first day class introduction,temp

7/30/2019 First Day Class Introduction,Temp

1/63

FIELD INSTRUMENTS


2/63

INTRODUCTION:-

Instrumentation is a field of sciencewhich deals with measurement of

different parameters and controlling

them.WHAT IS FIELD ?

INDUSTRY :Systematic Economic Activity that

could be related to Manufacture/Service/Trade


3/63

Various measurements generally in an industry

Fluid pressure

Fluid flow rate

The temperature of an object

Fluid volume stored in a vessel

Chemical concentration

Machine position, motion, or acceleration

Physical dimension (s) of an object

Count (inventory) of objects

Electrical voltage, current, or resistance


4/63

Automation

Automation is the process of operationof machines and systems without

significant human intervention by

technical equipment to achieves

performance superior to manual

operation.

(Automation is basically the delegation of human control function

to technical equipment for) Increasing Productivity

Increasing Quality

Reducing Cost

Increasing Safety in working conditions


5/63


6/63

Elements of automation

Process -a sequence of actions that take place in a set up or a plant where wewant to control.

Instrument- it is a physical means to determine the quantity orvariable

Sensors Primary Sensing Element orPSE :-A device that directly senses the

process variable and translates that sensed quantity into an analog

representation (electrical voltage, current, resistance, mechanical

force, motion, etc )

Transmitter:A device that translates the signal produced by a primary sensingelement (PSE) into a standardized instrumentation signal such as 3-15 PSI air

pressure, 4-20 mA DC current, 0 10 V DC , into Field bus digital signal packet,

etc., which may then be conveyed to an indicating / controlling device, or both.

transducer :- A device that converts one standardized instrumentationsignal into another standardized instrumentation signal, and/or performs somesort of processing on that signal, often referred to as a converterexample:- P/I


7/63

Controller-A device that receives a process variable (PV) signal from aprimary sensing element (PSE) or transmitter, compares that signal to the

desired value for that process variable (called the set point), and calculates

an appropriate output signal value to be sent to a final control element

(FCE) such as an electric motor or control valve.

Final control element-A device that receives the signal from acontroller to directly influence the process.( Examples: variable-speed electric

motor, control valve, electric heater. )


8/63

Instrumentation terms and their definitions(Industrial measurement and control systems terms and standards)

Process Variable

Set poin t

Lower- and Upper-range values

Automat ic mode

Manual mode Measur ing signal

Control l ing signal


9/63

Process Variable (PV): The specific quantity we are

measuring in a process. Examples:pressure, level,

temperature, flow, electrical conductivity, Ph,position, speed, vibration.

Set po in t (SP):The value at which we desire the

process variable to be maintained at. In other words,

the target value of the process variable.


10/63

Lower- and Upper-range values (LRV and URV):

the values of process measurement deemed to be

0% and 100% of a transmitters calibrated range.

Example - if a temperature transmitter is calibrated to

measure a temperature starting at 300 degrees

Celsius and ending at 500 degrees Celsius- 300

degrees would be the LRV and 500 degrees the

URV.

Relating 4 to 20 m A signals to instrument variables


11/63

Autom at ic mode:When the controller generates an

output signal based on the relationship of process

variable (PV) to the set point (SP).

Manual mode:When the controllers decision-

making ability is bypassed to let a human operator

directly determine the output signal sent to the finalcontrol element.


12/63

CASE STUDY


13/63

CHEMICAL REACTOR TEMPERATURE CONTROL SYSTEM


14/63

A steam-heated jacket envelops the reactor vessel ,transfers heat from the steam into the chemical solutioninside. the temperature transmitter, located near the

bottom of the vessel. The special connector (solid dots with lines in between

sometimes referred to as a fieldbus ) is used to connectthe temperature transmitter (TT) with the temperature

indicating controller (TIC). The TT send a digitalelectronic instrument signal using digital bits ofinformation rather than an analog signal (such as 4 to20 m A or 3 to 15 PSI).

The controller / TIC send an analog signal of 4 to 20

milliamps DC shown by dashed line a device labeled TYwhich is a transducer to convert the 4 to 20 mAelectronic signal into a 3 to 15 PSI pneumatic signalwhich then actuates the valve.


15/63

Various instruments used in an

industry Fluid pressure- can be measured by using basic sensing

elements such as bourdon tube , capsule, bellow , diaphragm

made of metal such as berelium copper and phosphorus bronze

, silicon diaphram .

