Measurement Techniques and Devices Flashcards
(120 cards)
1
Q
How can measurement techniques and devices be classified?
A
-In terms of the data acquired, they can be continuous or discrete
-In terms of the time or frequency they can be continuous or sampled
-In terms of their process applicability, they can be on-line or off-line
2
Q
What should we consider when determining an instruments suitability?
A
purpose, measurement range, accuracy, repeatability, resolution, measurement uncertainty, and turndown ratio
3
Q
Continuous Measurements
A
The output of a continuous measurement device, often called a transmitted, indicates the current value of the variable being measured. Temperature (thermocouple) and pressure (transducer) measurements are typically continuous, but they can be discrete too
4
Q
Discrete/Sampled Measurements
A
The output of a discrete measurement device is one of two states, depending on the value of the variable being measured on or off. Compositional measurements are typically discrete (but pH measurements can be continuous too)
5
Q
Terms that define the measurement range
A
-Lower limit of the measuring range
-Upper limit of the measuring range
-Span: difference between upper and lower limits
6
Q
Measurement Range
A
covers the maximum and minimum measured values for which the error of a measuring instrument will be within the specified limits. Within this range, the measurement has a well defined accuracy, outside the range it does not.
7
Q
Accuracy
A
degree of conformity of the measured value with either a standard, reference or other accepted value for the variable being measured. It is usually stated as the error in the measured value - frequent instrumentation calibration is required
8
Q
Repeatability
A
applies to the same measurement device in a given application
9
Q
Reproducibility
A
applies to different measurement devices in the same application
10
Q
Precision
A
May not mean accuracy, could be repeatability. The quality of being exact. To qualify a measurement or specification, especially by the number of significant figures.
11
Q
Resolution
A
To the least count of an instrument. For example, the resolution of a ruler with millimeter markings is 1mm.
12
Q
Turndown Ratio
A
Relates the maximum measured value to the minimum (non-zero) measured value
13
Q
Why is temperature important?
A
-Chemical reactions (rate and conversion)
-Physical/transport properties
-Determines state of matter (PVT diagram)
-Phase equilibria
-Plant equipment because material properties change with temperature
14
Q
Thermal Equilibrium
A
When two objects are at the same temperature are brought into thermal contact, no heat will flow from one object to the other because they are in thermal equilibrium
15
Q
Kinetic Theory of Gases
A
Temperature is a manifestation of measure of average kinetic energy of all particles (atoms and molecules) in a system. T = f (KE)
-Adding thermal energy (or heat) to a system causes an increase in the average kinetic energy and leads to an increase in temperature (no phase change)
16
Q
Difference in Temperature
A
1C = 1K = 1.8F = 1.8R
17
Q
How are reported variables related to measured
A
Instruments often measure a variable other than what is reported with the reported value related to the measured value by way of calibration
18
Q
Thermowell
A
Temperature probe is shielded from the actual process fluids by placing it in a thermowell. Then due to time lag and/or resistance Tp might be different from Tf.
19
Q
Thermometers (what does it encompass, how does it work)
A
-sensor and indicator
-under constant pressure the liquid volume changes with a change in temperature
20
Q
Simplest Thermometer
A
Liquid bulb thermometer
21
Q
Key property of thermometers/Bimetallic Strips
A
Volume coefficient of expansion, B = (1/V) (∂V/∂T)P
22
Q
Bimetallic Strips
A
Consists of two metal strips with differing thermal properties, bound or held together
-when the temperature changes the bimetallic strip deforms which can be correlated to the temperature
-typically made into a coil that is attached to an indicator gauge
23
Q
Two examples of Bimetallic Strips
A
-Old household thermostats
-Automobile temperature gauges
24
Q
Are bimetallic strips continuous?
A
-inexpensive way to obtain single temperature readings for on/off type of applications
-not useful for continuously monitoring
25
Thermocouples
Temperature difference produces a known voltage
-in order to measure temperature, T sense, the measured voltage and the value of T ref have to be known for each metal pair
26
What principle do thermocouples operate on?
Seebeck principle "Applying a temperature difference across two dissimilar metals can produce a voltage across a resistor"
27
Junctions
metals in a thermocouple are only in contact at two single points which are known as junctions
28
Wires of thermocouples
The wires (connecting the thermocouple to data logger) have to be of the same metals as the thermocouple. If not the reading will be affected by temperature differences along the length of the wire.
