INSTRUMENTATION 1

Sensors used for measuring different parameters:

A sensor; is a device that measures a physical quantity and converts it into a signal which can be read by

an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured

temperature into expansion and contraction of a liquid which can be read on a calibrated glass tube. A

thermocouple converts temperature to an output voltage which can be read by a voltmeter. For accuracy,

most sensors are calibrated against known standards.

A good sensor obeys the following rules:

Is sensitive to the measured property

Is insensitive to any other property likely to be encountered in its application

Does not influence the measured property

Resolution: The resolution of a sensor is the smallest change it can detect in the quantity that it is

measuring. Often in a digital display, the least significant digit will fluctuate, indicating that

changes of that magnitude are only just resolved. The resolution is related to the precision with

which the measurement is made.

1. Thermal, heat, temperature:

Bolometer, Calorimeter, Exhaust gas temperature gauge, Gardon gauge, Heat flux sensor

Infrared thermometer , Micro bolometer, Microwave radiometer, Net radiometer

Resistance temperature detector, Resistance thermometer, Silicon bandgap temperature sensor

Temperature gauge, Thermistor, Thermocouple, Thermometer

2. Pressure

Barograph, Barometer, Boost gauge, Bourdon gauge, Hot filament ionization gauge, Ionization

gauge, McLeod gauge, Oscillating U-tube, Permanent Downhole Gauge, Pirani gauge, Pressure

sensor ,Pressure gauge, Tactile sensor, Time pressure gauge

3. Flow, fluid velocity

Air flow meter, Anemometer , Flow sensor, Gas meter, Mass flow sensor, Water meter

4. Force, density, level

Bhangmeter, Hydrometer, Force gauge, Level sensor, Load cell, Magnetic level gauge

Nuclear density gauge, Piezoelectric sensor, Strain gauge, Torque sensor ,Viscometer

5. Proximity, presence

Alarm sensor, Motion detector, Occupancy sensor ,Passive infrared sensor ,Reed switch

Stud finder, Triangulation sensor , Touch switch , Wired glove

6. Optical, light, imaging

Charge-coupled device, Colorimeter , Contact image sensor , Electro-optical sensor

Flame detector, Infra-red sensor , LED as light sensor, Nichols radiometer, Fiber optic sensors

Photodetector, Photodiode, Photomultiplier tubes, Phototransistor, Photoelectric sensor

INSTRUMENTATION 2

Photoionization detector , Photomultiplier, Photoresistor, Photoswitch, Phototube, Proximity sensor

Scintillometer, Shack-Hartmann, Wavefront sensor

7. Acoustic, sound, vibration

Geophone , Hydrophone, Lace Sensor a guitar pickup , Microphone , Seismometer, Accelerometer

Transmitter:

In industrial process control, a "transmitter" is any device which converts measurements from a sensor into

a signal, conditions it, to be received, usually sent via wires, by some display or control device located a

distance away. Typically in process control applications the "transmitter" will output an analog 4-20 mA

current loop or digital protocol to represent a measured variable within a range. For example, a pressure

transmitter might use 4 mA as a representation for 50 psig of pressure and 20 mA as 1000 psig of pressure

and any value in between proportionately ranged between 50 and 1000 psig. (A 0-4 mA signal indicates a

system error.) Older technology transmitters used pneumatic pressure typically ranged between 3 to 15 psig

(20 to 100 kPa) to represent a process variable.

Why 4-20ma signal is preferred over 0-10v signal

The most popular standards worldwide are (not in any particular order):- 0-5V, 0-10V, 1-5V, 2-10V, 1-

5mA, 0-20mA, 4-20mA, 10-50mA. The first four are DC voltage signals whose uses are fairly obvious.

0-5V is especially popular with microprocessor applications as 5V is the popular supply voltage for these

products. It does however suffer from a drawback that it is very difficult to tell when you are at zero.

Take the example where a thermometer is calibrated so that 0°C = 0V and 100°C = 5V.

Typically the circuit is powered by a single 5V supply so that the output cannot go negative.

If we are reading 0V are we at 0°C or even lower. Could the output voltage have failed or perhaps a faulty

sensor is giving us a false reading?

If instead we use a 1-5V signal we can see if the temperature starts to go below zero as the reading drops

below 1V and we can use a level close to 0V to warn us of loss of power. This type of offset is referred to

as an ELEVATED ZERO and is the preferred type of signal in industry.

The advantage of a voltage signal is simplicity. Almost everyone understands the concept. The signal is on

two wires, one positive the other negative. Power must be provided to the electronics which drives the

voltage signal and this may be provided as two extra wires, or a single extra wire using one of the signal

wires (usually the negative) as a common.

The disadvantages of voltage signals are primarily loss of accuracy caused by the input impedance of the

measuring instrument and electrical interference from nearby power cables and radio transmitters.

To overcome both of the above problems a current signal is more effective. Assume a situation where 0-

20mA is used to represent 0-100°C. In our measuring instrument we have a 250ohm resistor through which

the current passes and we measure the voltage across it.

At 100°C we have 20mA passing through the resistor giving us a voltage of 5V. Ah-ha you say. This is just

a 0-5V signal. Not so. This 5V signal remains the same even if the resistance of the wires carrying it

changes, whereas with a voltage signal a change in wire resistance causes a change in signal at the

measuring instrument unless it has an infinite impedance.

INSTRUMENTATION 3

This means that a current signal may be transmitted long distances through conductors of varying

resistance with no loss of accuracy. Neat!

The other advantage of the current signal is improved noise immunity. Spurious signals are superimposed

on the signal we wish to measure by varying magnetic fields (maybe generated by AC power cables) which

cut the conductors and induce currents in the same fashion as a generator. If our conductors are small and

close together (theoretically in the same place) these fields generate equal and opposite currents in the two

conductors which cancel. In practice a twisted pair of conductors of telephone cable sized wire works well.

