Question Regulator Physics

[garywong007]

What changes would occur inside the 1st and 2nd stage if a diver descends for 10 meters while holding his breath? Will the 2nd-stage starts to free flow as the volume of air in the housing gets smaller? What about the IP in the 1st stage? Will the IP remain the same as before the descend or it will nevertheless increase as the ambient pressure increases?

 

[TrimixToo]

First stages have all been depth-compensated for a really, really long time. So the IP should remain at the same gauge pressure as one descends. Likewise, sealing off the 2nd stage chamber (via breath holding with a closed airway) will increase the pressure across the diaphragm until it hits cracking pressure. At that point, the valve will open enough to keep the 2nd stage full at something close to ambient pressure until the diver stops descending.

Free flow--the uncontrolled release of gas from a flowing second stage--should not occur under these circumstances for the regulator in the diver's mouth. The probability of a freeflow occurring on the other regulator is unchanged by what's happening with the primary.

 

[Nemrod]

As you descend the second stage diaphragm will be depressed by increasing water pressure until approximately 1.5 inches of H2O ( a typical average cracking force) and the LP seat will be moved from the orifice and air will flow into the second stage. And as air flows into the second stage the IP pressure will drop (in the hose) causing the first stage ambient diaphragm or piston to open the HP seat from the HP orifice and air will flow from the tank and into the hose connecting the stages and the set IP will be restored. The IP is constant over ambient (well, it may increase, decrease slightly with tank pressure depletion over the course of a dive depending upon design). When ambient pressure is restored in the second stage the diaphragm will equalize, the LP seat will close on the orifice, IP will be restored to set pressure and the first stage seat and orifice will then close.

 

[garywong007]

First stages have all been depth-compensated for a really, really long time. So the IP should remain at the same gauge pressure as one descends. Likewise, sealing off the 2nd stage chamber (via breath holding with a closed airway) will increase the pressure across the diaphragm until it hits cracking pressure. At that point, the valve will open enough to keep the 2nd stage full at something close to ambient pressure until the diver stops descending.

Free flow--the uncontrolled release of gas from a flowing second stage--should not occur under these circumstances for the regulator in the diver's mouth. The probability of a freeflow occurring on the other regulator is unchanged by what's happening with the primary.

Thank you John. In other words, since the body of air inside the 1st-stage at 10 meters is the same body or air inside the 1st-stage when diver is on the surface, there is no exchange of air and thus no change of IP, right?

 

[garywong007]

As you descend the second stage diaphragm will be depressed by increasing water pressure until approximately 1.5 inches of H2O ( a typical average cracking force) and the LP seat will be moved from the orifice and air will flow into the second stage. And as air flows into the second stage the IP pressure will drop (in the hose) causing the first stage ambient diaphragm or piston to open the HP seat from the HP orifice and air will flow from the tank and into the hose connecting the stages and the set IP will be restored. The IP is constant over ambient (well, it may increase, decrease slightly with tank pressure depletion over the course of a dive depending upon design). When ambient pressure is restored in the second stage the diaphragm will equalize, the LP seat will close on the orifice, IP will be restored to set pressure and the first stage seat and orifice will then close.

Thank you Nemrod. Let me try to visualize what you describe here. That means the 2nd-stage will never go into an uncontrolled free flow because once the cracking point is reached, only a small amount of air (just enough to equalize the pressure inside the housing and the ambient pressure) will "seep" into the housing and then the orifice is closed again until the next cycle. Am I correct?

 

[Nemrod]

Thank you Nemrod. Let me try to visualize what you describe here. That means the 2nd-stage will never go into an uncontrolled free flow because once the cracking point is reached, only a small amount of air (just enough to equalize the pressure inside the housing and the ambient pressure) will "seep" into the housing and then the orifice is closed again until the next cycle. Am I correct?

Yes, that is the gist of it. And no, your regulator will not free flow with it inserted in your mouth (or flooded out if your octopus) unless it is in need of service or some other defect. No, it will not free flow. You are correct.

 

[garywong007]

Yes, that is the gist of it. And no, your regulator will not free flow with it inserted in your mouth (or flooded out if your octopus) unless it is in need of service or some other defect. No, it will not free flow. You are correct.

Thank you again. This is helpful.

Have a good day.

 

[TrimixToo]

Thank you John. In other words, since the body of air inside the 1st-stage at 10 meters is the same body or air inside the 1st-stage when diver is on the surface, there is no exchange of air and thus no change of IP, right?

The IP being relative to ambient pressure, the first stage valve will open to maintain the IP without the 2nd stage valve opening during a breath-holding descent. When the 2nd stage valve opens, the first stage valve will also open to maintain the IP.

 

[Zef]

If water is allowed to enter the 2nd stage (no mouth on the 2nd stage regulator) the pressure will be equalized in and outside the 2nd stage, the diaphragm won't move and depress the demand lever and the valve (poppet against orifice) will remain shut.

If there is a mouth on the 2nd stage that keeps water from ingressing and the same amount of air/same air that there was on the surface remains in the body of the 2nd stage, then the ambient pressure while decending will push against the diaphragm and cause the demand lever to depress which will open the demand valve and allow air to flow from the hose connecting the 2nd stage to the 1st stage. Air will flow out the hose into the 2nd stage until the pressure in the 2nd stage balances with the ambient pressure outside the 2nd stage. The first stage HP valve will open as a result of the drop in pressure in the hose between the 1st and 2nd stage, and will subsequently close when that pressure equals the IP pressure the 1st stage is set to...that coincides with the 2nd stage demand valve closing.

-Z

 

[garywong007]

If water is allowed to enter the 2nd stage (no mouth on the 2nd stage regulator) the pressure will be equalized in and outside the 2nd stage, the diaphragm won't move and depress the demand lever and the valve (poppet against orifice) will remain shut.

If there is a mouth on the 2nd stage that keeps water from ingressing and the same amount of air/same air that there was on the surface remains in the body of the 2nd stage, then the ambient pressure while decending will push against the diaphragm and cause the demand lever to depress which will open the demand valve and allow air to flow from the hose connecting the 2nd stage to the 1st stage. Air will flow out the hose into the 2nd stage until the pressure in the 2nd stage balances with the ambient pressure outside the 2nd stage. The first stage HP valve will open as a result of the drop in pressure in the hose between the 1st and 2nd stage, and will subsequently close when that pressure equals the IP pressure the 1st stage is set to...that coincides with the 2nd stage demand valve closing.

-Z

Thank you! This explanation is all helpful!