Process Instrumentation basics (1).pdf the basic

PROCESS
INSTRUMENTATION
Dr. Asma Iqbal
Assistant Professor
NIT Srinagar

PROCESS INSTRUMENTATION
 Raw material Costly Products
 Commercial-scale operation
Refinery Control Room

 Process instrumentation is about measurement and
control.
 Instrumentation engineering is the engineering
specialization focused on the design and configuration
of process systems.
 Instruments are devices which are used in measuring
attributes of process systems.
 It provides the various indications to the operator for
controlling the process.
 Instrumentation is the basis for the control of a
process.

 It provides the various indications to the operator for
controlling the process.
 In some cases, operator records these indications for
evaluating the current condition of the process and to
take actions if the conditions are not as expected.
 Because of the continuous interactive nature of the
most of the processes, manual control is not feasible
and is unreliable.
 With instrumentation, automatic control of such
processes can be achieved.

INSTRUMENT
 An instrument can be defined as a device for determining
the value or magnitude of a quantity or variable.
 Measuring instruments, and formal test methods which
define the instrument’s use, are the means by which the
variables and the relations between variables are found.
Need of Measurement
In chemical process industry, we always want to know:
I. Temperature: Boiler, Reactor, furnace, etc.
II. Pressure: High pressure reactor vessel, etc.
III. Flow Rate: Process Fluid in a pipe, reflux stream in
the distillation column, etc.

NEED OF MEASUREMENT
IV. Concentration: Reactor, etc.
V. Others: pH, moisture content, conductivity,
density of a sample, etc.
The fundamental need of measurement in
industrial manufacturing and processing is to
obtain a numerical value corresponding to the
variable being measured so that we can determine
the quantity of a product (or) the efficiency of
production.

CLASSIFICATION OF MEASURING INSTRUMENTS
 The instrument used for measuring the physical and
electrical quantities is known as the measuring
instrument.
 The term measurement means the comparison between
the two quantities of the same unit.
 The magnitude of one of the quantity is unknown, and
it is compared with the predefined value.
 The result of the comparison obtained regarding
numerical value.
 The measuring instrument categorised into three
types:

Process Instrumentation basics (1).pdf the basic

 The mechanical instrument uses for measuring the
physical quantities. This instrument is suitable for
measuring the static and stable condition because the
instrument is unable to give the response to the
dynamic condition.
 The electronic instrument has quick response time.
The instrument provides the quick response as
compared to the electrical and mechanical instrument.
 The electrical instrument is used for measuring
electrical quantities likes current, voltage, power, etc.
The ammeter, voltmeter, wattmeter are the examples
of the electrical measuring instrument. The ammeter
measures the current in amps; voltmeter measures
voltage and Wattmeter are used for measuring the
power.

TYPES OF ELECTRICAL INSTRUMENT
1. Absolute instrument
 Absolute instrument measures the process variable directly
from the process without the use of conversion.
 Such instruments do not require comparison with any
other standard.
 The tangent galvanometer is an example for the absolute
instrument.
 These instruments are used as standards in labs and
institution.
2. Secondary instrument
 These instruments are so constructed that the deflection of
such instruments gives the magnitude of the electrical
quantity to be measured directly.
 These instruments required to calibrated with respect to
the standard instrument.
 These instruments are usually used in practice.
 It has two types:

Digital and Analog instrument:
Digital Instrument
 The digital instrument gives the output in the numeric
form.
 The instrument is more accurate as compared to the
analogue instrument because no human error occurs in
the reading.
Analog instrument
 The instrument whose output varies continuously is
known as the analogue instrument.
 The analogue instrument has the pointer which shows
the magnitude of the measurable quantities.
 The analogue device classifies into two types:
 Null Type Instrument
 Deflection Type Instrument

 Null Type Instrument
 In this instrument, the zero or null deflection
indicates the magnitude of the measured quantity.
 The instrument has high accuracy and
sensitivity. In null deflection instrument, the one
known and one unknown quantity use.
 When the value of the known and the unknown
measuring quantities are equal, the pointer shows
the zero or null deflection.
 The null deflection instrument is used in the
potentiometer and in galvanometer for obtaining
the null point.

