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Instrumentation Fundamentals. Module 1 – Pressure Scales Units of Pressure Pressure Scales & Conversions Atmospheric, PSIG, PSIA, PSID, Bar Manometers. Pressure . This module will cover: The physics of pressure Units of measure (SI, Metric, Imperial) Pressure scales and conversions

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Instrumentation fundamentals l.jpg

Instrumentation Fundamentals

Module 1 – Pressure Scales

Units of Pressure

Pressure Scales & Conversions

Atmospheric, PSIG, PSIA, PSID, Bar

Manometers


Pressure l.jpg

Pressure

This module will cover:

  • The physics of pressure

    • Units of measure (SI, Metric, Imperial)

    • Pressure scales and conversions

  • How pressure is measured

    • Elastic elements (bourdon, bellows diaphragm)

    • Electrical elements (strain gauge, piezoelectric)

    • Sensors, Switches and Transmitters


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What is it?

Pressure is an operating parameter that is relevant in many applications. (similar to voltage in an electric circuit)

  • Pressure applied over a given area can be used for useful work.

    • Steam pressure, Water pressure

  • Pressures can be measured to infer the condition of other process parameters.

    • Flow, level


The physics of pressure l.jpg

Force

Area

Pressure =

The Physics of Pressure

Pressure is defined as “force per unit area”

Therefore any object or material having a weight will exert a pressure over the area the force is acting on.


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Force

Area

Pressure =

The Units of Pressure

Pressure is defined as “force per unit area”

Pound force, Kilogram force

Newton, dyne

Square Inches, Square feet

Square Centimeters, Square Meters

  • Common units include:

    • Pounds per Square Inch (psi)

    • KiloPascals (kPa)


Example of pressure from a 1 cubic foot pound force acting on a surface l.jpg

550 lbs

708 lbs

62.4 lbs

849 lbs

Example of pressure from a 1 cubic foot pound force acting on a surface

Each base has an area of 144 in2

1 cubic foot of copper

550 lbs

144 in2

= 3.8 psi

1 cubic foot of lead

708 lbs

144 in2

= 4.9 psi

1 cubic foot of water

62.4 lbs

144 in2

= 0.43 psi

1 cubic foot of mercury

849 lbs

144 in2

= 5.9 psi


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More Pressure Scales

PSI and kPa are the most common pressure scales but there a few more:

  • Inches* of water

  • Inches* of mercury

  • Bar

  • Atmos

  • Torr (vacuum)

    * or millimeters when using metric


Example of the various pressure scales l.jpg

Example of the various pressure scales

The same process pressure is being applied to each gauge. Each gauge has a different scale calibration.

27.6806

“H20

2.03602

“Hg

0.068947

Bar

0.068046

Atmos

Applied process pressure is

1 psi or

6.89 kPa

  • The choice of scales will depend on

  • the amount of pressure being measured (high pressure = psi/kPa, low pressure = inches H20)

  • The type of application ( flow = inches H20, blood pressure = inches of Hg.)


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Conversion Factors

Need to Know:psiand kPaconversion

Imperial vs Metric vs SI

  • 1 cubic foot of water that weighs 62.4 lbs acting over an area of 144 in2 produces a pressure of 0.433 pound per square inch (psi)

  • The same volume of water weighs 28.3 Kilograms over an area of 929 cm2, therefore the pressure is 0.03 kilograms per square centimeter. (30.46 g/cm2)

  • SI use Newton per sq. meter and call it the Pascal

    1 psi = 0.006894757 Pascals

    = 6.895 kiloPascals


Need to know l.jpg

Need to Know

Ball Parking:

1 psi ≈ 7 kPa

3 psi ≈ 21 kPa

15 psi ≈ 105 kPa

20 psi ≈ 140 kPa

3 to 15 psi is a common pressure range

20 kPa to 100 kPa is also a common pressure range


Inches of water scale l.jpg

12 inches of water exerts a pressure of 0.433 psi

Inches of Water Scale

This scale is used to measure small pressures.

The properties of water are known and constant and can be used as a primary standard.

Pressure is proportion to the height of the water column (hydrostatic head pressure)


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Water Column

The hydrostatic head produced by a column of liquid is proportional to the height and density of the liquid.

P = height x Density

(Density = Mass/Volume)

Density of water is 0.0361 lbs/in3

P = 12 x 0.0361

= 0.433 lbs/in2

12 “ H20

0.433

psi


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Water Column

The greater the height the greater the hydrostatic head.

