Micrometer U-Tube Manometers for Medium-Vacuum Measurements

A family of U-tube manometers, suitable for use as standard instruments, has been designed and constructed at the National Bureau of Standards to cover the range of pressures from 100 to $1 × 10−2$ Torr. In these instruments the levels of the two liquid surfaces are measured by adjusting micrometers with conical points ground on the ends of the spindles. One hundred-millimeter and 50-mm range mercury manometers, with spindles approaching the liquid surface from above, are used for differential or absolute pressure measurements up to 100 Torr. Pressures less than 3.5 Torr are measured with oil manometers, in which the micrometer spindles approach the surfaces from below. Measurements made with the mercury manometer have an uncertainty of about $4 × 10−3$ Torr plus 8 parts in $105$ of the reading. Measurements made with the oil manometer have an uncertainty of about $4 × 10−4$ Torr plus 1 part in $104$ of the reading.