Deflection Type Instrument
 The instrument in which the value of measuring
quantity is determined through the deflection of
the pointer is known as the deflection type
instrument.
 The measuring quantity deflects the pointer of the
moving system of the instrument which is fixed on
the calibrated scale.
 Thus, the magnitude of the measured quantity is
known.
a) Indicating instrument
b) Integrating instrument
c) Recording instrument

(a) Indicating instrument:
 The instrument which indicates the magnitude of the
measured quantity is known as the indicating
instrument.
 The indicating instrument has the dial which moves on
the graduated dial.
 The voltmeter, ammeter, power factor meter are the
examples of the indicating instrument.
(b) Integrating instrument:
 The instrument which measures the total energy
supplied at a particular interval of time is known as the
integrating instrument.
 The total energy measured by the instrument is the
product of the time and the measures electrical
quantities.
 The energy meter, watt-hour meter and the energy
meter are the examples of integrating instrument.

(c) Recording instrument:
 The instrument records the circuit condition at a
particular interval of time is known as the
recording instrument.
 The moving system of the recording instrument
carries a pen which lightly touches on the paper
sheet.
 The movement of the coil is traced on the paper
sheet.
 The curve drawn on the paper shows the variation
in the measurement of the electrical quantities.
 The response time of the electronic instrument is
very high as compared to the electrical and
mechanical device.

CHARACTERISTICS OF INSTRUMENTS
There are two types of characteristics of instruments:-
1.Static characteristics,
2.Dynamic characteristics.
1. Static Characteristics
 The static characteristics of an instrument are
required to be considered for the instruments which
measure unvarying process conditions.
 The static characteristics are defined for the
instruments which measure quantities which do not
vary with time.

THE MAIN STATIC CHARACTERISTICS ARE :-
1. Accuracy
2. Sensitivity
3. Reproducibility
4. Drift
5. Static error
6. Dead zone
7. Precision
8. Threshold
9. Linearity
10. Stability
11. Range or Span
12. Bais
13. Tolerance
14. Hysteresis

1. Accuracy
 It is the degree of closeness with which an
instrument reading approaches the true value of
the quantity being measured.
 The accuracy of a measurement indicates the
nearness to the actual/true value of the quantity.
2. Sensitivity
 Sensitivity is the ratio of change in output of an
instrument to the change in input.
 The manufactures specify sensitivity as the ratio
of magnitude of the measured quantity to the
magnitude of the response.
 This ratio is called as Inverse sensitivity or
deflection factor.
 Sensitivity Meter

3.Reproducibility
 Reproducibility is defined as the degree of closeness by
which a given value can be repeatedly measured.
 The reproducibility is specified for a period of time.
 Perfect reproducibility signifies that the given readings
that are taken for an input, do not vary with time.
4.Drift
 The drift is defined as the gradual shift in the
indication over a period of time where in the input
variable does not change.
 Drift may be caused because of environment factors
like stray electric fields, stray magnetic fields, thermal
e.m.fs, changes in temperature, mechanical vibrations
etc.

 Drift is classified into three categories:
a. Zero drift
b. Span drift or sensitivity drift
c. Zonal drift
5. Static error
 It is the deviation from the true value of the
measured variable.
 It involves the comparison of an unknown quantity
with an accepted standard quantity.
 The degree to which an instrument approaches to its
excepted value is expressed terms of error of
measurement.

6.Dead zone
 It is the largest changes of input quantity for which there is
no output.
 For e.g. the input that is applied to an instrument may not
be sufficient to overcome friction. It will only respond when
it overcomes the friction forces.
7.Precision
 It is a measure of the reproducibility of the measurement
that is given a fixed value of variable.
 Precision is a measure of the degree to which successive
measurements differ from each other.
 For example consider an instrument on which readings can
be taken upto 1∕100th of unit.
 The instrument has zero adjustment error. So, when we
take a readings, the instrument is highly precise. However
as the instrument has a zero adjustment error the readings
obtained are precise, but they are not accurate.

8.Threshsold
 Threshold is the smallest measurable input, below
which no output change can be identified.
 While specifying threshold, manufactures give the
first detectable output change.
9.Linearity
 Linearity is defined as the ability of an instrument to
reproduce its input linearly.
 Linearity is simply a measure of the maximum
deviation of the calibration points from the ideal
straight line.
 Linearity is defined as,
linearity=Maximum deviation of o/p from
idealized straight line ∕ Actual readings

10.Stability
The ability of an instrument to retain its performance
throughout its specified storage life and operating life
is called as Stability.
11.Range or Span
 The minimum and maximum values of a quantity for
which an instrument is designed to measure is called
its range or span.
 Sometimes the accuracy is specified in terms of range
or span of an instrument.
12.Bais
 The constant error which exists over the full range of
measurement of an instrument is called bias.
 Such a bais can be completely eliminated by
calibration.
 The zero error is an example of bais which can be
removed by calibration.

