It is the standard atmospheric pressure at sea level. It is equal to 1 atm.
A compound pressure gauge measures two scales, one in positive pressure and one in negative (below atmospheric) pressure or vacuum positive pressure is measured in pounds per square inch (psi) or kilopascals or in some cases bars (1 bar = 14.35 psi) [ the average atmospheric pressure at 1000 feet above sea level] vacuum is measured in millimeters of mercury mmHg or inches of mercury inHg
1 atmosphere, at sea level.
Because that is the definition of 1 atmosphere at standard temperature.
The basic principle of barometer is Torricelli's experiment. Imagine that you fill a tube of one metre length (100cm) with mercury up to the brim. Then close the mouth of the tube with your thumb and invert the tube and place it in a cistern of mercury with its end immersed in the mercury say some 4 cm. Now you remove your thumb. Mercury would come down but it stops at one level to our astonishment. This level would be some 20 cm from the closed end of the tube. So there will be almost 76cm column of mercury in the tube. Why is it so? As mercury comes down the space left vacant will not be having any air molecule and so it is almost vacuum. This is named as Torricelli's vacuum. Now the air outside in the atmosphere would exert a pressure and wants to push the mercury towards this vacuum place. But the weight of the mercury column (76cm) will balance this pressure. Hence we say the atmospheric pressure will be 76 cm of mercury. This is how we have mercury scale for measuring the pressure. Usually pressure is defined as the force per unit area. Hence the unit will be N/m2 or pascal. But we give pressure only in cm or mm of mercury. The reason is explained properly. Same technique is applied in Fortin's barometer and vernier arrangement is made to measure the pressure so accurately.
standard pressure is 760 mm Hg.
A standard atmosphere, denoted by the symbol atm, is a unit of pressure measurement equal to 101,325 pascals, 760 millimeters of mercury, 29.9212 inches of mercury, or 14.696 pounds per square inch. The relationship between atmospheric pressure change and height above the earth's surface
The standard unit for measuring blood pressure is millimeters of mercury (mmHg). Blood pressure is typically recorded as two numbers: systolic pressure over diastolic pressure, for example 120/80 mmHg.
The two ways to express the atmospheric pressure at sea level are in millimeters of mercury (mmHg) or in hectopascals (hPa).
The three units of pressure equal to atmospheric sea level are atmospheres (atm), millimeters of mercury (mmHg), and kilopascals (kPa). At sea level, atmospheric pressure is approximately 1 atm, 760 mmHg, and 101.3 kPa.
There are a lot of possible units. Many TV stations report pressure in inches of mercury, where standard sea level pressure = 29.92 inHg. You can also use millimeters of mercury, which is a simple conversion from inches of mercury (1 inch is about 25.4 mm). Atmospheric scientists usually use hectoPascals (hPa) or millibars (mb), which are the same numerically. Standard sea level pressure is 1013.25 hPa, or 1013.25 mb. 1 hectoPascal is equal to 100 Pascals, while 1 millibar is one-thousandth of a bar. A couple of less common units are Torrs, where sea level pressure is 760 Torr, or atmospheres, where standard sea level pressure = 1 atmosphere.
Normal atmospheric pressure is enough to support a column of mercury approximately 760 millimeters (29.92 inches) high in a barometer.
The standard value for air pressure at sea level is around 1013.25 millibars, or 29.92 inches of mercury (inHg).
The normal partial pressure of oxygen in arterial blood is 75-100 millimeters of mercury. In comparison the partial pressure of oxygen at sea level is 750 millimeters of mercury.
A normal human breath exerts a pressure of around 760 mmHg (millimeters of mercury). This pressure is equivalent to atmospheric pressure at sea level.
When the height of the mercury in a barometer is 29.92 inches, the barometric pressure is equivalent to standard atmospheric pressure at sea level, which is 1013.25 millibars or 14.7 pounds per square inch (psi).
Standard air pressure at sea level is 760 millibars.