Fluid flow rate- can be measured by using basic sensing

elements such as orifice plate , venture tube , pitot tube . The temperature of an object- can be measured by using basic

sesnsing elements such as, RTD, thermocouple, thermister.

Fluid volume/level stored in a vessel- can be measured by using

float level,hydro static pressure measurement,by bubbler

system, by displacer method,ultra sonic method

Chemical concentration

Machine position, motion, or acceleration-

Physical dimension (s) of an object

Count (inventory) of objects


16/63

TYPES OF SENSORS

All the sensors can be classified in to two types

based up on the type of signals they provide.1. Digital instruments/sensors - instruments which

provide digital signals are called digitalinstruments. Digital signal are in the form of low

/high voltage /0 /1.{ absence / presence ofvoltage }.

2. Analog instruments/sensors- instruments whichprovide analog out put signal are called analog

instruments . Analog signal vary in continuousfashion and take on infinite number of values inany given range. The industrial standardsignals are 4 to 20 milliamp DC,0-10 volt DC,3-15PSI .


17/63

SOURCING AND SINKING

CIRCUITS

switch sinks current in from the LED through itsoutput terminal, through its transistor, and down

to ground


18/63

switch sources current from the positive supply

terminal

through its transistor and out to the LED through its

output terminal


19/63

CLOSE LOOP OPEN LOOP CONTROL OF PROCESS

OPEN LOOP CONTROL


20/63

OPEN LOOP CONTROL

The control system where the output variable does not have

any influence on the input variable is called open loop control.Fig-At constant applied pressure, the volumetric flow can be maintained

by the position of the control valve. Here the output variable is

"volumetric flow" and the input variable is "control valve setting" .if the

applied pressure fluctuates, the volumetric flow will also fluctuate.

In other words one or more input variables of a system act on a processvariable and causes the possible deviations . e.g. disturbances

caused by other in put variable (pressure) are not compensated for in

the open loop control process. Thus, the characteristic feature of

open loop control is an open action flow .


21/63

CLOSE LOOP CONTROL


22/63

CLOSE LOOP CONTROL

It is a process where the controlled variable is

continuously monitored and compared with thereference variable. The input variable for the systemis influenced to adjust the output variable to thedesired value despite any disturbing influences. This

feedback results in a closed-loop action. In other words in a closed loop control system, the

variable to be controlled is continuously measuredand then compared with a predetermined value

(reference variable ). If there is a difference betweenthese two variables, adjustments are being madeuntil the measured difference is eliminated and thecontrolled variable equals the reference variable.Hence, the characteristic feature of closed loopcontrol is a closed action flow


23/63

Measurement is a process of assigning a number to

an attribute (or phenomenon) according to a rule or

set of rules.(Or )

(it is an act of comparison between the un known

quantity and predefined standard)

MEASUREMENT


24/63

METHODS OF MEASUREMENT

1. Direct measurement: Un known quantity isdirectly compared against a standard.

2. Ind irect measu rement:measuring something

by measuring something else


25/63

ELEMENTS OF MEASURING SYSTEM


26/63


27/63

MEASUREMENT SYSTEMA measuring system consist of

Sensor / primary sensing element - to sense andmeasure the process variable by producingdisplacement , change in resistance)

Variable conversion element to convertdisplacement , change in resistancetoanalogous form( in to a voltage .. Etc exwhenpressure changes a strain transducer converts the deformationof the diaphragm into an electrical resistance change.)

Variable manipulation / processing element toprocess analog signal and feed the data for thepurpose of display/control. All elements may bein one box / separated.


28/63

ACTIVE AND PASSIVE INSTRUMENTS

ACTIVE INSTRUMENTS:- those which does notneed external power supply for excitation.

PASSIVE INSTRUMENTS:- those which needexternal power supply for excitation


29/63

DIGITAL SENSORS

A discrete variable or measurement refers to a

true-or-false condition. Thus, aDiscreteSensor is one that is able to indicatewhether the measured variable is

above or below a specified set point. DiGITAL sensors typically take the form of

Switches andare less sophisticated than so-calledcontinuous sensors


30/63

NORMAL status of a switch

Normal means not being pressed Electrical switch contacts are typically classified

as

normally-open

normally-closed


31/63

VARIOUS DISCRETE SENSORS AND THEIR NORMAL POSITION

Hand switch: no one pressing the switch

Limit switch: target not contacting the switch Proximity switch: target far away

Pressure switch: low pressure (or even a

vacuum)

