended flow rate? Would it not be better for the intermediate pressure to be compensated so that it stayed 225 PSI above ambient???

Thanks

 

[pipedope]

You want constant mass flow of O2.

For a sonic oriface you want the upstream pressure constant, i.e. uncompensated regulator. The mass flow will remain pretty well constant until the downstream pressure reaches the critical point of about half of the upstream pressure.