If we are using a 0-20mA signal we have to supply power to our electronics in a similar way as to power a

voltage signal using one or two extra power supply lines.

With a 4-20mA signal we always have at least 4mA flowing in the loop. This means that if we can power

our electronics from the residual 4mA we can have our power supply AND signal on the same pair of

conductors. This technique simplifies installation, especially in large plants, as only a twisted pair is needed

to transmit a signal from a sensing device to a measuring or control instrument.

An instrument which transmits a 4-20mA signal is usually referred to as a TRANSMITTER and if it

derives its power from the 4mA residual current it is called a LOOP POWERED TRANSMITTER.

With 0-20 mA you can not distinguish between minimum field value and connection break. With 2-20

mA, internal circuit can distinguish between connection break of minimum value. Normally when the value

is minimum the transmitter will give you 4 mA while in case of connection breakage it will give 0 mA.

Difference between 2 wire, 3 wire & 4 wire transmitter:

In 2 wire transmitter the power and signal are transmitted through same cable.

In 3 wire transmitter the data signal and power are with respect to common ground.

In 4 wire transmitter two wires for power supply and two for signals.

Only current transmitters can be used as 2 wire transmitters.

smart transmitter:

A “Smart” transmitter is a transmitter that uses a microprocessor as the heart of the electronics. in addition,

a “Smart” transmitter will output some type of remote digital communications allowing you to read and

set-up the device from a remote position.

They can convert analog signals to digital signals (A/D), making communication swift and easy and can

even send both analog and digital signals at the same time as denoted by D/A.

A smart transmitter has a number of other capabilities as well. For instance, inputs can be varied, as

denoted by A/D. If a temperature transmitter is a smart transmitter, it will accept millivolt signals from

thermocouples and resistance signals from resistance temperature devices (RTDs), and thermistors.

The controller takes the output signal from the transmitter and sends it back to the final control element.

The communicator is shown on the right.

The communicator is a hand-held interface device that allows digital "instructions" to be delivered to the

smart transmitters. Testing, configuring , and supply or acquiring data are all accomplished through the

communicator. The communicator has a display that lets the technician see the input or output information.

The communicator can be connected directly to the smart transmitter, or in parallel any where on the loop.

INSTRUMENTATION 4

Smart transmitters also have the following features:

Configuration

Re-ranging

Characteristics

Signal conditioning

Self-diagnosis

4.4.1 Configuration

Smart transmitters can be configured to meet the demands of the

process in which they are used. For example, the same transmitter

can be set up to read almost any range or type of thermocouple,

RTD, or thermistor.

4.4.2 Re-ranging

The range that the smart transmitter functions under can be easily changed from a remote location, for

example by the technician in a control room. The technician or the operator has access to any smart device

in the loop and does not even have to be at the transmitter to perform the change. The communicator could

be a PC, a programmable logic controller (PLC), or a hand-held device. The type of communicator depends

on the manufacturer.

There is a wide range of inputs that a smart transmitter will accept. For instance, with pressure units, the

operator can determine ahead of time whether to use inches of water, inches of mercury, psi, bars,

millibars, pascals, or kilopascals.

4.4.3 Characteristics

Another characteristic of a smart transmitter is its ability to act as a stand-alone transmitter. In such a

capacity, it sends the output signal to a distributed control system (DCS) or a PLC.

4.4.4 Signal conditioning

Smart transmitters can also perform signal conditioning, scanning the average signal and eliminating any

"noise" spikes. Signals can also be delayed (dampened) so that the response does not fluctuate. This is

especially useful with a rapidly changing process.

4.4.5 Self-diagnosis

Finally, a smart transmitter can diagnose itself and report on any problems in the process. For example, it

can report on a circuit board which is not working properly.

field bus:

fieldbus (or field bus) is the name of a family of industrial computer network protocols used for real-time

distributed control, now standardized as IEC 61158.

INSTRUMENTATION 5

A complex automated industrial system — such as a manufacturing assembly line — usually needs an

organized hierarchy of controller systems to function. In this hierarchy there is usually a Human Machine

Interface (HMI) at the top, where an operator can monitor or operate the system. This is typically linked to

a middle layer of programmable logic controllers (PLC) via a non-time-critical communications system

(e.g. Ethernet). At the bottom of the control chain is the fieldbus which links the PLCs to the components

which actually do the work such as sensors, actuators, electric motors, console lights, switches, valves and

contactors.

Fieldbus is an industrial network system for real-time distributed control. It is a way to connect instruments

in a manufacturing plant. Fieldbus works on a network structure which typically allows daisy-chain, star,

ring, branch, and tree network topologies. Previously computers were connected using RS-232 (serial

connections) by which only two devices could communicate. This would be the equivalent of the currently

used 4-20 mA communication scheme which requires that each device has its own communication point at

the controller level, while the fieldbus is the equivalent of the current LAN-type connections, which require

only one communication point at the controller level and allow multiple (100's) of analog and digital points

to be connected at the same time. This reduces both the length of the cable required and the number of

cables required. Furthermore, since devices that communicate through fieldbus require a microprocessor,

multiple points are typically provided by the same device. Some fieldbus devices now support control

schemes such as PID control on the device side instead of forcing the controller to do the processing.

Standards

There are a wide variety of concurring fieldbus standards. Some of the most widely used ones include:

AS-Interface

CAN

Industrial Ethernet based:

Interbus , LonWorks , Modbus , PROFIBUS, BITBUS, Compu Bus , Safety BUS p

Cost advantage

The amount of cabling required is much lower in Field bus than in 4-20mA installations. This is because

many devices share the same set of cables in a multi-dropped fashion rather than requiring a dedicated set

of cables per device as in the case of 4-20mA devices. Moreover, several parameters can be communicated

per device in a Fieldbus network whereas only one parameter can be transmitted on a 4-20mA connection.

