In a region such as outer space, which is virtually void of gases, the pressure is essentially zero. Such a condition can be approached very nearly in a laboratory when a vacuum pump is used to evacuate a bottle. The pressure in a vacuum is called absolute zero, and all pressures refernced with respect to this zero pressure are termed absolute pressures.
Many pressure-measuring devices measure not absolute pressure but only
difference in pressure. For example, a Bourdon-tube gage indicates only
the difference between the pressure in the fluid to which it is tapped and
the pressure in the atmosphere. In this case, then, the reference pressure
is actually the atmospheric pressure. This type of pressure reading is
called gage pressure. For example, if a pressure of 50 kPa is
measured with a gage referenced to the atmosphere and the atmospheric
pressure is 100 kPa, then the pressure can be expressed as either
p = 50 kPa gage or p = 150 kPa absolute.
Whenever atmospheric pressure is used as a reference, the possibility exists that the pressure thus measured can be either positive or negative. Negative gage pressure are also termed as vacuum pressures. Hence, if a gage tapped into a tank indicates a vacuum pressure of 31 kPa, this can also be stated as 70 kPa absolute, or -31 kPa gage, assuming that the atmospheric pressure is 101 kPa absolute.