CapnLoki wrote:You just proved my point: when the pump is used in a way the it was NOT designed, and performs NO useful purpose, then it creates a breeze. Otherwise, it creates a pressure. Its really just that simple.
I'm afraid you don't understand the concept.
Pressure is a result of resistance to flow. if you have no resistance to the flow, you have *NO PRESSURE*. no matter what the amount of flow coming from (what is it that cpap manufacturers call that blowy thing? pressure pump? no, pressurizer? no, pressure generator? no...
FLOW GENERATOR that's it!) from the
flow generator, until you obstruct that flow, you have
now pressure.
hook a manometer up to the hose of your cpap, turn it on with nothing on the end of the hose, set the thing to 10cm... what pressure will your manometer read? nothing, or very near it, even though the FLOW will be at the maximum, because the pump will be running at full speed, because the pressure is below the set point.... even though it may be blowing 300 lpm, or more.
put a blocked mask on the end of the hose, with a standard vent, the flow will encounter the blockage, and then you'll get pressure, and the blower will slow way down to where the flow is only some 30lpm.
the machine generates *FLOW*, not pressure. it's set for pressure because setting it for a given flow (which, actually, is how old cpaps worked) is greatly ineffective, because as the resistance to flow changes (decreasing when you inhale, increasing when you exhale) the
pressure would swing wildly, it would be more difficult to inhale and much harder to exhale, and you'd have to adjust the machine based on altitude.
CapnLoki wrote:palerider wrote:CapnLoki wrote: An electric outlet is known by its voltage (electrical pressure), the current it supplies depends on the resistance of the receiving device.
not an apt analogy. flowing water is appropriate. flowing electrons, not so much.
To a physicist, the differential equations are identical. In fact, understanding the analogy is one of the first steps in becoming a physicist.
Pumps, whether they pump air, water, or electrons, are defined in terms of pressure and flow rate depending on their purpose. Some are rated in PSI or Gallons per minute or Head or Volts or Watts. In our case, the key spec, essentially the only spec of import for us, the users, is pressure. The specs may mention volume of air somewhere, but almost everything I see relates to the pressure.
ya know, you're right about the second part, however you're *again* wrong about your first comment.
take your initial idea. an electrical outlet has, say, 120v rms on the hot wire, and zero v on the neutral, referenced to ground.
you can measure that with a multimeter, from hot to ground.
now, plug in your toaster, which has a fairly low resistance. measure the hot side, still 120vrms to ground, because you've got a huge amount of flow available. measure it AFTER THE RESISTANCE... what do you get? zero, because there's no more resistance, even though there's still plenty of current flowing.
get in your car, turn on the headlights, crank it, notice the lights dimming? that's because your 12v is being pulled lower, the pressure is leaking out through the less than infinite resistance of the starter, the pressure is dropping, more electrons are going through than the battery can supply, so the pressure drops.
there are tables in the back of the resmed clinical manuals that show the maximum flow that the flow generator can produce to maintain a pressure against excessive leaks. if it generated "PRESSURE" then none of that would matter. it wouldn't matter how big a leak the mask had.