-the wires are color coded for easy identification
29
Grounded Junction
Wires are welded to the sheath and electrically grounded. The remaining circuit much not have another path to the ground.
30
Ungrounded Junction
Wires are not electrically grounded with the sheath. The data acquisition hardware must be grounded.
31
Exposed
The measuring junction is exposed directly to the medium. This provides for a fast response.
32
Rules of thumb for installing thermocouples
1. Use the least length of connecting wires
2. Avoid laying thermocouple wiring along the same path as high voltage cables and near power machinery
3. Use shielded wiring
4. Ground the electrical circuit only at one point
5. Avoid filtering noise with hardware as this can lead to excessive smoothing (smoothing can be done by data acquisition software)
-susceptible to electrical/magnetic noise due to small micro-to-milli volt range
33
How expensive are thermocouples
-Relatively inexpensive ($100 per probe)
34
Can many thermocouples be connected to a single data acquisition machine or terminal?
yes
35
RTDs
-RTDs consist of a length of fine wire wrapped around a ceramic or glass core
-RTD wire is a pure material, typically platinum, nickel, or copper which has an accurate resistance vs temperature relationship
-RTD elements are fragile often housed in protective probes
36
Three main categories of RTDs
-Thin-Film
-Wire Wound
-Coiled Elements
-Carbon resistors are used at low temperatures (-273 to -173C)
37
Thermocouple in comparison to RTD
RTDs have higher accuracy and repeatability than thermocouples, hence they are preferred in industrial applications at temperatures below 600C
38
The selection between thermocouples and RTDs is influenced by which 4 factors
1. Temperature range (RTDs are limited)
2. Response Time (Thermocouples have faster response time)
3. Size (Thermocouple diameters are smaller)
4. Accuracy and stability requirements (RTDs offer high accuracy, lower drift, longer life, and precision)
39
Which temperature can RTDs not be used at
not to be used about 600-650C because the platinum wire can become contaminated by impurities from the metal sheath
40
RTDs compared to thermistors
platinum RTDs are less sensitive to small temperature changes and have a slower response time. Thermistors have a smaller temperature range and stability.
41
Thermistors
-Resistor whose resistance depends on temperature more than a resistor
-When a current flows through a thermistor, it generates heat, which raises the temperature of the thermistor (T) above that of its environment (To)
42
What equation is used for thermistors?
-At steady state, the rate of heat generation (from Ohm's law) becomes equal to the rate of heat loss from convection (Newton's law of cooling)
iV = V^2/R = hA(ΔT) = K (T-To), K is called the dissipation constant and T is related to R
-This equality of the two steady state rates is used for temperature measurement. To = T-V^2/KR
43
Negative Temperature Coefficient (NTC) Thermistors
Resistance decreases as temperature increase, K is negative
44
Positive Temperature Coefficient (PTC) thermistors
Resistance increases as temperature increases, K is positive
45
What temperatures are thermistors used for
low temperatures -50C to 150C
46
Pyrometers
Remote sensing devices for measuring the temperature of a surface based on the amount of emitted thermal radiation
47
Infrared Thermometers (Modern Pyrometers)
measure the temperature of relatively cooler objects, by detecting their infrared radiation flux
48
How do pyrometers work?
Pyrometers have an optical system which focuses the thermal radiation onto a detector
-the output is used to detect the temperature from a distance without any thermal contact with the object
49
Equation for Pyrometers
the output signal of the detector at temperature T, is related to the thermal radiation, j of the target object through the Stefan - Boltzmann law: j =δET^4
50
Why is pressure an important variable?
-Chemical reactions (rate and conversion), especially gas phase
-Many physical, thermodynamic, and transport properties
-Pressure (with temp and composition) determines phase
-Phase Equilibria
-Plant equipment must be operated in accordance with its pressure specifications to avoid leakages, ruptures, accidents, and explosions
51
Mechanical Equilibrium (Pressure)
Flow of fluids is governed by pressure difference. No net flow can take place when two fluid bodies are at the same pressure (including KE and PE components)
52
Pressure definition
amount of force applied normal to the surface of an object per unit area (shear stress acts parallel to the surface)
53
Where is absolute pressure used vs pressure difference
Absolute pressure is needed for estimating physical and thermodynamic properties, whereas pressure difference is useful for flow and mechanical design calcs
54
gauge pressure
absolute pressure - atmospheric pressure
55
Steam supply in most process plants is of which two types
-Low pressure (p < 0.7 MPa, for thermal energy or heating applications)
-High pressure (0.7 MPa
56
Static Pressure
Static pressure in a still fluid is the same in all directions.