13.Tolerance
 It is the maximum allowable error that is specified in terms
of certain value while measurement, it is called as tolerance.
 It specifies the maximum allowable deviation of a
manufactured device from a mentioned value.
14.Hysteresis
 Hysteresis is a phenomenon which depicts different output
effects while loading and unloading.
 Hysteresis takes place due to the fact that all the energy put
into the stressed parts when loading is not recoverable while
unloading.
 When the input of an instrument is varied from zero to its
full scale and then if the input is decreased from its full scale
value to zero, the output varies.
 The output at the particular input while increasing and
decreasing varies because of internal friction or hysteric
damping.

2. DYNAMICS CHARACTERISTICS
 Instruments rarely respond to the instantaneous
changes in the measured variables.
 Their response is slow or sluggish due to mass,
thermal capacitance, electrical capacitance,
inductance etc. sometimes, even the instrument has to
wait for some time till, the response occurs.
 These type of instruments are normally used for the
measurement of quantities that fluctuate with time.
 The behaviour of such a system, where as the input
varies from instant to instant, the output also varies
from instant to instant is called as dynamic response
of the system.
 Hence, the dynamic behaviour of the system is also
important as the static behaviour.

 The dynamic inputs are of two types:
1. Transient
2. Steady state periodic.
 Transient response is defined as that part of the response
which goes to zero as the time becomes large.
 The steady state response is the response that has a
definite periodic cycle.
 The variations in the input, that are used practically to
achieve dynamic behaviour are:
I. Step input:-The input is subjected to a finite and
instantaneous change. E.g.: closing of switch.
II. Ramp input:- The input linearly changes with respect to
time.
III. Parabolic input:- The input varies to the square of time.
This represents constant acceleration.
IV. Sinusoidal input:- The input changes in accordance with
a sinusoidal function of constant amplitude.

 The dynamic characteristics of a measurement system are:
1) Speed of response
2) Fidelity
3) Lag
4) Dynamic error
1) Speed of Response
It is defined as the rapidity with which an instrument,
responds to the changes in the measured quantity.
It shows how active and fast the system is.
2) Fidelity
It is defined as the degree to which a measurement system
is capable of faithfully reproducing the changes in input,
without any dynamic error.
3)Lag
 Every system requires its own time to respond to the
changes in input. This time is called as lag.
 It is defined as the retardation or delay, in the response of
a system to the changes in the input.

 The lags are of two types:
I. Retardation lag: As soon as there is a changes in the
measured quantity, the measurement system begins to
respond.
II. Time delay: The response of the measurement system
starts after a dead time, once the input is applied.
They cause dynamic error.
4)Dynamic error
It is the difference between the true value of the
quantity that is to be measured, changing with time
and the measured value, if no static error is assumed.

BASIC TERMINOLOGIES
 Process: Series of continuous or regularly recurring
steps or actions intended to achieve a predetermined
result, as in heat treating metal, or manufacturing
acid.
 Transducer (sensor): Element which converts one
form of Energy to Other form.
 Primary Transducer: Transducer which converts
the Process parameter to a form readable by
Secondary Transducer. Eg: Orifice plate
 Secondary Transducer: Transducer or
transmitter which responds to a measured variable
and converts it to a standardized transmission
signal which is a function only of the
measurement. Eg: DP Transmitter

BASIC TERMINOLOGIES
 Conveyor:
Conveyors are used as components in automated
distribution and warehousing.
A belt conveyor consists of two or more pulleys, with a
continuous loop of material - the conveyor belt - that
rotates about them.
One or both of the pulleys are powered, moving the belt
and the material on the belt forward.
The powered pulley is called the drive pulley while the
unpowered pulley is called the idler.
 Beltweigher:
Material flowing over the belt may be weighed in transit
using a beltweigher.
 A belweigher or belt weigher is a piece of industrial
control equipment used to gauge the mass or flow rate of
material travelling over a conveyor belt.