Level switch: low level (empty)

Temperature switch: low temperature (cold)

Flow switch: low flow rate (fluid stopped)


32/63

INSTRUMENTS

Analog electronic instrument-An instrument can be classified as

analog if it produces an analog signal standard to communicateinformation, even if the internal construction and design of the instrument

may be mostly digital in nature.

analog current signals-in modern industrial instrumentationsystems the 4 to 20 milliamp DC is an analog signal standard ,this electric

current is used to proportionately represent measurements or commandsignals. 4 milliamp current value represents 0% of scale, a 20 milliamp current valuerepresents100% of scale, and any current value in between 4 and 20 milliamps

represents a commensurate percentage in between 0% and 100%.

Most of the industrial control systems use at least two different 4-20 mA

signals: one to represent the process variable (PV) and one to represent the

command signal to the final control element.


33/63

4-wire or self-poweredTransmitter connection to plc/controller

Power source near to transmitter method :- 4-20 mA DC

electric current signals are used to communicate processmeasurement information from transmitters to controllers. Thetransmitter has two terminals for the 4-20 mA signal wires to connect,and two more terminals to connect a power source. The current signalfrom the transmitter connects to the process variable input terminalsof the controller to complete the loop. BUT process controllers areunable to directly accept milliamp input signals, however acceptsvoltage signals. For this reason a precision resistor of 250 ohmsconnected across the input terminals of controller convert the 4-20 mAsignal into a standardized analog voltage signal of 1 to 5 volts andfurther this voltage is converted in digital by ADC which can be

processed by the controllermicro processor. Some models of controller use different voltage ranges so require

different precision resistor values.


34/63

POWER SOURCE NEAR TO CONTROLLER METHOD

As shown in the fig same 4 wire transmitter is powered through additional

wires in the cable from a power source located in the same panel as the

controller.

Disadvantages:1. requirement of two more conductors in the cable

2. cable will be larger in diameter

3. more expensive for a given length.

4. Cables requires larger electrical conduit to fit in to

5. all field wiring panels will have to contain more terminal blocks.

6. Note :-this method of connection can be used If no suitable electrical power source exists at the

transmitter location.


35/63

2-wire (Loop-powered) Transmitter Method

In this method both electrical power to transmitter and 4 to 20 milliamps DC

analog information are communicated over the same two wires so called loop

powered.

Loop powered 2-wire transmitters circuitry acts as a current regulator, limiting current in

the series loop to 4 to 20 ma DC representing the process measurement.

The current source in loop-powered transmitter actually behaves as an electrical load.

Power source being 24 volts DC, the maximum voltage dropped across the

controllers 250 ohm resistor being 5 volts DC, the transmitter should alwayshave at least 19 volts available at its terminals to operate, while regulating

current to signal the process measurement.


36/63

The transmitter is designed with sensing, scaling, and output conditioningcircuitry inside and run on less then 4 mA of DC current, and at a modest

terminal voltage.

The transmitter circuitry uses a transistor to shunt (bypass) extra current when

needed to make the total current indicative of the process measurement

If the transmitters internal operating current is only 3.8 mA, and it must regulate

loopcurrent at 16 mA to represent a condition of 75% process measurement, the

transistor will bypass 12.2 mA of current.


37/63

ADVANCEMENT IN SENSOR TECHNOLOGY:-

Olden days current-based industrial transmitters

were using a current signal standard: 10 to 50milliamps DC and Loop power supplies for these

transmitters ranged upwards of 90 volts to

provide enough power.

Safety concerns made the 10-50 mA standardunsuitable for some industrial installations, and

advancement in technology reduced power

consumption in modern industrial

instrumentation systems to 4 - 20 milliamp DC asan analog signal standard , for nearly all types of

process transmitters.


38/63

INTELLIGENT INSTRUMENTS

In the past few years the development of

measurement systems made intelligent devices inwhich the computational element (usually called a

microcomputer or microprocessor) was integrated

into the measurement system.

SMART SENSORS:-Over years the size of intelligent instruments has

gradually reduced and the functions performed

have steadily increased.

they included a microprocessor within the sensoritself, are usually called as smart sensors instead

ofintelligent instruments.


39/63

Introduction to Temperaturemeasurement:-

Thermal conductivity:the flow of heat from a hightemperature region to a low temperature region.

