Atmospheric Pressure

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.  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’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.  As altitude increases, air density decreases and there will be fewer molecules in the shorter column above the measurement point.  It is easy to see why atmospheric pressure decreases with increasing altitude.  At an altitude of 62 miles (100 kilometers) and beyond, atmospheric pressure approaches zero.  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.

International Standard Atmosphere (ISA)

The International Standard Atmosphere (ISA) is defined as a mean atmospheric pressure of 29.92  inHg (760 mmHg) at 59°F (15°C) in dry air at sea level.  Other equivalent units are 14.72 psi, 1 bar and 101.3 kPa.  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.

In addition to altitude, atmospheric pressure is affected by air temperature, local weather conditions and other variables to a lesser extent.  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.  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.

 

Vacuum Pressure

Vacuum is simply a pressure that is less than the surrounding atmospheric pressure.  Essentially it is a difference in pressure, or differential, that can be used to do work.  Since vacuum is by definition a negative pressure, the common terminology of high-vacuum and low-vacuum can be confusing. The preferred terminology is deep-vacuum or shallow-vacuum.  Both of which are relative to local atmospheric pressure.  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.   

A 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.  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.



EVU-ATMOSPHERIC-GAUGE

What is the relationship betweenThe relationship between atmospheric pressure, positive gauge pressure, sub-atmospheric pressure (vacuum) and absolute zero is shown in the previous drawing.  An absolute measurement is always positive because it is referenced from absolute zero.  A sub-atmospheric pressure line is shown where the absolute pressure is constant over a three-day period.  A sine curve represents the normal variation in atmospheric pressure that could occur over the same three-day period.  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.  In effect, the ability to do work (pressure differential), changes in accordance with the atmospheric (barometric) pressure.  This is why we recommend using a mid-range rather than a deep vacuum pressure when designing vacuum systems.

On Earth, a vacuum is not self-sustaining since seals leak and most materials are minutely permeable.  Over time, enough air molecules will be pulled through the material that the vacuum will be “lost” due to equalization with atmospheric pressure.  To maintain a vacuum for a long time period, a vacuum pump must periodically evacuate air molecules to maintain a desired vacuum pressure.  Depending on material permeability (porosity), continuous evacuation may be required to maintain a desired vacuum pressure.


Vacuum Flow

The performance of a vacuum pump is defined by its’ performance curve which is simply a plot of the vacuum flow rate that it is capable of producing at a particular vacuum pressure.  As vacuum pressure increases, it becomes more difficult to remove (pump out) additional air molecules, so vacuum flow rate decreases until it becomes zero at the deepest attainable vacuum pressure.  Vacuum flow rate will always be highest at atmospheric pressure (zero vacuum) where the pump is under no load.  Many pump manufacturers advertise the efficiency of their pumps with this misleading number.  In reality this specification is meaningless since force can’t be developed and work can’t be done unless vacuum pressure is being created.



EVU-SEALED
Vacuum pressure determines the amount of force that can be developed to hold a work piece or to carry a load.  For a sealed system with no leakage, the two main concerns are; how much vacuum pressure is needed and how quickly can the system be evacuated to the required vacuum pressure?  Since the system is sealed, using a larger vacuum pump will reduce evacuation time but will not increase the system vacuum pressure since, given enough time, even a small vacuum pump will attain maximum vacuum pressure.  A larger vacuum pump will consume more energy without increasing the system load capacity so it is important to not over-specify vacuum pump capacity for a sealed system.

EVU-POROUS
However, when the work piece is porous (permeable) or the system otherwise leaks, the vacuum pump must produce enough vacuum flow rate to overcome the leakage and still attain the necessary vacuum pressure.  The pump must also have enough excess capacity to overcome possible future variations in work piece porosity – we have found corrugated board porosity variations of 4:1 among vendors supplying boxes to the same end user.

System porosity flow increases directly with increased vacuum pressure while pump flow decreases with increased vacuum pressure in accordance with its’ performance characteristics.  As a result, doubling the vacuum pump capacity in a porous system will double the energy usage (air consumption) but will only cause a smaller incremental increase in vacuum pressure.  At deeper system vacuum pressures the diminishing-returns effect becomes more pronounced so this is another reason to design systems for proper operation at mid-vacuum pressure by simply increasing the effective area upon which the vacuum pressure acts.    

We offer free porosity evaluation and assistance with vacuum pump selection.  EDCO USA will do the calculations for you and help you select the correct pump for your application.