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Let's say I have a closed, non-transparent metal container with pressurized gas and I'd like to measure the pressure inside without changing that pressure (or changing it as little as possible).

The thickness of the (metal) container walls isn't known, and there is no replica of it you could take apart to measure that; for example it may be a military tamper-proof device with a pressure sensor in the middle that destroys valuable data if that pressure changes, so the only way to "defuse" it would be to open it in a room which pressure is as close as possible to the box's pressure.

The gas in the container has no special properties and is only designed to keep that pressure; nitrogen gas seems to be a plausible option.

The sensor of course isn't perfect and has to leave a tolerance margin for vibrations and temperature changes, however we shouldn't rely on that and if possible the measurement shouldn't change the pressure at all.

There is a similar example on the Wikipedia page for the observer effect, but it's just a short example and a search for measure pressure observer effect doesn't find anything relevant. This similar question - Find pressure of liquids without making contact with it - is only about liquids and the answers suggest to look at the quantity of liquid inside to estimate its quantity, however for a compressible gas that doesn't seem possible.

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  • $\begingroup$ Define "altering" -- you might pick an EM wavelength which does pass thru the container, or you might send an acoustic ping & measure the response parameters. $\endgroup$
    – Carl Witthoft
    Commented Dec 21, 2014 at 21:26
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    $\begingroup$ Yes, and even a tiny ping will adjust the energy inside the container, leading to a change in either pressure, temperature, volume or some combination thereof. You don't seem to grasp the concepts involved here. $\endgroup$
    – Carl Witthoft
    Commented Dec 22, 2014 at 3:06
  • $\begingroup$ @CarlWitthoft I edited my question, is it clear now ? Thanks. $\endgroup$
    – user68162
    Commented Dec 22, 2014 at 14:20

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Perhaps you could use the expansion of the pressurized container to measure the internal pressure? Even a metal container will expand by a few microns if it pressurized - this expansion could be measured using lasers. Does that meet your definition of 'not altering'? Although I suppose even lasers would heat the container by a miniscule amount, which could be classed as altering ...

If you can't measure the size of the container beforehand then the ultrasonic 'ping' idea might work. The speed of sound in the gas will increase if it is pressurized (higher density), so perhaps you could measure the time it takes for a ping to bounce off the opposite wall and calculate from that?

Another idea is that if the container is pressurized then the walls will be in a state of elastic strain. This will affect stiffness/resonance properties, which could be measured, although in this case you would probably need to conduct experiments on a similar, unpressurized container first.

Do you know what the gas is inside?

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  • $\begingroup$ @AndréDaniel your comment makes no sense. $\endgroup$
    – Carl Witthoft
    Commented Dec 22, 2014 at 3:06
  • $\begingroup$ @AndréDaniel you can calibrate the setup by putting known pressures inside the container. It is actually more about the pressure difference between the inside and outside, so you could also apply different pressures on the outside. $\endgroup$
    – fibonatic
    Commented Dec 22, 2014 at 3:26
  • $\begingroup$ I assume you must first measure the dimensions of the container with no pressure in it, then measure the dimensions of the pressurized container and compare the two, right ? $\endgroup$
    – user68162
    Commented Dec 22, 2014 at 3:27
  • $\begingroup$ Yes, that was my idea. I'll post another couple of ideas, in case you can't measure the container beforehand. Are you able to experiment with another similar container - pressurize/depressurize, etc.? $\endgroup$
    – Time4Tea
    Commented Dec 22, 2014 at 5:12
  • $\begingroup$ Also, why do you need to know? Do you actually need to do this for real, or is it a theoretical assignment question? $\endgroup$
    – Time4Tea
    Commented Dec 22, 2014 at 5:13