All is relative
Everything is relative
Oh yeah, yeah, yeah
Everything is relative

("Relative,"  by Gavin DeGraw, 2008)

THE 3 PV PRESSURE CLASSIFICATIONS ARE DEFINED BY THEIR RELATIONSHIP TO ATMOSPHERIC PRESSURE

The nearby graphic illustrates the relationship of Atmospheric Pressure to the 3 classifications of the PV Pressure.

In the middle of the graphic ...

A dark blue upwardly pointing arrow begins at "Absolute Zero Pressure" and extends to a THICK black horizontal line that represents  where the detected pressure has attained 14.7 psia (101.3 kPa) ..."Atmospheric Pressure."

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order expertly understand these facts about  atmospheric pressure ...

• it is abbreviated P_atm.
• it is equal to 14.7 psi and 101.3 kPa at sea level and changes depending upon the density of air and the height of the "air head."
• at sea level P_atm is also equal to 760 mm Hg = 760 Torr = 1 atm = 29.92 inches Hg and ... newly added here ... 1013.5 millibar.

Now that PTOA Readers and Students expertly understand what P_atm is, they are ready to learn about the three PV Pressure classifications:

• Gauge Pressure.
• Absolute Pressure.
• Vacuum Pressure.

The three classifications of the PV Pressure are defined from the basis of their relationship to P_atm.

 

GAUGE PRESSURE (P_gauge)

The vast majority of pressure indicators (PIs) that Process Operators will glean pressure information from will be displaying "gauge pressure (P_gauge)."

As stated above and repeated here ...

In the nearby graphic, Atmospheric Pressure is drawn as a THICK black horizontal line.

Gauge Pressure is represented as the olive green upwardly pointing arrow that extends above  the blue upwardly pointing  Atmospheric Pressure arrow to the higher horizontal line that represents "Any Pressure Above Atmospheric Pressure."   

In PTOA Segment #149, PTOA Readers and Students learned that they are surrounded by  atmospheric pressure (P_atm) and have grown so accustomed to it that they don't consciously feel it bearing down on their shoulders and head anymore.

Ergo..

who is going to be surprised to learn that the vast majority of pressure gauges used in the process industries don't sense nor measure P_atm either!

In other words ...

Most PIs are manufactured to indicate the pressure that is above atmospheric pressure (P_atm) because that is pressure that must be artificially created by mechanical means over and above what exists in the natural world at sea level.

The PTOA PV Pressure Focus Study Area will soon explain in detail how process line pressures are created ...

like the line pressure that is being indicated as 84 psi by the nearby gauge ...

so don't fret about understanding that now.

At this point just remember that ...

A PI that is indicating gauge pressure (P_gauge) may state the pressure being indicating in units of "psig" or even "barg" with the "g" meaning "gauge" ...

The abbreviation "psig" verbally translates into "pounds per square inch gauge."

The abbreviation "barg" verbally translates into "bars of  gauge pressure."

Since "gauge pressure" is the default pressure that is typically indicated by  PIs ...

Modern pressure gauges often list the indicating units as simply "psi" or "bar"... meaning no "g" is added.

So it is okay to assume that any gauge labelled to be reading in units of "psi" is indicating Gauge Pressure(P_gauge).

 

 

Once more with gusto ...

The PTOA Department of Redundancy Department reiterates:

Gauge Pressure is any pressure that exceeds Atmospheric Pressure (P_atm) and might be indicated in units of "psi" or "psig."

The below PIs can display gauge pressures scaled from 0 to 30 up through 0 to 1500.

One quick glance reveals that all of these gauges are manufactured to indicate gauge pressure (P_gauge).

How can Your Mentor tell?

All of the gauges are correctly indicating "0" psi(g).

They are all disconnected from a process line and therefore their sensors only detect P_atmthrough their detached fitting.

Since all of the gauges are purposely manufactured to sense and indicate only gauge pressure ...

the pressure created above atmospheric pressure ...

they are each accurately indicating "0" psi.

All PIs that are manufactured to indicate "Gauge Pressure" will indicate "0 psi" when disconnected and open to atmospheric pressure (P_atm).

 

ABSOLUTE PRESSURE (P_abs)

A PI that can indicate all of the existing Atmospheric Pressure (P_atm) plus the existing Gauge Pressure (P_gauge) is indicating the Absolute Pressure (P_abs).

Otherwise stated:

P_abs = P_gauge + P_atm

In the nearby graphic, Absolute Pressure is represented by the long red arrow that points upward from the "Absolute Zero Pressure" datum to the horizontal line labelled "Any Pressure Above Atmospheric."

On the right side of the long red Absolute Pressure arrow is a dark blue-olive green compound arrow that shows the two components of Absolute Pressure ...

"Atmospheric Pressure" and "Gauge Pressure."

Gauges that indicate Absolute Pressure will be labelled with "psia" or will have the word "Absolute" on the dial face.

The "a" in "psia" stands for "Absolute" and the abbreviation verbally translates into "pounds per square inch absolute."

In English pressure-indicating gauges, Absolute Pressure is right around 14.7 psia greater than gauge pressure.

In SI pressure-indicating gauges, Absolute Pressure is 101.3 kPa greater than gauge pressure.

For example ... in English measuring units ..

A gauge that indicates 10 psi Gauge Pressure will indicate 24.7 psia on an Absolute Pressure Gauge because ...