Disadvantages

There are disadvantages to using fieldbus compared to the 4-20 mA analog signal standard (or to 4-20 mA

with HART):

Fieldbus systems are more complex, so users need to be more extensively trained or more highly

qualified

The price of fieldbus components is higher

Fieldbus test devices are more complex compared to a (high-spec) multimeter that can be used to

read and simulate analog 4-20 mA signals

Slightly longer reaction times with fieldbus, depending on the system

INSTRUMENTATION 6

Device manufacturers have to offer different versions of their devices (e.g. sensors, actuators) due

to the number of different (incompatible) fieldbus standards. This can add to the cost of the devices

and to the difficulty of device selection and availability.

One or more fieldbus standards may predominate in future and others may become obsolete. This

increases the investment risk when implementing fieldbus.

Modbus:

Modbus is a serial communications protocol published by Modicon in 1979 for use with its programmable

logic controllers (PLCs). It has become a de facto standard communications protocol in industry, and is

now the most commonly available means of connecting industrial electronic devices. The main reasons for

the extensive use of Modbus over other communications protocols are:

1. It is openly published and royalty-free

2. Relatively easy industrial network to deploy

3. It moves raw bits or words without placing many restrictions on vendors

Modbus allows for communication between many devices connected to the same network, for example a

system that measures temperature and humidity and communicates the results to a computer. Modbus is

often used to connect a supervisory computer with a remote terminal unit (RTU) in supervisory control and

data acquisition (SCADA) systems.

Limitations

Since Modbus was designed in the late 1970s to communicate to programmable logic controllers,

the number of data types is limited to those understood by PLCs at the time. Large binary objects

are not supported.

No standard way exists for a node to find the description of a data object, for example, to determine

if a register value represents a temperature between 30 and 175 degrees.

Since Modbus is a master/slave protocol, there is no way for a field device to "report by exception"

(except over Ethernet TCP/IP, called open-mbus)- the master node must routinely poll each field

device, and look for changes in the data. This consumes bandwidth and network time in

applications where bandwidth may be expensive, such as over a low-bit-rate radio link.

Modbus is restricted to addressing 247 devices on one data link, which limits the number of field

devices that may be connected to a master station (once again Ethernet TCP/IP proving the

exception).

Modbus transmissions must be contiguous which limits the types of remote communications

devices to those that can buffer data to avoid gaps in the transmission.

Modbus protocol provides no security against unauthorized commands or interception of data.

PROFIBUS (Process Field Bus) is a standard for field bus communication in automation technology and

was first promoted (1989) by BMBF (German department of education and research). It should not be

confused with the PROFINET standard for Industrial Ethernet.

Protocol:

INSTRUMENTATION 7

A protocol is a set of guidelines or rules. A communications protocol is a formal description of digital

message formats and the rules for exchanging those messages in or between computing systems and in

telecommunications. Protocols may include signaling, authentication and error detection and correction

capabilities. A protocol describes the syntax, semantics, and synchronization of communication and may be

implemented in hardware or software, or both.

The information exchanged between devices on a network or other communications medium is governed

by rules that can be set out in a technical specification called a communication protocol standard. The

nature of the communication, the actual data exchanged and any state-dependent behaviors are defined by

the specification. This approach is often taken for protocols in use by telecommunications.

In digital systems, the rules can be expressed by algorithms and datastructures, raising the opportunity of

hardware independency. Expressing the algorithms in a portable programming language, makes the

protocol operating system independent. In other words, a protocol is a machine independent description of

communications between systems.

To implement a protocol, the protocol software modules are to be interfaced with a framework assumed to

be implemented on the machine's operating system. This framework implements the networking

functionality of the operating system. Obviously, the framework needs to be as simple as it can be, to allow

for an easier incorporation into the operating systems.

At the time the Internet was formed, layering had proven to be a succesful design approach for both

compiler and operating system design and given the similarities between programming languages and

communication protocols, it was intuitively felt that layering should be applied to the protocols as well.

This gave rise to the concept of layered protocols which nowadays forms the basis of protocol design.

Systems do not use a single protocol to handle a transmission. Instead they use a set of cooperating

protocols, sometimes called a protocol family or protocol suite. To cooperate the protocols have to

communicate with each other, so there is an unnamed 'protocol' to do this. A technique used by this

'protocol' is called encapsulation, which makes it possible to pass messages from layer to layer.

The protocols can be arranged on functionality in groups, for instance there is a group of transport

protocols. The functionalities are mapped on the layers, each layer solving a distinct class of problems

relating to, for instance: application-, transport-, internet- and network interface-functions. To transmit a

message, a protocol has to be selected from each layer, so some sort of multiplexing/demultiplexing takes

place. The selection of the next protocol, also part of the aforementioned 'protocol' is accomplished by

extending the message with a protocolselector for each layer.

RS 232, RS485:

In telecommunications, RS-232 (Recommended Standard 232) is a standard for serial binary single-ended

data and control signals connecting between a DTE (Data Terminal Equipment) and a DCE (Data Circuit-

terminating Equipment). It is commonly used in computer serial ports. protocol is one or a few sets of

hardware and software rules agreed to by all communication parties for exchanging data correctly and

efficiently.

INSTRUMENTATION 8

Synchronous and Asynchronous Communications

Synchronous Communication requires the sender and receiver to share the same clock. The sender provides

a timing signal to the receiver so that the receiver knows when to "read" the data. Synchronous

Communication generally has higher data rates and greater error-checking capability. A printer is a form of

Synchronous Communication.

Asynchronous Communication has no timing signal or clock. Instead, it inserts Start/Stop bits into each

byte of data to "synchronize" the communication. As it uses less wires for communication (no clock

signals), Asynchronous Communication is simpler and more cost-effective. RS-232/RS-485/RS-422/TTL

are the forms of Asynchronous Communications.

Drilling Down: Bits and Bytes

Internal computer communications consists of digital electronics, represented by only two conditions: ON

or OFF. We represent these with two numbers: 0 and 1, which in the binary system is termed a Bit. A Byte

consists of 8 bits, which represents decimal number 0 to 255, or Hexadecimal number 0 to FF. As

described above, a byte is the basic unit of Asynchronous communications.