Conduction :flow of heat in a material is transferred from one

molecule to the next

Convection :the transfer of heat due to motion of elevated

temperature particles in a material (such as liquid and gases)

Radiation :the emission of energy by electromagnetic waves thattravel at the speed of light through most materials that do not conduct

electricity. Thermistors:a class of metal oxide (semiconductor material)

which have a high negative temperature coefficient of resistance, but

can also be positive .


40/63

BASICS ON ELECTRICITY .

Resistance :It is the property of a substance to oppose theflow of electricity through it .it is measured in resistancevaries directly with length and inversely with cross section. It also

depends up on material , temperature. R=.I /A Wheatstone bridge circuit :it is used to measure resistance and

consist of four arms and operates upon null indication principle.attwo points an EMF is applied and at other two points a null

indication instrument/ galvanometer is connected.


41/63

See beck effect:the voltage produced in a thermocouple isproportional to the temperature between the two junctions.

Peltiereffect:if a current flows through a thermocouple one junction is

heated (puts out energy) and the other junction is cooled (absorbs energy).

Thermopile : a number of thermocouples connected in series, toincrease the sensitivity and accuracy by increasing the output voltage when

measuring low temperature differences.

Pyrometers are devices that measure temperature by

sensing the heat radiatedfrom a hot body through a

fixed lens that focuses the heat energy on to a

thermopile;this is a noncontact device. Furnace

temperatures, for instance, are normally measured

through a small hole in the furnace wall. The distance

from the source to thepyrometer can be fixed and the

radiation should fill the field of view of the sensor


42/63

TEMPERATURE SWITCHES Bi-metal temperature switch - it is a bond of two dissimilar metals of

equal sizes having different temperature expansion of coefficient .when

subjected to temperature the bond bends. Temperature switches often use

bimetallic strips as temperature sensing element, the motion of which actuates

one or more switch contacts.


43/63

Temperature can be measured by using variousinstruments. they are Glass thermometer

Bimetallic thermometers

Pressure gauge /filled bulb thermometers

RTD Thermistors

Thermocouples/ Thermopiles

Pyrometers

Optical pyrometers

M i l Thi i di t i l i di t Th ti


44/63

Mercury in glass-This gives direct visual indicator The operating range

of the mercury thermometer is from 30 to 800F (35 to 450C)


45/63

Bi-metal temperature sensors:-

This bending motion of bi metal can to drive a pointermechanism, activate an electromechanical switch, or

perform any number of other mechanical tasks, making this

a very simple and useful primary sensing element fortemperature.


46/63

CONTINUOUS TEMPERATURE MEASUREMENT

Filled-bulb temperature sensors :-it works on the

principle of expansion of fluid when temperature changes. Class I and Class V systems use a liquid fill fluid (class V is mercury),

Class II, which uses a volatile liquid/vapor combination,

Class III systems use a gas fill fluid, to generate a temperature-dependent fluid

expansion.

Th i t (th l i t )


47/63

Thermistors (thermal resistors) :-Theyare made of metallic oxides such as manganese ,nickel,cobalt ,

copper, iron, uranium, which either increase in resistance with

increasing temperature (a positive temperature coefficient) or

decrease in resistance with increasing temperature (a negative

temperature coefficient). They are in the form of beads ,rods,

discs. They are suitable for measurement range from -60 to +15

deg c. They are highly sensitive but non linear and detect very

small change in temperature.

Resistance Temperature Detectors (R T


48/63

Resistance Temperature Detectors (R T

Ds):- It works on the principle of change in resistance of aconductor with change in temperature. RTD s are made of pure

metal (usuallyplatinum or copper) which always increase in

resistance with increasing temperature. (170 to 780C).

100 and 1000 is a very common reference resistance for

industrial RTDs.

Analog electronic RTD instruments use operational amplifiers to

convert the RTDs voltage drop into a standard instrument output

signal, such as 4-20 mA DC. self-heating of RTD is removed bysending pulse current through them.

Nickel = 0.00672 /oC Tungsten = 0.0045 /oC

Silver = 0.0041 /oC Gold = 0.0040 /oC Platinum = 0.00392 /oC Copper = 0.0038 /oC

RTD four-wire connection,


49/63

RTD three-wire connection

,

RTD R i t l b d i Wh t t


50/63

RTD s Resistance can also be measured using a Wheatstone

bridge, but are supplied from a constant current source. heating

of the device is prevented by using a pulse technique. In this

method the current is turned ON for say 10 ms every 10 s, and

the sensor resistance is measured during this 10 ms time period. In this circuit, when the current flow in the meter is zero then the

bridge is said to be in null balance. As the RTD temperature

increases, the voltage read by the voltmeter increases. If a

voltage transducer replaces the voltmeter, a 4-20 mA signal,

which is proportional to the temperature range being monitored,can be generated.


