Basics of Process Instrumentation

1. Introduction
to
Process Instrumentation

2. OBJECTIVES
At the end of this chapter, students should be able to:
1.Explain the static and dynamic characteristics of an
instrument.
2. Calculate and analyze the measurement error,
accuracy, precision and limiting error.
3. Describe the basic elements of electronic
instrument.

25. TEMPERATURE MEASUREMENT

26. OVERVIEW
 Types of Sensors and how they work
1. Liquid-in-glass thermometres
2. Bimaterial thermometres
3. Electrical thermometres
4. IR-thermometres
5. Pyrometres
7. Other measurement methods
 Sensor Applications
 Advantages and Disadvantages

27. Liquid-in-glass thermometres
The “traditional” thermometers
Measurement scale from
-190 °C to +600 °C
Used mainly in calibration
 Mercury: -39 °C … +357 °C
 Spirit: -14 °C … +78 °C

28. Structure
 Thermal expansion:
Method is based on the
expansion of a liquid
with temperature
The liquid in the bulb
is forced up the
capillary stem

29. Causes of inaccuracies / errors
 Temperature
differences in the
liquid
 Glass temperature
also affects
 The amount of
immersion (vs.
calibration)

30. Bimetallic /Bimaterial Thermometres
 Method based on different thermal expansions of
different metals
◦ Every metal expands more than other: twisting
◦ Inaccurary ± 1 ° C
◦ Industry, sauna thermometers

31. TYPES OF TEMPERATURE SENSORS
 Thermocouples
 Resistance
Temperature
Detectors (RTDs)
 Thermistors
 Infrared Sensors
 Semiconductors

32. THERMOCOUPLES
 Two wires of different
metal alloys.
 Converts thermal
energy into electrical
energy.
 Requires a temperature
difference between
measuring junction and
reference junction.
 Easy to use and obtain.

34. THERMOCOUPLE APPLICATIONS
 Plastic injection molding
machinery
 Food processing equipment
 Semiconductor processing
 Heat treating
 Medical equipment
 Industrial heat treating
 Packaging equipment

35. THERMOCOUPLES
 Simple, Rugged
 High temperature
operation
 Low cost
 No resistance lead wire
problems
 Point temperature
sensing
 Fastest response to
temperature changes
 Least stable, least
repeatable
 Low sensitivity to small
temperature changes
 Extension wire must be
of the same
thermocouple type
 Wire may pick up
radiated electrical noise
if not shielded
 Lowest accuracy
Advantages Disadvantages

36. RESISTANCE TEMPERATURE DETECTORS
(RTDS)
 Wire wound and thin
film devices.
 Nearly linear over a
wide range of
temperatures.
 Can be made small
enough to have
response times of a
fraction of a second.
 Require an electrical
current to produce a
voltage drop across the
sensor

37. RTD APPLICATIONS
 Air conditioning and
refrigeration
servicing
 Furnace servicing
 Foodservice
processing
 Medical research
 Textile production

38. RTDS
 Most stable over time
 Most accurate
 Most repeatable
temperature
measurement
 Very resistant to
contamination/
 corrosion of the RTD
element
 High cost
 Slowest response time
 Low sensitivity to small
temperature changes
 Sensitive to vibration
(strains the platinum
element wire)
 Decalibration if used
beyond sensor’s
temperature ratings
 Somewhat fragile
Advantages Disadvantages

39. THERMISTORS
 A semiconductor used as a temperature sensor.
 Mixture of metal oxides pressed into a bead, wafer
or other shape.
 Beads can be very small, less than 1 mm in some
cases.
 The resistance decreases as temperature increases,
negative temperature coefficient (NTC) thermistor.

40. THERMISTORS
 Most are seen in
medical equipment
markets.
 Thermistors are also
used are for engine
coolant, oil, and air
temperature
measurement in the
transportation
industry.

41. THERMISTORS
 High sensitivity to small
temperature changes
 Temperature
measurements become
more stable with use
 Copper or nickel
extension wires can be
used
 Limited temperature
range
 Fragile
 Some initial accuracy
“drift”
 Decalibration if used
beyond the sensor’s
temperature ratings
 Lack of standards for
replacement
Advantages Disadvantages

42. INFRARED SENSORS
 An infrared sensor intercepts a portion of the infrared energy
radiated by an object.
 Many types Optical Pyrometers, Radiation Pyrometers, Total
Radiation Pyrometers, Automatic Infrared Thermometers, Ear
Thermometers, Fiber optic Thermometers, Two-Color
Pyrometers, Infra-Snakes, and many more.