Baud rate, Data bits, Parity, and Stop bit

RS-232 Logic Waveform (8N1)

The baud rate is the communication speed that measures the number of bit transfers per second. For

example, 19200 baud is 19200 bits per second. Data bits are a measurement of the actual data bits in a

communication packet. For example, the above graphic shows eight (8) data bits in a communication

packet. A communication packet refers to a single byte transfer, including Start/Stop bits, Data bits and

Parity. If you are transferring a standard ASCII code (0 to 127), 7 data bits are enough. If it is an extended

ASCII code (128 to 255), then 8 data bits are required.

Parity is a simple way to error-check. There are Even, Odd, Mark and Space indicators. You can also use

no parity. For Even and Odd parity, the serial port sets the parity bit (the last bit after the data bit) to a

value to ensure that the data packet has an Even or Odd number of logic-high bits. For example, if the data

is 10010010, for Even parity, the serial port sets the parity bit as 1 to keep the number of logic-high bits

Even. For Odd parity, the parity bit is 0 so that the number of logic-high bits is Odd. Mark parity simply

sets the parity bit to logic-high and Space sets the parity bit to logic-low, so that the receiving party can

determine if the data is corrupted.

Stop bits are used to signal the end of a communication packet. This also helps to synchronize different

clocks on the serial devices.

Handshaking (Flow Control):

Handshaking is also called "Flow Control". The main purpose of Handshaking is to prevent receiver

overloading. By using Handshaking signals, receivers will be able to tell the sending device to pause data

INSTRUMENTATION 9

transmission if the receiver is overloaded. There are three types of handshaking: Software handshaking,

Hardware handshaking and Both.

Software handshaking uses two control characters: XON and XOFF. The receiver sends these control

characters to pause transmitter during communication. XON is decimal 17 and XOFF is decimal 19 in the

ASCII chart. The drawback of Software handshaking is that these two control characters can not be used in

data. This is quite important when you are transmitting Binary data as you might need to use these two

codes in your data.

Hardware handshaking makes use of actual hardware lines, such as RTS/CTS, DTR/DSR, and DCD/RI

(for modem).

In DTE/DCE communication, RTS (Request to Send) is an output on the DTE and input on the DCE. CTS

(Clear to Send) is the answering signal coming from the DCE. Before sending a data, the DTE asks

permission by setting its RTS output to high. No data will be sent until the DCE grants permission by using

the CTS line.

The DTE uses the DTR (Data Terminal Ready) signal to indicate it is ready to accept information, whereas

the DCE uses the DSR signal for the same purpose. DTR/DSR are normally ON or OFF for the whole

connection session (e.g. Off-hook), while RTS/CTS are ON or OFF for each data transmission.

DCD (Data Carrier Ready) is used by the modem when a connection has been established with remote

equipment, while RI (Ring Indicator) is used by the modem to indicate a ring signal from telephone line

Data formats (Binary, Hex, Dec, Oct, and ASCII)

Serial devices use Binary for communication, which consists of just two unique numbers: 0 and 1. Binary

is the Base-2 numbering system. One byte of data consists of 8 binary digits, from 0000 0000 to 1111

1111.

Hexadecimal is the base-16 system, which consists of 16 numbers: 0 to 9 and the letters A to F (decimal

number 15). The Hexadecimal numbering system is useful because it can represent every byte as two

consecutive hexadecimal digits, and it is easier for humans to read Hexadecimal numbers than Binary

numbers. Most of the manufacturers use Hexadecimal in their protocol documentation. It is simple to

convert a value from Hexadecimal to Binary. Just translate each Hexadecimal digit into its 4-bit binary

equivalent. E.g. Hexadecimal number F3 equals Binary number 1111 0011.

Octal refers to the base-8 numbering system, which uses just eight unique symbols (0 to 7). Programmers

often use Octal format because it is relatively easy for people to read and can easily be translated into

binary format: each Octal digit represents 3 binary digits. E.g. Octal number 73 equals to Binary number

111 011.

Decimal refers to numbers in base 10, which is the numbering system we use most in everyday life. It's not

as easy as Hexadecimal and Octal to converter Decimal to Binary number, but it is easier for us to

understand Decimal.

ASCII (American Standard Code for Information Interchange) is a character encoding based on the English

alphabet. ASCII codes (both readable and unreadable) are widely used in communications, such as Modem

communications. Letters A to Z and numbers 0 to 9 are readable ASCII codes. Some ASCII codes are

unreadable, such as the control codes: XON and XOFF, which are used in Software flow control.

INSTRUMENTATION 10

Checksum: Many serial protocols use checksum (additional bytes added at the end of the data string) to

check the data integrity, as errors might occur during data transmission.

There are many types of checksum, from the simplest uses of it in Modula or BCC to sophisticated CRC

calculation. Using Modula as an example, we learn that before data transmission, the sender adds all

command bytes together then mod it by 255 (decimal) to get an additional byte. This is to be added at the

end of the command string. When the receiver receives the command string, it will first check the added

byte to see whether data remain unchanged or not. If that is the case, it will accept the data, and if not, it

will ask the sender to resend the data.

Rs 485:

EIA-485, also known as TIA/EIA-485 or RS-485, is a standard defining the electrical characteristics of

drivers and receivers for use in balanced digital multipoint systems. The standard is published by the ANSI

Telecommunications Industry Association/Electronic Industries Alliance (TIA/EIA). Digital

communications networks implementing the EIA-485 standard can be used effectively over long distances

and in electrically noisy environments.

Actuator: An actuator is a mechanical device for moving or controlling a mechanism or system. It takes energy,

usually transported by air, electric current, or liquid, and converts that into some kind of motion.

Examples and applications

Mechanical actuators operate by coversion of rotary motion into linear motion, or vice versa. Conversion is

commonly made via a few simple types of mechanism including:

Screw: Screw jack, ball screw and roller screw actuators all operate on the principle of the simple machine

known as the screw. By rotating the actuator's nut, the screw shaft moves in a line. By moving the screw

shaft, the nut rotates. Etc.

