= Gauge pressure
{title2=$p_g=p-p_{\mathrm{atmosphere}}$}
<Gauge pressure> is <pressure> measured relative to the surrounding <atmospheric pressure>; absolute <pressure> is instead measured relative to an ideal <vacuum>. At an open surface $p_{\mathrm{surface}}=p_{\mathrm{atmosphere}}$, so the constant-<mass density> <hydrostatic pressure> law becomes $p_g=\rho_fgh$. Here $p_g=0$ at the surface denotes equality with <atmospheric pressure>. It does not denote zero absolute <pressure>. <University Physics Volume 1 by William Moebs and others>[OpenStax, University Physics, §14.2: gauge and absolute pressure]{ref}
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