BASIC TERMINOLOGIES
 Signal: The signal is the event or phenomenon that
conveys data from one point to another.
 Loop: A Loop is a combination of one or more
interconnected instruments arranged to measure a
process variable. It shall comprises the whole chain
from Primary element to Correcting Element.
 Controller: A device that operates automatically by
use of some established algorithm to regulate process
variable(PV) according to the set point(SP). The
controller input receives information about the status
of the process variable and then provides an
appropriate output signal(MV-manipulated variable)
to the final control element(eg-valves etc.,).

BASIC TERMINOLOGIES
 Interlock: It refers to the set of plant conditions (eg. Level
of a tank, temp of furnace, position of furnace or a valve,
flow of a fluid, etc) which are to be satisfied before
operating(starting, stopping, opening ,closing ,etc)of any
instrument or equipment.
 Analyser:
 Monitor pollutant gas emissions from industrial
processes.
 Gas analyzer is a gas comparator providing high
linearity of signal transformation function.
 Weighfeeder:
 Controller multiplies the signal from the load cell (belt
load, kg/m) with that from the speed transducer(belt
speed, m/s) to get the feed rate.
 The controller then either changes the belt speed or belt
load to get the set feed rate.

MEASUREMENT OF INSTRUMENTS
 The Instruments duty is to measure the given specimen
according to the given dimensions.
 The units of measurement in both C.G.S., M.K.S and F.P.S. are
as follows:
1. Metric system (C.G.S)
 Length is measured in centimeter (cm)
 Mass is measured in grams (gm)
 Time is measured in second (s)
2. S.I. system (M.K.S)
 Length is measured in meter (m)
 Mass is measured in Kilograms (kg)
3. F.P.S. Systems
 Length is measured in foot (ft)
 Mass is measured in pound (lb)

TYPES OF MEASUREMENT
 Direct Measurement
The quantity to be measured is determined directly by
instrument.
Example: Measure the distance by scale, micrometer,
vernier callipers; weight of substance.
 Indirect Measurement
The quantity to be measured is not measured directly.
But other related parameter is measured and inference
is drawn from there.
Example: Measure the distance by optical method
where we use telescope to calculate distance

FUNCTIONS OF AN INSTRUMENT
The primary job of an instruments is to measure a
process variable. While doing so, it performs several
functions such as
 Transmitting
 Signalling
 Registering
 Indicating
 Recording
 Transmitting:
Instrument conveys the information concerning the
measured quantity over some distance/to remote point.

Example:
(1) Thermocouple which measures the temperature in a
reactor (or) distillation column and we can see results from
central panel.
(2) Transmitters are used in Telephone in process control loop.
 Signalling
Instrument indicate the general value or range of values of
its measuring quantity.
Example: Weighing balance
 Registering
Instrument indicates by numbers or some other symbol of
discrete increments, the value of the quantity being
measured.
Example: Cash Register

 Indicating
An indicating instrument indicates the instantaneous
value of the variable being measured at that time.
Such instrument normally uses a calibrated scale and
a pointer.
Example: Clock, Ammeter, Voltmeter, Wattmeter, Lab
pressure gauge.
 Recording
Instrument makes a written record of the value of the
measured quantity against other variable or against
time.
Example: Modern computer, a circular chart recorder,
etc.

ELEMENTS OF MEASUREMENT
 The purpose of analysis and synthesis, the instruments
are considered as systems.
 The different components are called elements and they
perform certain definite and required steps in the act of
measurement.

FUNCTION OF EACH ELEMENT
1. Primary Sensing Element:
An element that is sensitive to the measured
variable. The sensing element sense the condition,
state or value of the process variable.
2. Variable conversion element (Transducer Element):
An element that converts the signal from one hysical
form into another without changing the information
contents of the signal.
Example: Bourdon Tube, Bellow, etc.

3. Data Transmitter Element:
An element that transmits the signal from one
location to another without changing its information
content. Data may be transmitted over long
distances.
Example: Shaft and gearing assembly, telemetry
system for transmitting signals from missiles to
ground equipment.
4. Variable Manipulation Element:
An element that operates on the signal according to
some mathematical rule without changing physical
nature of the variable.
Input x Constant = Output

5. Processor
An element that modifies the data before it is
displayed or finally recorded. It converts the data into
useful form.
6. Data Presentation Element:
An element that provides a record or indication of the
output from the data processing element output stage
constitute the data display, record or control.

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