Wheel and axle: Hoist, winch, rack and pinion, chain drive, belt drive, rigid chain and rigid belt actuators

operate on the principle of the wheel and axle. By rotating a wheel/axle (e.g. drum, gear, pulley or shaft) a

linear member (e.g. cable, rack, chain or belt) moves. By moving the linear member, the wheel/axle rotates.

In engineering, actuators are frequently used as mechanisms to introduce motion, or to clamp an object so

as to prevent motion. In electronic engineering, actuators are a subdivision of transducers. They are devices

which transform an input signal (mainly an electrical signal) into motion. Specific examples include:

electrical motors, pneumatic actuators, hydraulic actuators, linear actuators, comb drive, piezoelectric

actuators and amplified piezoelectric actuators, thermal bimorphs, micromirror devices and electroactive

polymers.

Motors are mostly used when circular motions are needed, but can also be used for linear applications by

transforming circular to linear motion with a bolt and screw transducer. On the other hand, some actuators

are intrinsically linear, such as piezoelectric actuators.

INSTRUMENTATION 11

Thermodynamic efficiency

The efficiency of an actuator is a standard tool used to calculate or estimate the usefulness of any actuating

mechanism. It is a dimensionless quantity which is generally lower than 1 expressing the energy

conversion factor. For better explanation see Thermodynamic efficiency. Most of the wasted energy (due to

friction, magnetic losses, eddy currents etc.) is thermally dissipated.

Actuators for valves can be classified up several characteristics:

By the type of movement

Multi-turn.

Quarter-turn.

Linear.

Lever.

By the energy source

Manual

Electric: they can be drived by direct and alternate current.

Pneumatic: they use pressured air or gas to create motion. They are widely used in the industry due

to their low cost. In case of failure they are easy to diagnose or repair in field, rather than electric

actuators.

Oleo-Hydraulic

Functionality

On / Off valve service

Positioning to % open

Modulating to control changes on flow conditions

Emergency Shut Down (ESD)

Other characteristics

Explosion-proof

Safety integrity level (SIL)

Enclosure: IP and NEMA.

NEMA stands for the National Electrical Manufacturers Association.

IP stands for International Protection

How RTD Works:

A resistance temperature detector (RTD) is a temperature measuring device made of materials such as

platinum whose resistance change is known at different temperatures. The R v/s T curve is used to

determine the temperature. The platinum RTD's are the most stable, linear and accurate among all the

INSTRUMENTATION 12

RTD's

The Common material used to make RTD's are

Platinum

Nickel

Copper

However Tungsten and Balco are also used to make RTD's but they are very rare. RTD's are commonly

categorized by their nominal resistance at 0 degree celcius. Typical nominal resistance values for platinum

RTD's include 100ohm and 1000ohm. The most common RTD's are PT100 and PT1000.

The Advantages of using RTD's for temperature measurement are

Accurate

Provide good stability and repeatability

RTD's are also relatively immune to electrical noise and therefore well suited for temperature

measurement in industrial environments, especially around motors, generators and other high

voltage equipments.

The resistance changes linearly with temperature so the measurement is quite simple.

The Disadvantages of RTD's are

Lower temperature measuring range

High cost

The RTD's are not well suited to use in harsh environments compared to thermocouple.

Resistance Temperature Detectors (RTDs) are temperature sensors that contain a resistor that changes

resistance value as its temperature changes.They have been used for many years to measure temperature in

laboratory and industrial processes,and have developed a reputation for accuracy, repeatability, and

stability. Why use an RTD instead of a thermocouple or thermistor sensor? Each type of temperature

sensor has a particular set of conditions for which it is best suited. RTDs offer several advantages:

• A wide temperature range (-50 to 500°C for thin-film and -200 to 850°C for wire-wound)

• Good accuracy (better than thermocouples)

• Good interchangeability

• Long-term stability

With a temperature range up to 850°C, RTDs can be used in all but the highest-temperature industrial

processes. When made using metals such asplatinum, they are very stable and are not affected

by corrosion or oxidation.Other materials such as nickel, copper, and nickel-ironalloy have also been used

for RTDs. However, thesematerials are not commonly used since they havelower temperature capabilities

and are not as stable orrepeatable as platinum.

Wiring Configuration:

The wiring configuration is the last of those parameters typically specified by the instrument manufacturer,

although the system designer does have some control based on the application. An RTD is inherently a 2-

wire device, but lead wire resistance can drastically reduce the accuracy of the measurement by adding

additional, uncompensated resistance into your system. Most applications therefore add a third wire to help

the circuit compensate for lead wire resistance, and thus provide a truer indication of the measured

temperature.

INSTRUMENTATION 13

Four-wire RTD's provide slightly better compensation, but are generally found only in laboratory

equipment and other areas where high accuracy is required. When used in conjunction with a 3-wire

instrument, a 4-wire RTD will not provide any better accuracy. If the fourth wire is not connected, the

device is only as good as the 3-wire RTD; if the fourth wire is connected, new errors will be introduced.

Connecting a 3-wire RTD to a 4-wire instrument can cause serious errors or simply not work at all,

depending on the instrument circuitry. A 2-wire RTD can be used with either a 3 or a 4 -wire instrument by

jumping the appropriate terminals, although this defeats the purpose and reintroduces the un compensated

resistance of the leads. To get the optimum performance, it is generally best to specify the RTD according

to the instrument manufacturer's recommendations.

The advantages of 2, 3 and 4 wire Pt100 measurement.

INTRODUCTION Two, Three and Four wire measurement techniques have been developed for measuring accurately the

resistance of resistive temperature detectors (RTD). This application note looks at the new techniques

being adopted in „Smart‟ instruments where, using the intelligence of a microprocessor, the traditional

drawbacks of 3 wire systems no longer apply.