51/63

THERMOCOUPLE:- it is an arrangement of two dissimilar


52/63

THERMOCOUPLE: it is an arrangement of two dissimilar

metal wires joined to form two junctions by vacuum brazing

, induction brazing ,and argon arc welding. One junction is

called cold junction and other is called hot junction.


53/63


54/63


55/63

Type C


56/63

Type K (chromelalumel) is the most common general purpose

thermocouple. It is inexpensive and available in a wide variety of probes.

They are available in the 0 C to +1250 C range. Sensitivity is

approximately 41 V/C.

Type E (chrom elconstan tan ) has a h igh outp ut (68 V/C)which makes

it well suited to cryogenic use. Additionally, it is non-magnetic.

J typelimited range (40 to +750C). The Curie point of the iron (770 C)

causes an abrupt change to the characteristic and it is this that provides the

upper temperature limit. Type J thermocouples have a sensitivity of about55 V/C

Type N (nici ron) thermoco uples are sui table for us e at high

temperatures, exceeding 1200C, due to their stability and ability to resist

high temperature oxidation. Sensitivity is about 39 V/C at 900 C, slightly

lower than type K. Designed to be an improved type K, it is becoming more

popular.

Type B thermocouples use a platinum rhodium alloy


57/63

Type Bthermocouples use a platinumrhodium alloy

for each conductor. One conductor contains 30%

rhodium while the other conductor contains 6%

rhodium. These thermocouples are suited for use atup to 1800 C. Type B thermocouples produce the

same output at 0 C and 42 C, limiting their use

below about 50 C.

Type R thermocouples use a platinumrhodium alloy

containing 13% rhodium for one conductor and pure

platinum for the other conductor. Type R

thermocouples are used up to 1600 C.

Type S thermocouples are constructed using one wire of 90%

Platinum and 10% Rhodium (the positive or "+" wire) and asecond wire of 100% platinum (the negative or "-" wire). Like

type R, type S thermocouples are used up to 1600 C. In

particular, type S is used as the standard of calibration for the

melting point of gold (1064.43 C).


58/63

Type T (copperconstantan) thermocouples are suited formeasurements in the200 to 350C range. Often used as adifferential measurement since only copper wire touches the

probes. Since both conductors are non-magnetic, there is noCurie point and thus no abrupt change in characteristics. TypeT thermocouples have a sensitivity of about 43 V/C.

Type C (tungsten 5% rhenium tungsten 26% rhenium)

thermocouples are suited for measurements in the 0 C to2320 C range. This thermocouple is well-suited for vacuumfurnaces at extremely high temperatures and must never beused in the presence of oxygen at temperatures above 260C.

Application and selection of RTD and Thermocouple:-

Thermocouples are extremely rugged and have fargreater temperature measurement ranges than

thermistors, RTDs, and other primary sensing

elements. However, if the application does not

demand extreme ruggedness or large measurement

INSTALLATION GUIDE LINES:


59/63

INSTALLATION GUIDE LINES:TEMPERATURE SENSOR ACCESSORIES


60/63

One of the most important accessories for any

temperature-sensing element is a pressure-tight sheath

known as a thermo well.

This may be thought of as a thermally conductive

protrusion into a process vessel or pipe that allows a

temperature-sensitive instrument to detect process

temperature without opening a hole in the vessel or pipe.

Thermo wells are critically important for installationswherethe temperature element (RTD, thermocouple,

thermometer, etc.) must be replaceable without de-

pressurizing the process.

Thermo wells may be made out of any material that is

thermally conductive, pressure-tight, and not chemically

reactive with the process.


61/63

If the temperature gauge is removed for service

or replacement, the thermowell maintains

pressure integrity of the pipe (no process fluidleaking out, and no air leaking in)


62/63

real (stainless steel) thermowelldevice where the temperature

sensor is inserted

RTD

Pyrometers : pyrometer is a non contacting device that intercepts


63/63

Pyrometers :- pyrometer is a non-contacting device that interceptsand measures thermal radiation. This device can be used todetermine the temperature of an object's surface

first day class introduction,temp

Documents