Full range mechanical bourdon tube gauges for determining system vacuum. Much more accurate than a compound gauge. All gauges are 1/8" NPT Male bottom connection. 3-1/8" gauges are larger to show vacuum more accurately in inches of mercury and Torr (or millibar). 1-1/2" gauges are used on Yellow Jacket vacuum pumps. CAUTION: Vacuum gauges must be handled carefully. Dropping, pressurizing and rough handling reduce their life.