THREE WIRE MEASUREMENT The traditional method of accurately measuring a resistance, is to incorporate the resistance into a

Wheatstone bridge circuit (see figure below).

A voltage excites the bridge and the voltage across the bridge is proportional

to the resistance of the RTD.

A problem occurs when we introduce lead resistances (See figure

below). It is apparent that any resistance in the lead looks as

though there is additional resistance in the element to be

measured.

To minimise these errors the three wire compensated bridge was introduced (see figure below).

This has the effect of removing the error introduced by the lead

resistance as long as lead resistances RL1 and RL3 are matched.

However the effect of the lead resistance can be to cause less

current to flow in the detector leg and hence introduce a small but

possibly significant span error. This can be eliminated by exciting

the bridge from a constant current source rather than a constant voltage and so whatever the lead resistance,

the same current always flows through the detector. With this method there are no lead resistance errors

introduced as long as the lead resistances are equally matched. In practice they are very closely matched as

long as the wire used is part of the same multi-core cable.

INSTRUMENTATION 14

The exception to this is when the sensor is used in a Hazardous area and connected to the bridge circuit via

a Zener Barrier. Here any mis-match in the resistance of the two legs of the Zener barrier can appear as a

sensor error. Although still small, this error can be as much as 0.15 ohm or approx. 0.3°C.(MTL 155

Barrier).

For analogue transmitters Status Instruments Ltd. have traditionally used a variation of this technique using

an in house active bridge circuit. The exception being the new „Smart‟ series of instruments which use a

different technique which will be explained later.

Another way of measuring Pt100 elements is to use a 4 wire current and voltage method (see figure below).

Here the detector is excited by a constant current and the voltage

across the detector measured by an amplifier with a high

impedance input. If the current source is perfect and the input

impedance of the voltage measuring circuit is infinite, then there

is no error whatsoever introduced by the lead resistances even if

they are mismatched.

THE ‘SMART’ WAY FORWARD This paragraph describes the method used to measure Pt100 on the new DM3000 series instruments.

The current trend for so called SMART instruments is to have a universal input capable of supporting a

wide range of inputs. It is inconvenient (and unnecessary) to dedicate input pins and electronics to support

a constant current supply and a bridge arrangement. The input circuit measures voltages to a high degree of

accuracy and the microprocessor performs the calculation in the figure below.

Rc is used solely to limit the current flowing and Rs is a stable

reference resistor.

Having computed the resistance, the microprocessor applies the

corrections required and translates the resistance to an accurate

temperature reading.

In addition, the microprocessor can determine which if any, of the

RTD inputs has become disconnected and detect other errors such as RTD short circuit. This is an

improvement over both conventional three and four wire circuits because you can now have a predictable

failure mode which does not depend upon which of the three wires has become disconnected.

This technique removes lead resistance effects as long as they are equal. Again, we have the problem when

using Zener barriers, in that if the legs of the barrier are not accurately matched, then a small error could be

introduced.

Temperature range supported by RTD:

INSTRUMENTATION 15

PT100 Platinum Resistance Thermometers

Platinum resistance thermometers (PRTs) offer excellent accuracy over a wide temperature range (from

-200 to +850 °C). Standard Sensors are are available from many manufacturers with various accuracy

specifications and numerous packaging options to suit most applications. Unlike thermocouples, it is not

necessary to use special cables to connect to the sensor.

The principle of operation is to measure the resistance of a platinum element. The most common type

(PT100) has a resistance of 100 ohms at 0 °C and 138.4 ohms at 100 °C. There are also PT1000 sensors

that have a resistance of 1000 ohms at 0 °C.

The relationship between temperature and resistance is approximately linear over a small temperature

range: for example, if you assume that it is linear over the 0 to 100 °C range, the error at 50 °C is 0.4 °C.

For precision measurement, it is necessary to linearise the resistance to give an accurate temperature. The

most recent definition of the relationship between resistance and temperature is International Temperature

Standard 90 (ITS-90).

This linearisation is done automatically, in software, when

using Pico signal conditioners. The linearisation equation

is:

Rt = R0 * (1 + A* t + B*t2 + C*(t-100)* t3)

Where:

Rt is the resistance at temperature t, R0 is the resistance at

0 °C, and

A= 3.9083 E-3

B = -5.775 E-7

C = -4.183 E -12 (below 0 °C), or

C = 0 (above 0 °C)

For a PT100 sensor, a 1 °C temperature change will cause a 0.384 ohm change in resistance, so even a

small error in measurement of the resistance (for example, the resistance of the wires leading to the sensor)

can cause a large error in the measurement of the temperature. For precision work, sensors have four wires-

two to carry the sense current, and two to measure the voltage across the sensor element. It is also possible

to obtain three-wire sensors, although these operate on the (not necessarily valid) assumption that the

resistance of each of the three wires is the same.

The current through the sensor will cause some heating: for example, a sense current of 1 mA through a

100 ohm resistor will generate 100 µW of heat. If the sensor element is unable to dissipate this heat, it will

report an artificially high temperature. This effect can be reduced by either using a large sensor element, or

by making sure that it is in good thermal contact with its environment.

Using a 1 mA sense current will give a signal of only 100 mV. Because the change in resistance for a

degree celsius is very small, even a small error in the measurement of the voltage across the sensor will

produce a large error in the temperature measurement. For example, a 100 µV voltage measurement error

will give a 0.4 °C error in the temperature reading. Similarly, a 1 µA error in the sense current will give 0.4

°C temperature error.

INSTRUMENTATION 16

Because of the low signal levels, it is important to keep any cables away from electric cables, motors,

switchgear and other devices that may emit electrical noise. Using screened cable, with the screen

grounded at one end, may help to reduce interference. When using long cables, it is necessary to check that

the measuring equipment is capable of handling the resistance of the cables. Most equipment can cope with

up to 100 ohms per core.

