[Karsten (OP)]

 I cannot think of a way to do this. Not really urgent, just curious. I was staring at some pipes in a  hotel room yesterday and began to wonder if they are pressurized, if yes how much, and how I could find out without drilling a hole in them or opening them up (which I have done in another place just to find out that I could not close it again and flooded my bathroom and hall with very hot water, soaked the downstairs neighbor's ceiling, and drenched the downstairs neighbor's bathroom as well  [:I]).

[Geezer]

Yes. The pressure does compress the fluid slightly, so its density will increase slightly also. This will alter the rate at which sound propagates through the fluid, so you should be able to determine the pressure by acoustic means.

(I've no idea if this is really true, but it sounds plausible  [])

[Mazurka]

It is possible to measure flow rates without putting a sensor into a pipe using an ultrasonic device based upon the doppler effect.  With enough of the other parameters, I think it would be possible to derive a pressure measurement for a specific set of pipes/ fluid, but I do not think that there is a general device that can measure pressure remotely.

If there is and it can work on a saturated mixed gas in a HDPE pipe in harsh conditions, my former employer would be very interested.  This is becasue one of the biggest problems with landfill gas extraction systems is balencing extraction rates across a site and detecting problems often involves drilling holes in pipework until it resembles swiss cheese!

[Karsten (OP)]

Interesting. It seems to be difficult to do. So it sounds like that sound will travel at a higher rate through a denser medium. Will it travel through the unchanged metal pipe faster though? If I tap the pipe with a hammer, will there be three "bings" arriving at my sound measuring device 6 feet away? One that arrived via the pipe, one via the contents of the pipe, one via surrounding air? May I assume that the direction and speed of flow of the contents have an influence as well?

[Geezer]

I think you hit the nail on the head, or the pipe with the hammer if you prefer  []

If there was a section of plastic pipe acting as an acoustic isolator, it might help. Alternatively, it may be a case of selecting the right frequency (or frequencies). We'd probably need to know the temperature of the fluid as well.

[Bored chemist]

There are circumstances where you "hear" two dings.
If someone sets of a big bomb far away from you you feel the shock-wave come through the ground before you hear the bang.

[Geezer]

There are circumstances where you "hear" two dings.
If someone sets of a big bomb far away from you you feel the shock-wave come through the ground before you hear the bang.

Yes. I think something would be required to exclude the wave in the actual pipe. BTW, even if the fluid is flowing, this method might still be made to work by measuring the speed in both directions and taking the average.

Somebody must have done this before.