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