43. Disapperaring filament pyrometer

45. Thermal radiation
 Every atom and molecule exists in perpetual motion
 A moving charge is associated with an electric field and thus
becomes a radiator
 This radiation can be used to determine object's temperature
 Waves can be characterized by their intensities and wavelengths
◦ The hotter the object:
 the shorter the wavelength
 the more emitted light

46. INFRARED APPLICATIONS
 Manufacturing process like
metals, glass, cement, ceramics,
semiconductors, plastics, paper,
textiles, coatings.
 Automation and feedback
control
 Improve safety in fire-fighting,
rescues and detection of
criminal activities.
 Used to monitor and measure
human body temperatures with
one second time response.
 Reliability and maintenance
needs from building heating to
electrical power generation and
distribution

47. INFRARED SENSORS
 No contact with the
product required
 Response times as fast
or faster than
thermocouples
 No corrosion or
oxidation to affect
sensor accuracy
 Good stability over time
 High repeatability
 High initial cost
 More complex - support
electronics required
 Emissivity variations
affect temperature
measurement accuracy
 Field of view and spot
size may restrict sensor
application
 Measuring accuracy
affected by dust,
smoke, background
 radiation, etc.
Advantages Disadvantages

48. SEMICONDUCTORS
 Are small and result from the fact that
semiconductor diodes have voltage-current
characteristics that are temperature sensitive.
 Temperature measurement ranges that are small
compared to thermocouples and RTDs, but can be
quite accurate and inexpensive.
Applications
 Hard Disk Drives
 Personal Computers
 Electronic Test Equipment
 Office Equipment
 Domestic Appliances
 Process Control
 Cellular Phones

58. Types of Flow Measurement Technologies
• Variable Area (rotameters)
• Rotating Vane (paddle & turbine)
• Positive Displacement
• Differential Pressure
• Vortex Shedding
• Thermal Dispersion
• Magnetic Magnetic
• Thermal Mass
• Coriolis Mass
• Ultrasonic

60. Some Facts About Variable Area
Flowmeters
• Called “float type float
type”, “rotameter’’, or
“variable area”
flowmeters.
• By far the most
common specified,
purchased, and
installed flowmeter in
the world

61. Variable Area Flowmeters
• Fluid flow moves the float
upward against gravity.
• Float will find equilibrium
when area around float
generates enough drag
equal to weight -
buoyancy.
• Some types have a guide
rod to keep float stable.
• Low Cost (pricing usually
starts < $50)
• Simple Reliable Design
• Can Measure Liquid or Gas
Flows
• Tolerates Dirty Liquids or
Solids in Liquid

63. (1) End fitting — flange
shown;
(2) flowmeter body;
(3) rotation pickup —
magnetic,
reluctancetype shown;
(4) permanent magnet;
(5) pickup cold wound on
pole piece;
(6) rotor blade;
(7) rotor hub;
(8) Rotor shaft bearing —
journal type shown;
(9) rotor shaft;
(10)diffuser support and
flow straightener;
(11)diffuser;
(12) flow conditioning plate
(dotted) — optional
with some meters.

64. Electromagnetic Flowmeters
• Magnetic flowmeters have been widely used in
industry for many years.
• Unlike many other types of flowmeters, they offer
true noninvasive measurements.
• They are easy to install and use to the extent that
existing pipes in a process can be turned into
meters simply by adding external electrodes and
suitable magnets.
• They can measure reverse flows and are insensitive
to viscosity, density, and flow disturbances.
• Electromagnetic flowmeters can rapidly respond to
flow changes and they are linear devices for a wide
range of measurements.
• As in the case of many electric devices, the
underlying principle of the electromagnetic
flowmeter is Faraday’s law of electromagnetic
induction.
• The induced voltages in an electromagnetic

65. • As is the case in many applications, if the pipe walls are
made from nonconducting elements, then the induced
voltage is independent of the properties of the fluid.
• The accuracy of these meters can be as low as 0.25%
and, in most applications, an accuracy of 1% is used.
• At worst, 5% accuracy is obtained in some difficult
applications where impurities of liquids and the contact
resistances of the electrodes are inferior as in the case of
low-purity sodium liquid solutions.
• Faraday’s Law of Induction
• This law states that if a conductor of length l (m) is
moving with a velocity v (m/s–1), perpendicular to a
magnetic field of flux density B (Tesla), then the induced
voltage e across the ends of conductor can be expressed
by:

66. Ultrasonic Flowmeters
• There are various types of ultrasonic flowmeters
in use for discharge measurement:
• (1) Transit time: This is today’s state-of-the-art
technology and most widely used type.
• This type of ultrasonic flowmeter makes use of
the difference in the time for a sonic pulse to
travel a fixed distance.
• First against the flow and then in the direction of
flow.
• Transmit time flowmeters are sensitive to
suspended solids or air bubbles in the fluid.
• (2) Doppler: This type is more popular and less
expensive, but is not considered as accurate as
the transit time flowmeter.
• It makes use of the Doppler frequency shift
caused by sound reflected or scattered from
suspensions in the flow path and is therefore
more complementary than competitive to transit

67. Principle of transit time flowmeters.

71. Orifice Plate

72. Orifice for High Viscose Liquids

73. Orifice
• Service: Clean Liquids, Gases
Steam,(no slurries or corrosive)
• Scale: Square Root
• Accuracy: 1% Full Scale
• Permanent Pressure Loss: High
• Cost: Low

75. Venturi & Flow Nozzle

76. Flow Nozzle
• Service: Liquids, Gases, Steam, High
Velocity Flows
• Scale: Square Root
• Accuracy: 1% Full Scale
• Rangability: 3:1
• Permanent Pressure Loss: High
• Cost: High

77. Venturi
• Service: Clean Liquids, Gases Steam
Slurries and Dirty Fluids
• Scale: Square Root
• Accuracy: 1% Full Scale
• Rangability: 3:1
• Permanent Pressure Loss: Low
• Cost: High

78. Variable Area Meters

79. Turbine Flowmeters