![]() Vacuum pressure is measured from the atmospheric pressure curve down to the sub-atmospheric pressure line and it can be readily seen that the magnitude of available vacuum pressure is different for each of the three days. A sine curve represents the normal variation in atmospheric pressure that could occur over the same three-day period. A sub-atmospheric pressure line is shown where the absolute pressure is constant over a three-day period. An absolute measurement is always positive because it is referenced from absolute zero. The relationship between atmospheric pressure, positive gauge pressure, sub-atmospheric pressure (vacuum) and absolute zero is shown in the previous drawing. This is convenient since the measured “gauge” vacuum level is the vacuum pressure differential that is available to do work and can thus be used directly for calculations of vacuum force which is directly proportional to vacuum pressure and the sealed area upon which it acts. The units of measure for positive pressure and vacuum pressure are the same but a minus sign (-) or the word “vacuum” signifies a negative pressure relative to atmosphere.Ī vacuum gauge has a calibrated mechanism that is referenced to local atmospheric pressure so the value displayed is the amount that the measured pressure is below atmospheric pressure. Both of which are relative to local atmospheric pressure. The preferred terminology is deep-vacuum or shallow-vacuum. ![]() ![]() Since vacuum is by definition a negative pressure, the common terminology of high-vacuum and low-vacuum can be confusing. Essentially it is a difference in pressure, or differential, that can be used to do work. Vacuum is simply a pressure that is less than the surrounding atmospheric pressure. What we usually hear from a weather forecaster is that the barometric pressure is “falling” and bringing in a storm, or, that the barometric pressure is “rising” so sunny days are forecast. The atmosphere is disturbed by weather systems which can cause either “high” or “low” pressure systems by increasing or decreasing the local atmospheric layer thickness. In addition to altitude, atmospheric pressure is affected by air temperature, local weather conditions and other variables to a lesser extent. To complicate matters, the instrument used to measure atmospheric pressure is a barometer and atmospheric pressure is commonly called barometric pressure so the two terms can be used interchangeably. Other equivalent units are 14.72 psi, 1 bar and 101.3 kPa. The International Standard Atmosphere (ISA) is defined as a mean atmospheric pressure of 29.92 inHg (760 mmHg) at 59☏ (15☌) in dry air at sea level. Even in deep outer space there are still a few gas molecules per cubic mile so a true absolute zero pressure is not achieved even though it is very close. At an altitude of 62 miles (100 kilometers) and beyond, atmospheric pressure approaches zero. It is easy to see why atmospheric pressure decreases with increasing altitude. As altitude increases, air density decreases and there will be fewer molecules in the shorter column above the measurement point. ![]() The Earth’s gravitational field holds the atmosphere so that it rotates in unison with the Earth and the atmospheric pressure exerted at any altitude is simply the sum of the weight of all the air molecules in a column above that point. This mixture of gases is comprised of 78% nitrogen and 21% oxygen plus trace amounts of many other gases which collectively make up the atmospheric “air” that we all breathe. The Earth is 7,900 miles (12,715 kilometers) in diameter and is enveloped by a layer of gases about 60 miles (96.6 kilometers) thick which is called the atmosphere.
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