The type of probe and cable should be chosen carefully to suit the application. The main issues are the

temperature range and exposure to fluids (corrosive or conductive) or metals. Clearly, normal solder

junctions on cables should not be used at temperatures above about 170 °C.

Sensor manufacturers offer a wide range of sensors that comply with BS1904 class B (DIN 43760): these

sensors offer an accuracy of ±0.3 °C at 0 °C. For increased accuracy, BS1904 class A (±0.15 °C) or tenth-

DIN sensors (±0.03 °C). Companies like Isotech can provide standards with 0.001 °C accuracy. Please note

that these accuracy specifications relate to the SENSOR ONLY: it is necessary to add on any error in the

measuring system as well.

Related standards are IEC751 and JISC1604-1989. IEC751 also defines the colour coding for PRT sensor

cables: the one or two wires attached to one end of the sensor are red, and the one or two wires at the other

end are white.

Differencies between PT1000 and PT100:

A PT1000 is however a temperature-dependent resistance. It has 1000 ohms with zero °C, with 1°C

1003.85 ohms with 10°C 1038.5 ohms and so on. The temperature coefficient amounts to depending

upon platinum material +3.85 Ohm/°C with the PT1000 and +0.385 Ohm/°C with the PT100. A PT1000

has therefore a larger slope and makes possible thereby a higher resolution. The sensor has small

currents, a self-heating of sensors small is minimized. With a PT100 it is easy to develop thermometers

for a large temperature range.

What is a Thermocouple:

Thermocouple is the devise used extensively for measurement of the temperature of the body. Temperature

is the fundamental property just like the mass and time and is frequently measured quantity. Thermocouple

comprises of at least two metals joint together to form two junctions. One is connected to the body whose

temperature is to be measured; this junction is called as hot or measuring junction. The other junction is

connected to the body of known temperature; this is called as cold or reference junction. Thus the

thermocouple enables measuring the unknown temperature of the body with reference to the known

temperature of the other body.

Principle of Working of Thermocouple

The working principle of thermocouple is based on three effects, discovered by Seebeck, Peltier and

Thomson. All these have been described in brief below.

1) Seebeck effect: The Seebeck effect states that when two different or unlike metals are joined together at

two junctions, an electromotive force (emf) is generated at the two junctions. The amount of emf generated

is different for different combinations of the metals.

2) Peltier effect: As per the Peltier effect when two dissimilar metals are joined together to form two

junctions, the emf is generated within the circuit due to different temperatures of the two junctions of the

circuit.

INSTRUMENTATION 17

3) Thomson effect: As per Thomson effect, when two unlike metals are joined together forming two

junctions, the potential exists within the circuit due to temperature gradient along the entire length of the

conductors within the circuit.

In most of the cases the emf suggested by Thomson effect is very small and it can be neglected by making

proper selection of the metals. The Peltier effect play prominent role in the working principle of the

thermocouple.

How Thermocouple Works?

The general circuit for the working of thermocouple is shown in the figure 1 above. It comprises to two

dissimilar metals A and B. These are joined together to form two junctions, p and q, which are maintained

at the temperatures T1 and T2 respectively. Remember that the thermocouple cannot be formed if there are

no two junctions. Since the two junctions are maintained at different temperatures the Peltier emf is

generated within the circuit and it is the function of the temperatures of two junctions.

If the temperature of both the junctions is same, equal and opposite emf will be generated at both junctions

and the net current flowing through the junction is zero. If the junctions are maintained at different

temperature, the emf‟s will not become zero and there will be net current flowing through the circuit. The

total emf flowing through this circuit depends on the metals used within the circuit as well the temperature

of the two junctions. The total emf or the current flowing through the circuit can be measured easily by the

suitable devise.

For measurement of the temperature of the body, one junction of the thermocouple is connected to the body

whose temperature is to be measured. This junction is called as hot junction or the measuring junction. The

other junction is connected to the body whose temperature is known. This junction is called as cold or

reference junction.

Within the circuit of the thermocouple the devise for measuring the current or emf flowing the circuit is

connected. It measures the amount of emf flowing through the circuit due to the two junctions of the two

dissimilar metals maintained at different temperatures. In the figure 2 above the two junctions of the

thermocouple and the devise used for measurement of emf (potentiometer) are shown.

Now, the temperature of the reference junctions is already known, while the temperature of measuring

junction is unknown. The output obtained from the thermocouple circuit is calibrated directly against the

unknown temperature. Thus the voltage or current output obtained from thermocouple circuit gives the

value of unknown temperature directly.

Devices Used for Measuring emf within the Thermocouple Circuit

The amount of emf developed within the thermocouple circuit is very small, which is usually in millivolts,

hence some highly sensitive instruments should be used for measuring the emf generated in the

INSTRUMENTATION 18

thermocouple circuit. The two devices used commonly for measuring emf within the thermocouple circuit

are ordinary galvanometer and voltage balancing potentiometer. The manually or automatically balancing

potentiometer is used more commonly.

The figure 2 above shows potentiometer connected in the thermocouple circuit. The junction p is connected

to the body whose temperature is to be measured. The junction q is the reference junction, whose

temperature can be measured by the thermometer. In some cases the reference junctions can also be

maintained at the ice temperature by connecting it to the ice bath (see figure 3). This devise can be

calibrated in terms of the input temperature so that its scale can give the value directly in terms of

temperature.

Materials used for Thermocouples:

For the formation of the thermocouple at least two metals should be joined together to form two junctions.

The thermocouple cannot be formed if the two junctions are not formed. Both the junctions are maintained

at different temperatures. One is at the known temperature and the other at the unknown temperature. There

are several combinations of the metals that can be used to form the thermocouple. However, there certain

metals and their combinations that work better than the other combinations and they have become standard

for certain ranges of temperature and for specific applications.

