An open tube manometer measures the gauge pressure, p = p0 + hρg, where p is the pressure being measured, p0 is the atmospheric pressure, h is the difference in height between the manometric liquid of density ρ in the two arms. For a given pressure p, the product hρ is constant. That is, ρ should be small for h to be large. Therefore, for noticeably large h, laboratory manometer uses a low density liquid.
