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From Wikipedia:

Bourdon tubes measure gauge pressure, relative to ambient atmospheric pressure, as opposed to absolute pressure;

But looking at pictures such as the one above, it seems there is only one connection into the tube, the one where the gas whose pressure is being measured enters. It would seem that the deformation of the tube is only dependent on the pressure of the gas. So why does the Bourdon tube measure gauge pressure relative to atmosphere instead of absolute pressure?

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The atmosphere is acting on the outside surface of the tube.

So any internal pressure is acting against the atmospheric pressure, which means if you need absolute pressure readings you need to accurately determine the atmospheric pressure at that height and compensate to sea level as necessary.

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  • $\begingroup$ In most gauges I've seen the casing of the gauge seems pretty airtight. Is there some kind of hole or a valve that allows ambient air inside it? $\endgroup$ – S. Rotos Sep 20 '19 at 19:38
  • $\begingroup$ Most gauges I have seen, taken apart and dealt with are definitely not airtight... $\endgroup$ – Solar Mike Sep 20 '19 at 19:41

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