57
Dynamic Pressure
Flowing fluid exerts additional pressure on a surface facing the flow (KE - ; this directional component is the dynamic pressure. It is like the extra pressure feels when facing wind)
58
Stagnation Pressure
A probe facing the flow direction will measure the sum of static and dynamic pressure, the total value is stagnation pressure (pitot tube)
59
Manometers
-U shaped manometers are (about, less than) half filled with a liquid
-One end is connected to the process while the other is exposed to the reference pressure (which might be atmospheric)
60
Preferred liquid in manometers
Mercury is the preferred liquid due to its high density and low vapor pressure
-For measuring low pressure differences, light oil or water can be used
61
Equation for a manometer
Pa = Po + pgh, If Po = ambient pressure it is obtained from a barometer
62
Barometer
-The conventional barometer involves a special application of the manometer equation for which Po = 0
-One end is exposed to the atmosphere where is other end is sealed and under absolute vacuum (Po = 0)
63
Corrections needed for Barometers
-p = f(T), a temperature correction is needed for a different Hg temperature
-since g = f(latitude and altitude), another gravity correction is needed
64
Modern electronic barometers
in smart cell phones are based on MEMs and piezoresistive pressure sensing technology
65
Aneriod (Without Fluid) Gauges
include a metallic pressure sensing element that flexes elastically under applied pressure
-can be used to measure the pressure of both liquids and gases
-the deflection of the pressure sensing element is read by a linkage and connected to a needle or may be read by a secondary transducer
-Common secondary transducers typically measure
a change in capacitance due to the mechanical
deflection
66
Types of pressure sensing elements in aneriod
Bourdon tube, a diaphragm, a capsule or a set of bellows
67
Bourdon Tube (Aneriod Gauge)
A flattened tube, when pressurized tends to straighten or regain its circular cross section
-The strain of the tube is magnified by making the tube into a C-shape spiral or helix, such that the entire tube tends to straighten out or uncoil, elastically
-A flattened, thin-wall, closed-end tube is connected to the fluid under pressure. The closed-end move in an arc and this motion is converted by connecting link into the rotation of a gear
-Another gear on the pointer shaft magnifies the rotation
-The indicator card which indicated the magnitude of the strain of the tube which is related to the fluid pressure
68
Diaphragm Gauge (Aneriod Guage)
-deflection of a thin flexible membrane that separates the two pressure regions
-the diaphragm deformation is dependent on the difference in pressure between its two faces
-They are sensitive to very small pressure changes
and the readings can be recorded continuously
69
How is the diaphragm deformation measured
Using mechanical, optical, or capacitive techniques
70
Different shapes of diaphragm gauge
Flat, corrugated, flattened tube, capsule
71
Differential Pressure (DP) cell
-is an electrical capacitance device
-applies a differential pressure across two sides of a metal diaphragm that is submerged in a non-conducting dielectric oil
-the diaphragm forms one plate of a capacitor and either side of the cell body form the stationary plates
-the movement of the diaphragm produced by the differential pressure alters the separation between the plates, and alters the electrical capacitance of the cell which results in a change in the electrical output signal
72
What is pressure difference correlated to in a DP cell?
the degree of diaphragm movement is directly proportional to the pressure difference
73
Metal Strain Gauge (electronic pressure instrument)
The strain gauge is glued or deposited onto a membrane, who's deflection due to pressure causes a resistance change in the strain gauge which can be measured electronically
74
Capacitive (electronic pressure measurment)
Uses a diaphragm and pressure cavity to create a variable capacitor to detect strain due to applied pressure
75
Piezoelectric (electronic pressure measurment)
uses the piezoelectric effect in certain materials, such as quartz to measure the strain on the sensing mechanism
-piezoelectric effect is the ability of certain materials to generate an electric charge in response to applied stress
76
Piezoresistive Strain Gauge
Uses the piezoresistive effect of bonded or formed strain gauges to detect strain due to applied pressure
77
Piezoresistive Silicon Pressure Sensor
Uses a temperature-compensated piezoresistive silicon pressure sensor, which is selected for its good sensitivity and long-term stability
78
Electronic Pressure Instruments - Magnetic
Measures the displacement of a diaphragm by means of change in inductance
79
Electronic Pressure Instruments - Optical
Uses the physical change of an optical fiber to detect strain due to applied pressure
80
Resonant
Uses the change in resonant frequency in sensing mechanism to measure the stress caused by applied pressure
81
Thermal Conductivity
Based on the principle that both density and thermal conductivity of gases vary with pressure. A wire filament is heated electrically, whose steady-state temperature is dependent on the rate at which the filament loses heat to the surrounding gas.
