Microns of Mercury (µHg)

Micron or Micrometre of Mercury is a very small pressure unit which is derived from the pressure due to gravity that is generated by a 1 micrometer (1/1000mm) column of liquid mercury. 1 micron of mercury at zero degrees Celsius equals 0.133322 pascals.
The main use for micrometers of mercury is to measure high vacuum.

The micron of Hg pressure unit is treated as being equal to 1 millitorr, however although the mtorr is a fixed value, the mercury measurement can vary slightly around the world depending what is assumed for the density of mercury and local gravity.