Should have updated the page prior to posting. But after all that typing it can stay.
Air comes into the 2nd stage via the valve spindle. At the other end of the spindle is the controls for the venturi & sometimes a cracking pressure control. In the middle of the spindle is an orifice to allow air into the body of the 2nd stage. These controls do completely different things.
The venturi is a vane that slides around the outside of the spindle. In the dive position it directs air at the mouthpiece. This creates a lower pressure at the diaphragm, which is pulled (sucked) inwards. As we inhale it moves the lever to open the shuttle valve & allow air to flow. At this point no further effort is required to maintain the flow. In fact to stop it we have to exhale into the 2nd stage & change the pressure to reverse the above & close the flow down. With the control set to the pre dive position the vane now redirects the air away from the mouth piece & so raising the pressure inside the 2nd stage. The increased pressure pushes (blows) the diaphragm out, raising the lever & closing the shuttle valve & stopping the flow. We can still breath with it in this position, but the flow will now stop when we stop inhaling, unlike in the dive position.
The cracking pressure control knob slides inside the valve spindle & as it's screwed in, it increases the pressure on the shuttle valve spring. This increased pressure means it will take more inhalation effort to open. The response you get from screwing the knob in & out out will depend on where it's tuned to at the present.
I have my main reg tuned to just short of free flowing when the knob is screwed all the way out. For 99% of my no stop diving I never have to alter this setting. Sometimes when deep, swimming into a current or zipping about on a DPV, it can become necessary to de tune the 2nd a little due to a slight free flow caused by pressure on the 2nd stage face plate. This is dealt with by screwing the knob in until the slight free flow stops.
Air comes into the 2nd stage via the valve spindle. At the other end of the spindle is the controls for the venturi & sometimes a cracking pressure control. In the middle of the spindle is an orifice to allow air into the body of the 2nd stage. These controls do completely different things.
The venturi is a vane that slides around the outside of the spindle. In the dive position it directs air at the mouthpiece. This creates a lower pressure at the diaphragm, which is pulled (sucked) inwards. As we inhale it moves the lever to open the shuttle valve & allow air to flow. At this point no further effort is required to maintain the flow. In fact to stop it we have to exhale into the 2nd stage & change the pressure to reverse the above & close the flow down. With the control set to the pre dive position the vane now redirects the air away from the mouth piece & so raising the pressure inside the 2nd stage. The increased pressure pushes (blows) the diaphragm out, raising the lever & closing the shuttle valve & stopping the flow. We can still breath with it in this position, but the flow will now stop when we stop inhaling, unlike in the dive position.
The cracking pressure control knob slides inside the valve spindle & as it's screwed in, it increases the pressure on the shuttle valve spring. This increased pressure means it will take more inhalation effort to open. The response you get from screwing the knob in & out out will depend on where it's tuned to at the present.
I have my main reg tuned to just short of free flowing when the knob is screwed all the way out. For 99% of my no stop diving I never have to alter this setting. Sometimes when deep, swimming into a current or zipping about on a DPV, it can become necessary to de tune the 2nd a little due to a slight free flow caused by pressure on the 2nd stage face plate. This is dealt with by screwing the knob in until the slight free flow stops.
