re: The Emperor's new cloths

[Roger Miller <rgrmill at rt66_com>]

Tom wrote:

> A painfully simple experiment disproves everything Dave has asserted. Set
> your regulator to 20 psi and your bubble rate to, say, 2 bubbles per second.
> Now, leave the needle valve alone and increase the pressure from the
> regulator until the bubble rate no longer increases. Got it? The valve is
> now running "sonic", aka choked.  Or at least close to it.

No.  I studied the article at Control Engineering that Dave refered you to.  
Your procedure is the wrong test.  The valve is running choked when the flow 
rate at a contant inlet pressure remains constant while the *outlet* pressure 
is changed.

To find out if your valve is running "choked" you would keep the regulator 
output pressure constant and move the outlet tube to different levels in the 
aquarium, or maybe insert it into a pump inlet to get different outlet 
pressures.  If those changes don't cause changes in the flow rate, then the 
valve is choked.  In realitiy, those pressure changes are probably too small 
to make very much difference.

> The assertion that a valve has no value in controlling a cylinder dump is
> simply astonishing. Anyone else care to comment on that while we wait for
> the expert?

Perhaps I'm not as easily astonished as you.  Just the same, I don't see any 
information in the article that Dave cited that supports the idea that the 
valve needs to run "choked" in order to prevent a dump, or that running 
"choked" would even be protective.   Perhaps Dave can be more instructive.

Looking at the example graph of  pressure drop vs flow rate it appears to me 
that a valve that is not necessarily running choked still provides dump 
protection.  The curve that describes the relationship between flow rate and 
pressure is concave downward.  An increase in the pressure drop across the 
valve (as during the end-of-tank dump) does lead to an increase in the flow 
rate, but the increase in flow rate is much smaller than it would be if the 
relationship were more linear -- i.e. if the valve were not there.

Now, would anyone like to discuss the color of pure water in a white bucket?

Roger Miller