Brendan1 said:
yes this is what is confusing me. I think is because the vessel diameter is 200mm length 2m and the pipe is 8mm and varies in length so if you were trying to move the the volume of air in that big vessel through a small pipe would this not cause a spike in pressure?
No. Under no circumstances should the pressure downstream of your reservoir end up higher in pressure than your reservoir. Assuming sufficiently high reservoir pressure, as soon as you open the valve (or rather, after some short but finite development time), you will have a shock wave that propagates downstream through your tube and an expansion wave that propagates upstream through your tank. That expansion wave is what essentially sets the gas in the tank in motion out through the tube. There will be some period of quasi-steady operation as the expansion propagates backward, at which point the total pressure in the tube (behind the shock) should match the total pressure in the reservoir in the region that has already been accelerated. In the region of the tube that hasn't seen the shock yet, you would still be at atmospheric.
This doesn't account for viscous losses, but should get you pretty close. There is no way for the pressure in the tube to rise above that of the tank, though, as viscosity would only serve to dissipate energy (and therefore total pressure) in the region with moving gas (i.e. the tube).
Brendan1 said:
Yes that was what i meant they won't detect steady pressure, yes they all have the safe cut off frequency.
Be careful with your terminology. Static pressure has a very specific meaning in fluid mechanics, and is typically synonymous with the thermodynamic pressure. That's the pressure felt by the surface of an imaginary object in the flow (without slowing down or otherwise changing the flow). Total pressure, in either case, is the pressure observed if you slow the flow down isentropically to zero velocity. It is also a measure of the total energy pool available to the flow. That's all pretty straightforward.
Here's where this is going to get tricky; the relationship between static, total, and dynamic pressure is going to be different for compressible and incompressible flows. In an incompressible flow, you use a Pitot tube to measure total pressure and a static pressure port to measure the static pressure, and the difference between the two is the dynamic pressure, ##\rho V^2/2##, which is essentially the kinetic energy in the flow due to the bulk fluid motion. For a compressible flow, this isn't true. You now have to deal with the fact that internal energy is not constant in the fluid and can also play a role in the energy balance. It's a term in the energy equation that vanishes for incompressible flows and leads directly to Bernoulli's equation, but can't be ignored in a compressible flow.
Your flow is absolutely compressible.
russ_watters said:
Note, that when it comes to air, every reading is a differential (change in) pressure. There's no problem with that; you just have to subtract-out the reference as needed (per my description of the dynamic pressure, above).
This is not correct. You can buy both absolute and differential pressure transducers. Not every measurement is differential. It is, of course, of paramount importance to know which you have.
russ_watters said:
I'm not certain of what you are trying to say, but air flows from areas of high total pressure to low total pressure - that's what makes it move. You might see a drop in static pressure and then a rise again if pipe size changes, but you should never see a total pressure above the vessel pressure.
This is also not true. Fluids
accelerate from areas of high pressure to low pressure. They move all sorts of ways depending on the pressure gradient, viscosity, gravity, and their inertia. A fluid with sufficient inertia can certainly move from a low pressure region to a high pressure region. It will just slow down when it does so.