# Instrumentation Fundamentals - PowerPoint PPT Presentation

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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

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

• How pressure is measured

• Elastic elements (bourdon, bellows diaphragm)

• Electrical elements (strain gauge, piezoelectric)

• Sensors, Switches and Transmitters

### 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

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.

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)

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

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

### 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

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.)

### 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

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

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)

### 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

### 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

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

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.

### 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.

### Inclined Plane Manometer

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)

### 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 (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

### 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.

### 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?

### 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

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

### 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

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)

Requires 2 inputs.

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

### 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

### 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)

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

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