Instrumentation Fundamentals

1
Instrumentation Fundamentals

• Module 1 Pressure Scales
• Units of Pressure
• Pressure Scales Conversions
• Atmospheric, PSIG, PSIA, PSID, Bar
• Manometers

2
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

3
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

4
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.
5
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)
6
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
7
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

8
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.)

9
Conversion Factors

• Need to Know psi and kPa conversion
• 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

10
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

11
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)
12
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
13
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
14
Manometers

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

15
Reading pressure with a U-tube Manometer

• If the total displacement h 3 the applied
  pressure would be 3H20 3WC 0.108 psi

Using Mercury as a filling liquid increases the
pressure range by 13.6 times.
16
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.
17
Inclined Plane Manometer

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

18
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)

19
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
20
Gauge Pressure (psig)
1 psi Atmos

• 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

h
21
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.
22
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?

23
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.
24
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

25
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

26
Differential Pressure (psid)

• Differential pressure 4 psid

27
Differential Pressure Gauge (psid)

• Requires 2 inputs.
• Must observe pressure polarity, i.e. hi side / lo
  side

28
Differential Pressure Cell Transmitter
4 20 mA output 2 wire transmitter

• The d/p cell is often used to measure level and
  flow.
• What is the maximum allowable input pressure in
  psi?

Differential Input
0 200 in. H20 Typical input range
29
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
30
Exercise (ball park is fine)
What is this in psi, kpa, inches of H20?
What is this in psig, psia, inches of Hg?
31
Pressure Sensing Elements

• Manometers
• Mechanical Gauges
• Electrical Transducers
• Switches Transmitters

32
Pressure Indicators

• Pressure is sensed and measured against some
  calibrated scale.

33
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
34
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 (lt40 psi) applications
• Can measure gauge, vacuum and differential
  pressure

35
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

36
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.
37
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.

38
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.
39
Spiral Helical Bourdon Tubes
This design provide a greater tip movement per
psi reducing the physical size of the gauge.
40
Bellows

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

41
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

42
Capsule

• Smallest pressure measurement of all elastic
  deformation elements.
• Used with electrical pressure sensors.

43
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.
44
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

45
Potentiometric Pressure Sensor

• Helical bourdon tube is used to turn a
  potentiometer.
• The output resistance is proportional to the
  applied pressure.

46
Inductive Pressure Sensor

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

47
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
48
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.