P = height x Density

(Density = Mass/Volume)

24 “ H20

Density of water is 0.0361 lbs/in3

P = 24 x 0.0361

= 0.866 lbs/in2

0.866

psi


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Applied Process Pressure

Atmospheric Press

Atmospheric Pressure

h

Height (h) of displaced water = applied pressure

U-Tube Manometer

Manometers

Manometers can be used as a primary standard to measure small pressures


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Applied Process Pressure

Atmospheric Press

h

Height (h) of displaced water = applied pressure

Reading pressure with a U-tube Manometer

If the total displacement h = 3“ the applied pressure would be 3”H20 = 3”WC = 0.108 psi

Using Mercury as a filling liquid increases the pressure range by 13.6 times.


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Well Type Manometers

The well type uses one measuring arm. Gives a larger pressure range

Mercury filled well type manometers can measure up to 30 psi and more. (6 footer)

Can be used as a primary standard.


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Inclined Plane Manometer

Used for very small pressure measurements. Very sensitive, often used to measure room pressures.


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psi inatmosphere

Gauge Pressure (psig)

The standard pressure measurement is referenced to atmospheric pressure and is called gauge pressure.

The scale units on the manometer could be calibrated in

  • inches of water (gauge)

  • inches of mercury (gauge)

  • psig

    And all measurements would be relative to atmospheric pressure 14.7 psi

    (varies slightly with elevation and weather)


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Gauge, Absolute and Atmospheric Pressure

Any pressure above atmosphere is called gauge pressure (psig)

Any pressure below atmosphere is a vacuum (negative gauge pressure)

Absolute pressure (psia) is measured from a perfect vacuum

Differential Pressure (psid) has no reference to either absolute vacuum or atmospheric pressure


Gauge pressure psig20 l.jpg

Gauge Pressure (psig)

Applying 1 psi would produce a displacement of about 2 in. Hg or 30 in. H2O

Since the reference side of the manometer is open to atmosphere, the applied pressure would be read as gauge pressure

i.e. 1psig or just 1 psi

1 psi Atmos

h


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Standard Gauge

  • When a gauge has no input applied, it will read 0 psig

  • The pressure range for this gauge is 0 – 100 psi

  • What is the range in kPa?

Some gauges may not include the “g” after psi, some will.


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Pressure Range & Scale

This gauge has a pressure range of 0 to 30 in. H2O

The pressure being measured is still gauge pressure.

What is the maximum psig that can be applied? kPa?


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Small pressure measurements

Dwyer differential pressure gauge registers a differential of 0 - 2 psi, 1/8" npt. High and low pressure input ports on side and back. Manual set point. Max 15 psi and 140ºF. 4-3/4" diameter x 2" high.


Examples of psig psia and vacuum l.jpg

Examples of psig, psia and vacuum

20 psig

= 20 + 14.7 = 34.7 psia

60 psia

= 60 – 14.7 = 45.3 psig

10 psia

= 10 – 14.7 =-4.7 psig

=-4.7=- 9.6 ”Hg

0.0361 x13.6


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PSIA – Absolute Pressure

A gauge with a psia scale will indicate 14.7 when no pressure is applied.

The compound gauge is more common than psia, it measures vacuum and gauge pressure.

This gauge has a range of 0 – 30 in. Hg vacuum and 0 to 15 psig


Differential pressure psid l.jpg

6 psi2 psi

4

0.0361 x 13.6

h =

= 8.15 inches of H20 differential

Differential Pressure (psid)

Differential pressure = 4 psid


Differential pressure gauge psid l.jpg

Differential Pressure Gauge (psid)

Requires 2 inputs.

Must observe pressure polarity, i.e. hi side / lo side


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Differential Pressure Cell Transmitter

The d/p cell is often used to measure level and flow.

What is the maximum allowable input pressure in psi?

4 – 20 mA output

2 wire transmitter

Differential Input

0 – 200 in. H20

Typical input range


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Pressure Conversion Chart

  • Ball Parking

    • 1 psi = 7 kPa

    • 1 inch Hg = 0.5 psi

    • 100 inch H20 = 3.5 psi

    • 1 Bar = 1 Atmos = 14.7 psi

  • Accurate

    • 1 psi = 6.89 kPa

    • 1 inch Hg = 0.49 psi

    • 100 inch H20 = 3.61 psi

    • 1 Bar = 14.5 psi = 100 kPa


Exercise ball park is fine l.jpg

Exercise (ball park is fine)

What is this in psi, kpa, inches of H20?

What is this in psig, psia, inches of Hg?


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