# Re: Aquatic Plants Digest V5 #112

```> Date: Thu, 23 May 2002 17:40:32 -0500
> From: "Tom Wood" <tomwood2 at flash_net>
> Subject: zxcvbob - Then prove it.
>
> The assertion: That a needle valve is not a valve unless the gas passing
> through it reaches the speed of sound.
>
> That is the assertion that I ridiculed. Prove me wrong.
>
> Don't conjure up some high-pressure, high volume jet engine example. Let's
> say around 20 - 40 psi in, around 2 - 5 bubbles per second out, using a
> typical off the shelf needle valve. You assert that the valve will not act
> *as* a valve unless the gas passing through it reaches the speed of sound.
> Prove it.

You're now trying to change the subject.  Your original assertion was
that the speed of the gas was irrelevant.  My assertion was that your
ignorant ridicule was arrogant and stupid.  I never commented on the
details of control valve theory or fluid dynamics; that has been
discussed here many times.  But to summarize it simply (so you can
understand it) the flow rate through the valve becomes less dependant on
the input pressure as the velocity through the valve approaches the
speed of sound.  When the velocity equals the speed of sound, increasing
the input pressure does not give you "sonic boomlets", it just increases
the backpressure on the valve.  The flow rate remains constant at that
point. (I think it's called the "choke point"?)

I won't really go into turbulent flow, because I don't understand the
theory well enough to explain it.  It really is rocket science.  But
with turbulent flow, the flow rate is nonlinear with respect to
pressure, and a large increase in input pressure will give a relatively
small increase in the flow rate.  I think most of us operate our needle
valves or other flow restrictors well into the turbulent flow range,
which helps make the CO2 delivery systems stable even if the regulator
pressure drifts up near end-of-tank.

Regards,
Bob
```