= Water-column wind-tunnel pressure measurement
NASA's Wind Tunnel Experiment Details, slide 27, describes a practical <manometer> measurement: partially fill a Tygon tube with <water> and connect its two ends to <total pressure> and <static pressure> ports in a <wind tunnel>. The <total pressure> connection pushes its <liquid meniscus> downward; the vertical <length> difference supplies the <pressure> difference. The slide predicts increasing separation as the tunnel <speed> increases. It supplies an apparatus and method, rather than a table of observed readings.<Wind Tunnel Experiment Details, slide 27 by NASA Glenn Research Center>[Wind Tunnel Experiment Details, slide 27: water-filled Tygon tube, total/static pressure ports and height readout]{ref}
A control for this practical is to expose both connections to the same <atmospheric pressure> before the run: settled <liquid meniscus> heights should agree in a symmetric column. Read each <liquid meniscus> at eye level, reconnect sealed tubing to the identified ports, and wait for settling. Recheck the equal-pressure zero afterwards; reverse connections to check the sign. These checks establish neither <mass density> nor ruler accuracy.<University Physics Volume 1 by William Moebs and others>[§14.2, Manometers, Figure 14.12(a): both arms open to atmosphere]{ref}
The inference is a differential <pressure> from <hydrostatic pressure>, with <stagnation pressure> on one side and <static pressure> on the other in NASA's practical. Converting that difference to a flow <speed> additionally needs the hypotheses discussed under <dynamic pressure> and <Pitot-static tube>. A changing <pressure> can leave the column oscillating; leaks, blocked <pressure tappings> or trapped <liquid> in a <gas> connection can alter or delay the readout. The column should be read after settling, and <temperature> should be recorded when choosing the measuring <liquid>'s <mass density>.
Two corrections clarify the limits. If both <gas> ports are at the same height and carry the same <gas> of constant <mass density> $\rho_g$, keeping its <hydrostatic pressure> gives $p_L-p_R=(\rho-\rho_g)g\Delta h$. Different port heights require their separate <gas> column heights. Also, a curved <liquid meniscus> can sustain a <capillary pressure> jump through <surface tension>. Write $J_L,J_R$ for <liquid> <pressure> minus <gas> <pressure> at the two surfaces. Neglecting <gas> weight, the balance becomes $p_L-p_R=\rho g\Delta h+J_R-J_L$. Identical meniscus shapes in matching clean bores cancel those jumps; unequal bores or different wetting conditions need not. Thus the useful simplicity of $\rho g\Delta h$ rests on identifiable physical conditions.
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