Inch of mercury
An inch of  (inHg) was a measurement unit of pressure.

The atmospheric pressure in the pressure chamber aboard the  was measured using this unit. 

In 2267, henchman loyal to Khan Noonien Singh placed Captain James T. Kirk in this chamber, threatening him with suffocation and as a warning to the command crew of the starship on what would happen to those who did not join Khan's side. Before the atmospheric pressure in the chamber zeroed out, Lieutenant Marla McGivers brought the pressure back up to 30 inHg and released him from the chamber. ()

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Measurements