Some of the elements used commonly for thermocouple are:

1) Copper as independent element

2) Iron as independent element

3) Platinum as independent element

4) Rhodium as an independent element

5) Iridium as an independent element

6) Constantan: combination of 60% copper and 40% nickel

7) Chromel: combination of 10% chromium, 90% nickel

8) Alumel: combination of 2% aluminum, 90% nickel and remainder silicon and manganese

Properties of Some Combinations of Materials Used for Thermocouples

Here some of the properties of commonly used combinations of the elements for thermocouples.

1) Copper - constantan: used for -300 to 650 F. Inexpensive, high emf output

2) Chromel - constantan: used for 0 to 1000 F. Highest emf output, good stability

3) Iron - constantan: used for 0 to 1500 F. Inexpensive, high emf output, iron oxidizes after 1500F

4) Chromel - alumel: used for 600 to 2000 F. Resistant to the temperature within the specified range

INSTRUMENTATION 19

5) Platinum - 10% rhodium: used for 1300 to 2850 F. Expensive and gives low output, resistant to

oxidation, stable, used only for high temp.

How to Choose the Thermocouple Elements:

Here are some of the factors that should be considered while selecting the material combinations for

thermocouples.

1) The combination of the materials should produce sufficiently high electromotive force (emf) when they

are connected. This ensures that the emf can be measured easily and the value of the temperature can be

obtained correctly.

2) The combination of materials should have high resolution or fast response time: This means for every

degree change in the temperature of the junctions there is sufficient emf generated within the circuit so that

it can be sensed and measured easily.

3) Higher stability: The properties of the combination should remains stable for the specified temperature

range.

4) Resistant to oxidation: The materials used for the thermocouple should be resistant to the oxidation, this

is especially very important for the materials that are used at high temperatures since they tend to oxidize at

those temperatures.

For all the material combinations the size of the wire is very important. If higher temperatures are to be

measured, the diameter of the wire should be more, but in that case time response to the temperature

change increases. Hence some compromise will have to be made between the response of the thermocouple

and its life.

Selecting a Device for Cold-Junction Temperature Measurement:

As explained above, to implement cold-junction compensation, the temperature of the cold junction must

be determined. This calculation can be accomplished with any type of temperature-measurement device.

Among the more popular devices are temperature-sensing ICs, thermistors, and RTDs. Each family of

devices offers advantages and disadvantages over the others, so the requirements of a specific application

will determine which type to use.

For applications requiring extreme accuracy, a calibrated platinum RTD offers the best performance across

the widest temperature range. This performance comes, however, with high cost.

Thermistors and silicon temperature-sensing ICs are cost-effective alternatives to RTDs for applications

that do not require such high accuracy. Thermistors operate across a wider temperature range than silicon

ICs. Nonetheless, temperature-sensing ICs are often preferred over thermistors because of their linearity;

correcting a thermistor's nonlinearity can require too much work from the system's microcontroller. The

temperature-sensing ICs provide excellent linearity but operate across a narrower temperature range.

In summary, a cold-junction temperature-measurement device must be selected to match the requirements

of the system. As with any temperature-measurement application, accuracy, temperature range, cost, and

linearity are all significant considerations in the selection process. Each requirement must be weighed

carefully in order to select the optimal combination of cost and performance.

INSTRUMENTATION 20

Conclusion:

When working with thermocouples, you must establish a reference point because thermocouples are

differential temperature-measurement devices. A thermocouple provides a voltage that represents the

temperature difference between the hot and cold junctions. If you know both the temperature of the cold

junction and the temperature of the hot junction relative to the cold-junction temperature, you can

determine the actual hot-junction temperature.

The main selection criteria for the appropriate cold-junction compensation device are accuracy, cost,

linearity, and temperature range. Some platinum RTDs offer the best accuracy, but at high cost.

Thermistors are inexpensive and operate over a wide temperature range, but their lack of linearity can be

problematic. Silicon temperature-sensing ICs operate over a narrow temperature range, but offer reasonable

accuracy, linearity, and low cost, thus making them a suitable choice for many thermocouple cold-junction

compensation applications.

How capacitive & Inductive sensors function:

Inductive Proximity Sensors

Inductive proximity sensors detect the presence of metallic objects. Their operating

principle is based on a coil and high frequency oscillator that creates a field in the

close surroundings of the sensing surface. The presence of metal in the operating

area causes a change in the oscillation amplitude. This change is identified by a

threshold circuit, which changes the output of the sensor. The operating distance of the

sensor depends on the coil‟s size as well as the target‟s shape, size and material.

Capacitive Proximity Sensors

Capacitive sensors are used for non-contact detection of metallic objects &

nonmetallic objects (liquid, plastic, wooden materials and so on). Capacitive

proximity sensors use the variation of capacitance between the sensor and the

object being detected. When the object is at a preset distance from the

sensitive side of the sensor, an electronic circuit inside the sensor begins to

oscillate. The rise or fall of such oscillation is identified by a threshold circuit

that drives an amplifier for the operation of an external load. A screw placed on the backside of the sensor

allows regulation of the operating distance. This sensitivity regulation is useful in applications, such as

detection of full containers and non-detection of empty containers.

Photoelectric Proximity Sensors

Photoelectric sensors use light sensitive elements to detect objects and are made up of an emitter (light

source) and a receiver. We offer three types of photoelectric sensors are available.

INSTRUMENTATION 21

Photoelectric sensor types:

Direct Reflection (Diffused) - emitter and receiver are housed

together and use the light reflected directly off the object for detection.

Reflection with Reflector (Retro-reflective) - emitter and receiver are housed together and require a

reflector. An object is detected when it interrupts the light beam between the sensor and reflector.

Thru Beam - emitter and receiver are housed separately and detect an object when it interrupts the light

beam between the emitter and receiver.

Magnetic Proximity Sensors

Magnetic sensors are actuated by the presence of a permanent magnet. Their operating principle is based

on the use of reed contacts, which consist of two low reluctance ferro-magnetic reeds enclosed in glass

bulbs containing inert gas. The reciprocal attraction of both reeds in the presence of a magnetic field, due to

magnetic induction, establishes an electrical contact.