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# Instrumentation Fundamentals Module 1 Pressure Scales - PowerPoint PPT Presentation

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

Module 1 – Pressure Scales

Units of Pressure

Pressure Scales & Conversions

Atmospheric, PSIG, PSIA, PSID, Bar

Manometers

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

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

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.

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)

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

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

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

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.)

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

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

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)

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

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

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

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.

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.

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

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)

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

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

• 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.

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?

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.

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

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

6 psi2 psi

4

0.0361 x 13.6

h =

= 8.15 inches of H20 differential

Differential Pressure (psid)

Differential pressure = 4 psid

Requires 2 inputs.

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

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

• 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

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

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