I can feel it coming in the air tonight ...
And I've been waiting for this moment for all my life.

("In the Air Tonight," Phil Collins, 1981)

 

UNDERSTANDING ATMOSPHERIC PRESSURE

The PV Temperature has four different scales that  are used for temperature indicating (°C, °F, °K, and °R) and the PV Pressure has three different classifications that are used for pressure indicating:

• Gauge Pressure
• Absolute Pressure
• Vacuum Pressure

A thorough understanding of atmospheric pressure (Patm) is a prerequisite to understanding the classifications of PV Pressure.

Atmospheric pressure (P_atm) is the focus of this PTOA Segment # 149.

 

ATMOSPHERIC PRESSURE ...
THE PRESSURE YOU DON'T EVEN FEEL ANYMORE

Once upon a time there you were ...

happily dozing off  ...

swimming around ... doing the backstroke in your mother's womb ...

buoyed up by amniotic fluid that followed Pascal's Law of contained liquids exerting pressure in all directions ... but, I digress ...

Sometimes you gave a good kick as if to say "Hello from the other side."

and then ... one day ... it was time to be born!

And the first thing you did when the air started flowing through your lungs was quite literally ...

cry ... well ...  like a baby!

Because the air pressure that surrounded you ...

14.7 psi (aka 101.3 kPa) ...

felt like a crushing force on every square inch of your body ...

You don't remember it ...

but you were one cranky fussbudget until you eventually became accustomed to that oppressive air pressure that eventually synchronized with the other essential systems of your being.

Shortly thereafter and up until nowadays ...  you don't even feel the pressure!

You don't even realize that you are bombarded by atmospheric pressure 24/7 and 365 days a year!

As stated above and repeated below ...

The atmospheric pressure that made each and every PTOA Reader or Student uncomfortable upon emerging from the womb was somewhere around 14.7 psi (101.3 kPa).

We know this because that's the atmospheric pressure (P_atm) measured at sea level.

 

THE GREATER THE ALTITUDE,
THE LESS THE ATMOSPHERIC PRESSURE (P_atm)

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order focused on the nearby graphic in PTOA Segment  #140  while learning how the boiling point of water (and all liquids) decreases when the atmospheric pressure above the liquid level decreases.

In the nearby chart atmospheric pressure (P_atm)  is measured in kiloPascals (kPa) which PTOA Readers and Students who do not live in the USA will immediately relate to.

For those who do live in the USA, 1 kPa = 0.145 psi.

At this juncture, PTOA Readers and Students should focus on how an increase in altitude (the column labelled "Feet above sea level")  corresponds to a decrease in atmospheric pressure (P_atm).

Had this chart included a measurement in a city that was exactly at sea level, the elevation would have been recorded as "0" ... but the location would have been in the water.

(Could happen with global warming!)

New York City is very nearly at sea level ... just 10 feet above. The corresponding P_atm recorded for New York City is 101.3 kPa (14.7 psi) ... exactly what would be recorded at exact sea level.

The highest elevation on the earth's surface is  the top of Mount Sagarmatha in Tibet (aka Mt. Everest). At 29,028 feet above sea level, the P_atm is 32 kPa (4.64 psi). If it were possible to build a processing facility at this altitude most of the equipment would have to be specially fabricated.

The above chart also shows that the atmospheric pressure (P_atm) increases when the altitude "is negative" ... meaning below sea level.  Death Valley is 282 feet below sea level and has an P_atm of 102.6 kPa (14.9 psi).

The below graph shows the continuous relationship between P_atm and the "Elevation above Sea Level" over the very wide range of -2500 meters (-8200 feet) through 20,000 meters (65,616 feet).

On the left hand side of the chart is P_atm in units of kPa and on the right side of the chart is P_atm in units of psi.  Pick your scale!

Regardless of the P_atm scale you picked ... both scales confirm that within the elevation range that processing facilities are built there is a one-to-one totally predictable inverse linear relationship summed up by the following:

As altitude increases, P_atm predictably decreases and vice versa.

 

THE DENSITY OF AIR AND "AIR HEAD" DETERMINE ATMOSPHERIC PRESSURE

We Are All Bottom Dwellers

Who amongst the brilliant PTOA Readers and Students noticed that the above P_atm-Altitude chart and graph also showed the pressure profile of the atmospheric blanket that surrounds the earth?

The magnitude of atmospheric pressure (P_atm) that surrounds us Eathlings depends only upon:

• the density of air measured at the elevation of interest.

and ... for lack of a better euphemism ...the PTOA will use the term ..

• the "Air Head."

The PTOA Department of Redundancy Department wants to make certain that you understand the following:

All of us Earthlings are living at the bottom of a head of gas that we call air.

Just like the hydrostatic head created by a pooled liquid, the magnitude of atmospheric pressure (P_atm) measured anywhere between the top layer of the Earth's atmosphere to the Earth's surface at sea level depends upon just two things:

• The density of the air where the P_atm measurement is being taken.
• The height of the air column where the measurement is being taken  ... aka the "Air Head."

Ergo ...

Since all of us Earthlings are living at the bottom of the Air Head we are experiencing the maximum pressure exerted by that mass of air on earth's surface ... 14.7 psi (101.3 kPa).

The situation would be different for a jet.

A jet traveling at an elevation of 12 kM above sea level (aka 7.5 miles) experiences significantly less atmospheric pressure because:

 

• 

  The higher the elevation, the less air molecules per volume. Otherwise stated: the higher the altitude the less density of air.