82
Pirani Gauge
Consists of metal wire exposed to the pressure being measured which is heated electrically and cooled by the gas surrounding it. A change in gas pressure alters the rate of heat transfer, which changes the steady‐state
temperature of the wire.
83
Ionization Gauge
It is a highly sensitive gauge for
very low pressures or extreme vacuum. It senses
the pressure indirectly by measuring the electrical
ions produced when the gas is bombarded with
electrons.
84
How to chose a flow meter
-Identify the ones capable of meeting the requirement,
-Identify the ones that are available in the required size
and materials of construction, and
-Consider cost, service life, delivery, reliability, etc.
-Consider specific performance requirements, such as the
turn‐down ratio within the metering range (i.e.,
rangeability; ratio of maximum to minimum flow limits).
-Consider the ease of installation, maintenance, etc.
-The final selection: the least expensive option that has
all, or substantially all, of the needed features and
characteristics for a reliable service.
85
Differential Pressure Flow Meters Pros
They can be used for both gases and liquids, including viscous and corrosive fluids. They lack moving parts and are suitable for a range of pipe/tube diameters
86
Differential Flow Meters Cons
Their rangeability (maximum/minimum flows) may be limited, K (discharge coefficient) can vary with the Reynolds number, the installation could be involved/time consuming
87
Differential Pressure Flow Meters
detection of pressure drop across a flow restriction
88
Orifice Meter (type of differential pressure flow meter)
simplest and least expensive flow meter, it is relatively more expensive in smaller pipe sizes but economical in pipe sizes over 6 inch
-pressure drop as the flow occurs through a small hole
88
Equation for differential pressure flow meters
If the density is constant (or corrected for its changes the pressure drop can interpreted into a flow reading)
Q (flow rate) = K (constant) (pressure drop/density)^0.5
89
Orifice Meter Cons
Low accuracy and low rangeability, high pressure loss, declining measurement accuracy and long-term repeatability as the edge wears and/or deposits build up
89
Orifice Meter Pros
Useful for a wide range of applications, worn on damaged plate can still provide reasonably repeatable (even though inaccurate) results, ease of servicing and replacement, could be used with dirty fluids at higher temperatures
90
Venturi Meter
-shape of these meters is designed to minimize the pressure drop
-installed to reduce the size of pumping equipment and save on energy costs
-streamlined shape, less empirical calibration
91
Pros of Venturi Meter
Low pressure loss, smaller size of pumping equipment and saving of pumping energy costs, less abrasion, can be used to measure the flow of dirty fluid and slurries, low maintenance
92
Cons of Venturi Meter
Considerably larger, bulkier, heavier, and more expensive than orifice meters, installation is also more difficult and tedious
93
Pilot Tube Meter
Measure fluid velocity at a radial location
-the probe tip opening (hole) faces the flow and the difference measured between stagnation pressure (at the probe tip) and the static pressure (At the side)
-Volumetric rate is obtained by measuring the velocity at different radial locations and integrating the velocity profile over the pipe radius
94
Pros of Pilot Tube Meter
Low cost with small pressure loss, convenient for temporal measurement at a radial location
95
Cons of Pilot Tube Meter
Provide lower accuracy volumetric readings, subject to plugging, requiring maintenance. Multiple probes can help accuracy
96
Elbow Taps Meter
Measure flow rate by detecting the differential pressure between taps located on the inner and outer radii of an elbow
-In larger pipes, this can be a low-cost installation because the pipe size doesn't affect the overall cost
-It is inaccurate measurement technique requiring high flow rates and long upstream straight pipe
97
Elbow Taps Pros
A low-cost simple measuring system with negligible pressure loss, other than 2 taps no other material or modification needed
98
Elbow Taps Cons
Much lower accuracy flow estimates
99
Rotameters (Most common variable area meters)
Measure flow rate by allowing the cross-sectional area of the device to vary with flow rate
-weighted float rises in a tapered tube as the flow rate increases
-the float becomes stationery when area between the float in tube is large enough that the net weight of the float is balanced by the drag force of the flowing fluid
-widely used especially low flow rates (hospitals)
100
Rotameters pros
-Low cost, low pressure loss, direct flow indication, ability to detect very low flow rates of both gases or liquids including viscous fluids
101
Rotameters cons
the need for vertical mounting, the glass tube design has low pressure rating
-The metallic tube units are readily available and can
be obtained in larger sizes, with higher pressure
ratings. Good rangeability (5:1) and a linear output,
but they, too, are limited to clean fluids and vertical
mounting.