P_abs = P_gauge + P_atm

so...

24.7 psia = 10.0 psi + 14.7 psia

More than one Process Operator I know of has had his/her ignorance of understanding Absolute Pressure exposed.

No PTOA Reader or Student should be surprised to learn that the industry applications of monitoring and indicating Absolute Pressure typically involve very low gauge pressures ... for example 2 to 3 psig (13.7 to 20.7 kPa gauge).

PTOA Readers and Students!

Stop right here and understand why the Absolute Pressure Gauge used for such a service would daily indicate 16.7 to 17.7 psia to the Process Operator.

More than one Process Operator has been schooled by the Instrument Technician that received a work order written by the Process Operator with the following gist:

"The system has been taken off line for inspection. The pressure gauge/transmitter still reads  approximately 14.7 psia (101.3 kPa) instead of "0" psi. The gauge is obviously broken. "

The Absolute Pressure Gauge in the above scenario is accurately detecting the P_atm component of pressure and does so even when there is no existing gauge pressure to detect/indicate.  It's blush time for the Process Operator!

The Correct Units for Barometric Pressure are also PSIA

PTOA Readers and Students were already introduced to a pressure-measuring instrument that measures Absolute Pressure in units of psia ... the barometer!

Which PTOA Readers and Students notice that the THICK Black "Atmospheric Pressure" line  in the nearby graphic is also labelled "Barometric Pressure?"

Since a barometer cannot sense any Gauge Pressure ...  the above expression that defines P_abs reduces to:

P_abs  = 0 psig + 14.7 psia

Thus, a barometer pressure reading of P_atm is measuring in absolute pressure units (psia) ...

and in this case the "Gauge Pressure" is "0."

 

VACUUM PRESSURE

A Vacuum Pressure is any pressure that is below Atmospheric Pressure.

In the nearby graphic, the Vacuum Pressure is illustrated as an olive green arrow pointing DOWNWARD from the THICK black horizontal line that represents Atmospheric Pressure to the lower horizontal line that represents "Any Pressure Below Atmospheric."

There is no simple "psig" or "psia" equivalent nomenclature for a Vacuum Pressure.

"Light to Mild" Vacuum Pressures are measured in psia (on a scale from  0 to 14.7 psia), millimeters of mercury (mm Hg on a scale from 0 to 760),  inches of mercury (in Hg on a scale from 0 to 30), inches of water (in H2O) and Torrs.

"Strong" Vacuum Pressures are measured in millibars.

 

 

A "perfect vacuum" would be a theoretical pressure of 0 psia ... the very bottom line in the nearby graphic that is labelled "Absolute Zero Pressure."

Guess what?

It is impossible to create a "perfect vacuum" and difficult to sustain a "strong vacuum."

It takes a lot of energy to maintain "pulling a vacuum" because a "vacuum pressure" is not the normal, everyday pressure that the natural environment maintains ...

Heck ...

all smart PTOA Readers and Students instantly recognize the natural environment wants to maintain simply "Atmospheric Pressure."

Hey! I see a pattern!

Great effort is required to accomplish both of these objectives:

• Create a Gauge Pressure above P_atm.
• Create a Vacuum Pressure below P_atm.

There are several dozen useful applications of Vacuum Pressures in the processing industries and one very bad outcome of an undesirable vacuum creation called "an implosion."

Implosions and the useful applications of Vacuum Pressures in the processing industries are featured in the next PTOA Segment because this PTOA Segment #150 is already too long to explore those subjects!

 

TAKE HOME MESSAGES: The three classifications of the PV Pressure are derived from their relationship to Atmospheric Pressure (P_atm).

Gauge Pressure (P_gauge) is any pressure above atmospheric pressure. In English measuring units, a PI that indicates gauge pressure may measure in "psi" or "psig."

Absolute Pressure (P_abs)is a pressure that includes both a Gauge Pressure component and the full amount of Atmospheric Pressure (typically 14.7 psia or 101.3 kPa).

In English measuring units a PI that indicates Absolute Pressure will state the reading to be in "psia" or will have the word "Absolute" on the dial.

Otherwise stated:

• P_abs = P_gauge + P_atm
• P_abs is typically 14.7 psi (101.3 kPa) greater than a Gauge Pressure indication.

The correct way to communicate about Absolute Pressure is to use the word "absolute" as in "I observed a pressure of 16.7 psi absolute."

Barometers also measure an Absolute Pressure in units of psia; their indicated measurement is obviously equal to the observed Atmospheric Pressure component of the Absolute Pressure equation and the Gauge Pressure component is equal to "0."

Vacuum Pressures are pressures that are lower than atmospheric pressure.

Vacuum Pressures can be identified by their uniquely weird units of indication: mmHg, inches Hg, inches H20, Torr, millibar and psia (in the range of strong vacuum 0 to weak vacuum 14.7).

A "full" or "perfect" Vacuum pressure achieves zero Absolute Pressure aka 0 psia and is theoretically impossible.

Both Gauge Pressures and Vacuum Pressures take energy to create.

There are several important applications of "Light to Mild" Vacuum Pressures in the processing industries.

A non-desirable Vacuum Pressure can cause an implosion.

©2016 PTOA Segment 0150
PTOA Process Variable Pressure Focus Study Area
PTOA Introduction to PV Pressure Focus Study

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