Decompression chamber controls, with atmospheric pressure measured in inches of mercury from 0 to 50
An inch of mercury (inHg) was a measurement unit of pressure.
The atmospheric pressure in the pressure chamber aboard the USS Enterprise 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. (TOS: "Space Seed")
For reference, 30 inHg, or more accurately 29.92 inHg, is equivalent to one standard atm at sea level.