This is the absolute measuring instrument for vacuum which gives pressure exerted by permanent gases. The gauge works on the principle of Boyel’s law of gases. Two parallel capillary tube are connected to a common mercury bowl. The rise of mercury column in the capillary gives the vacuum in mmhg.
Ranges :
1) 2 Torr to 0.001 Torr
2) 10 Torr to 0.01 Torr.