-Another design type replaces gravity with spring
loading, which can be mounted even horizontally,
as in pipelines. An increase in flow rate causes a
compression or deflection of a spring, and this
movement is used in the display.
102
Positive Displacement Meters
-PD pumps that deliver accurate quantities, either in batch processes, for injecting additives, or for sales/delivery.
-The PD meters trap a fixed volume of fluid in each cycle and transfer it from the inlet to the outlet side.
103
Volumetric Flow Calc - Positive Displacement Meters
Volumetric flow=volume of liquid trapped in each cycle frequency
104
Pros Positive Displacement Meters
Good accuracy and rangeability are particularly suited to measure the flow of high viscosity liquids; calibration is usually not needed and the output is linear with the frequency.
105
Cons Positive Displacement Meters
Limited to clean fluids (to avoid abrasion); require regular maintenance (due to wear and tear); can be bulky, heavy and costly.
106
Electromagnetic Meters
-operate in accordance with Faraday’s law (electricity
& magnetism)
-measure the velocity of electrically conductive liquids as their flow cuts the magnetic field maintained across the metering tube
-often used corrosive liquids and slurries
107
Electromagnetic Meters Pros
unobstructed fluid path and lack of moving parts;
no pressure loss; no wear and tear; chemical
compatibility with most liquids; can handle wide
variations of viscosity, pressure, temperature, and
density; available in
a wide range of sizes.
108
Electromagnetic Meters Cons
can only be used with electrically conductive
fluids (not for gases and hydrocarbon liquids); costly to
purchase and maintain; safety concerns.
109
Turbine Meters
The output signal is linear with the flow rate, as the
speed of rotation of the turbine (caused by flowing
fluid) is measured.
Can be used for both liquids and gases.
* Suitable for both low and high flow rates as well as for
low‐ to medium‐viscosity fluids.
* Rangeability of single turbine meters is about 10:1, for
dual‐turbine meters, it exceeds 100:1.
* Can be used for high‐accuracy measurements at all
pressures and temperatures.
110
Turbine Meters Pros
Easy to install, small in size, fast response
111
Turbine Meters Cons
Relatively high cost; incompatible with viscous or
dirty liquids; damaged if vapour slugs are present.
112
Vortex Meters (Less Common)
Fluid velocity is measured using the von Kármán effect,
which states that when flow passes by a bluff body (as
opposed to a streamlined body), a repeating pattern of
swirling vortices is generated.
-Vortices are created as the liquid moves through the
sensor; the imbedded electronics converts the fluid
oscillation signal into flow rate.
113
Three types of vortex meters
vortex shedding, the vortex precession, and the fluidic oscillation.
114
Pros vortex meters
Cost effective; no moving components; can be
used with gas, steam, or liquid; good accuracy and
repeatability; high rangeability; low maintenance.
115
Cons vortex meters
not used with viscous or dirty fluids; limited to
12 inch (300 mm) size; Reynolds number must be
higher than about 10,000.
116
Ultrasonic Meters
Can be used with corrosive and unclean liquids.
-In the Doppler type, an ultrasonic pulse is beamed into
the pipe and reflected by inclusions such as air or dirt.
-The velocity is detected only in a small region (where
the sonic beam enters the flowing stream); hence, that
velocity should be representative of the full cross
section of the pipe.
117
Ultrasonic Meters Pros
no moving components; low cost, which does not
change with pipe size; useful for corrosive liquids with
suspended particles; no contact with the flowing fluid;
good accuracy and repeatability.
118
Ultrasonic Meters Cons
not suitable for the measurement of clean fluids
or gases
