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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

Area

Pressure =

The Physics of PressurePressure 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 PressurePressure 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 surfaceEach 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

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 ScaleThis 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

Atmospheric Press

Atmospheric Pressure

h

Height (h) of displaced water = applied pressure

U-Tube Manometer

ManometersManometers 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 ManometerIf 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.

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

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?

kPa

Inches H20 (WC)

Inches Hg

Bar

Manometer

Mechanical or

Electrical Gauge

P

Pressure IndicatorsPressure is sensed and measured against some calibrated scale.

Pressure Switches and Transmitters

Pressure is also sensed and transmitted as an electrical or pneumatic signal.

3 – 15 psi

4 – 20 mA, 1-5 V, 0-5 V

Normally Closed / Normally Open Dry Contact or Digital signals

Field Bus Communications (Hart, ProfiBus, ModBus)

Switches

Transmitters

Transducers

P

Manometers

U-Tube Well Type Inclined Plane

- Manometers provide a simple method of accurately measuring pressure, can be used as a primary standard.
- Typically restricted to laboratory or calibration lab applications, mercury filled have restricted use.
- Limited to low pressure (<40 psi) applications
- Can measure gauge, vacuum and differential pressure

Mechanical Pressure Gauges

Use elastic or deformation elements such as

- Bourdon Tubes (C-type, spiral, helical)
- Bellows
- Diaphragms
- Capsules

The applied pressure creates a movement which drives a link and pointer mechanism across a calibrated scale.

Also called dry meters, aneroid meters

C-type Bourdon Tube

When pressure is applied at the inlet port the sealed tip of the tube will move in proportion to the applied pressure.

The small tip movement (1/4 to 1/8 inch) drives the link and gear assembly moving the pointer across a calibrated scale.

Cutaway view C-type bourdon tube

The pointer and links are moved to calibrate the scale. (zero & span adjustment)

High pressure gauges require a deadweight test for calibration.

Bourdon Tube Pressure Gauge

Ranges

Vacuum / Compound to 30"HG / 0 / 200 PSI

Pressures from

15 PSI to 15,000 PSI

Accuracy

+ 1.5% of span

Material

Tube made from copper alloys, brass.

Process Pressure

Liquid, Gases, Vapour

Issues: Repeatability, hysterisis, visual signal only.

Spiral & Helical Bourdon Tubes

This design provide a greater tip movement per psi reducing the physical size of the gauge.

Bellows

- Used for measuring lower pressures
- Spring is used to determine the range
- Bellows material may be brass, phosphor bronze, or stainless steel

Diaphragm

- Diaphragm gauges are used for small pressure measurements
- Diaphragm gauges are typically spring-loaded as a means of setting the range and sensitivity
- Diaphragm gauges can be used to measure absolute, gage, and differential pressures

Capsule

Smallest pressure measurement of all elastic deformation elements.

Used with electrical pressure sensors.

Specialty Gauges

GLYCERIN FILLED GAUGESThese types of gauges are suitable for the pressure measurement in the vibrating area. This gauge provides smooth running of the pointer. All standard ranges are available with good accuracy

Sanitary Gauge

The internal volume of the pressure instrument and the area from the top of the seal diaphragm are filled with a system fill fluid.Any pressure applied to the sanitary seal diaphragm is hydraulically transmitted to the pressure element of the gauge and generates a pressure reading.

Electrical Pressure Sensors

Early attempts to convert pressure into an electrical signal were crude.

These low cost, low performance devices had poor repeatability and hysteresis errors

Potentiometric Pressure Sensor

Helical bourdon tube is used to turn a potentiometer.

The output resistance is proportional to the applied pressure.

Inductive Pressure Sensor

A pressure sensing capsule or diaphragm is used to move an LVDT.(Linear Variable Differential Transformer)

Piezoelectric Pressure Sensors

When a piezoelectric material (crystals) is compressed a voltage is created which is proportional to the applied force.

Voltage is only produced when the pressure changes, static pressure measurement requires more electronics

Piezoresistive Pressure Sensor

A diaphragm formed from a piezoresistive material is etched on a silicon wafer.

Applying a force causes the resistance of the element to change which is proportional to the applied pressure.

The piezoresistive element requires a resistive bridge circuit to convert the change in resistance to voltage.

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