= Atmospheric pressure
{title2=$p_{\rm atm}$}
{wiki}
<Atmospheric pressure> often provides the reference for a <gauge pressure> measurement: $p_g=p_{\rm abs}-p_{\rm atm}$. The absolute <pressure> is measured relative to an ideal <vacuum>, whereas zero <gauge pressure> means equality to the chosen reference. The actual <atmospheric pressure> can vary; it must be specified when converting between these descriptions. https://openstax.org/books/university-physics-volume-1/pages/14-2-measuring-pressure[OpenStax: atmospheric, gauge and absolute pressure]{ref}.
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