  There is less mass of air, hence air is less dense. That's because the decreased gravity allows the air molecules to spread out.

• There is less Air Head because the distance from the top of the  Earth's atmospheric layer to the plane is 7.5 miles less than the Air Head that extends all the way to the Earth's surface from the top of the Earth's atmosphere.

The relationship between atmospheric pressure and altitude is so predictable that the airplane instrument called an altimeter is actually measuring atmospheric pressure and converting the reading into a corresponding altitude.

 

MEASURING ATMOSPHERIC PRESSURE (Patm)

Atmospheric pressure (P_atm) is measured with a barometer.

The simple Torricelli mercury barometer design makes it easy to describe how a barometer works but nowadays the instrument has been replaced with a much safer and less toxic design.

Frankly,  Process Operators will not frequently if ever use a barometer.

However the more modern barometer will be found in the processing facility's Quality Assurance/Quality Control laboratory because laboratory technicians must adjust some lab results to account for changes in P_atm.

The simple mercury barometer is made of a glass tube that is open on only one end.

The open end is submerged in a dish that contains mercury (Hg) ...

 

PTOA Readers and Students learned in PTOA Segment #101 that the element mercury has been assigned the abbreviation Hg.

Focus on the two blue downward arrows in the barometer graphic.

These arrows represent the force of the air that is spread out over the surface of the liquid mercury.

This force of air is disbursed over the mercury surface area and creates the pressure of the atmosphere which is labelled "Air Pressure" in the graphic. This is the same atmospheric pressure that impacts your skin and which you don't even notice.

The "Air Pressure"  force component pushes some of the mercury into the open end of the inverted glass tube to a height that is shown in the graphic as "H."

A height (H) of 760 mm Hg (aka 29.92 inches Hg) has been determined to be the pressure exerted by air at sea level and is also called "1 Atmosphere" (1 atm).

Here's a quick demonstration that shows how the hydrostatic head H is converted into English psi units:

P = (SG) * (.433) * H

where SG = specific gravity of mercury = 13.633

H = 29.92 inches * 1 foot / 12 inches = 2.49 feet

so

P = 13.633 * (0.433) psi / ft * 2.49 ft = 14.7 psi

 

Because a dude named Torricelli invented the barometer and figured out how to determine atmospheric pressure, he is honored by naming the following conversion factor after him:

760 mm Hg = 760 Torr

See what happened here?

Torr just means "a millimeter of mercury." So it saves syllables and ink to use "Torr" instead of "mil-li-meter-of-mercury."

In summary ... here's a list of conversion factors for P_atm at sea level:

760 mm Hg = 760 Torr = 1 atm  = 29.92 inches Hg = 14.7 psi

Your Mentor would not even bring up all the conversion factors for P_atm were it not for the fact that the PV Pressure can be indicated in all of the above units of measurement!

 

Barometer Measurements Forecast Weather

A barometer can be used to predict weather.

When P_atm is greater than 14.7 psi (101.3 kPa), the height of the mercury column (H) is greater than 29.92 inches Hg.

For example a barometer measuring a very high P_atm of 800 mm Hg column would be equivalent to:

800/760 mm Hg = 1.053 times the equivalent conversion factor so ...

800 mm Hg = 800 Torr = 1.053 Atm = 31.50 in Hg = 15.5 psi

When the P_atm is less than 14.7 psi (101.3 kPa), the height of the mercury column is less than 29.92 inches Hg (aka 760 mm Hg).

For example, a very low P_atm might result in a short mercury column of 26.5 inches Hg (aka 670 mm Hg).

670 mm Hg/760 mm Hg = .882 times the equivalent conversion factor

or

670 mm Hg = 670 Torr = 0.882 Atm = 26.4 in Hg = 13.0 psi

Note the inside scale of the below barometer could be reading in Torr or mmHg ... so we'll assume mmHg.

The scale indicates that a 760 mm Hg sea level P_atm would predict a partly cloudy-edging-to-sunny day.

This barometer would predict "high scale off the chart" sunny weather when the P_atm is much higher at 800 mm Hg. The weather prediction changes to "low scale off the chart" stormy should the P_atm decreases 670 mm Hg.

TAKE HOME MESSAGES: This PTOA Segment was devoted to understanding Atmospheric Pressure (P_atm) because the classification of PV Pressure measurement and indicators are based upon a thorough understanding of what P_atm is.

P_atm measured at sea level is 14.7 psi aka 101.3 kPa.

There is an extremely predictable inverse relationship between P_atm and altitude; the greater the altitude the less P_atm and vice versa.

P_atm is totally analogous to the hydrostatic pressure created by pooled liquids because the magnitude of P_atm likewise depends upon the density of air and the "Air Head" at the elevation where P_atm is being measured.

Human beings are living at the bottom of the "Air Head" that extends between the upper level of earth's atmosphere and the earth's surface and are therefore experiencing the maximum atmospheric pressure of 14.7 psi (101.3 kPa) even if they don't feel it.

P_atm is measured with a barometer.

There are several units in use to measure pressure.

760 mm Hg = 760 Torr = 1 atm  = 29.92 inches Hg = 14.7 psi

©2016 PTOA Segment 0149
PTOA Process Variable Pressure Focus Study Area
PTOA Introduction to PV Pressure Focus Study

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