Absolute Pressure: The measurement of pressure relative to the pressure in a vacuum. It is equal to the sum of the pressure shown on a pressure gauge and atmospheric pressure.

Absolute Zero: The temperature at which a system would undergo a reversible isothermal process without transfer of heat and at which the volume of an ideal gas would become zero, calculated to be - 273.16°C (-459.7°F) or zero degree Kelvin.

Ambient Pressure: The atmospheric pressure surrounding a sensor or instrument.

Ambient Temperature: The average temperature of the atmosphere surrounding an instrument

Accuracy: Degree of trueness, the closeness of the agreement between the result of the measurement and the conventional true value of the quantity.

Accuracy: The maximum deviation to be expected between a meter reading and the actual value being measured under specified operating conditions. Usually expressed in percent of full scale for analog instruments, percent of reading for digital instruments.

Analogue signal: A signal that continuously represents a variable or condition.(Typical analogue signal are 1-5 DC volts, 4 to 20 mA, 3 to 15 psi or .02 to 1.0 kg/cm2).

Analogue to Digital Converter (ADC): An electronic module or device that converts analogue signals to an equivalent digital value.

Avalanche break down: It is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents within materials which are otherwise good insulators. It is a type of electron avalanche. The avalanche process occurs when the carriers in the transition region are accelerated by the electric field to energies sufficient to free electron-hole pairs via collisions with bound electrons.

Automatic control: When a device is designed to perform without the need of human inputs for correction it is called automatic control.

Anti surge Control: Anti surge control system is a feedback system that protect compressor from instability (Surge). In order to avoid this, a by-pass valve is installed between the discharge of the compressor and the inlet. When higher pressure at the outlet is detected, the valve opens fully, usually in less than one second, to dump the pressure onto the inlet side. .The instability occurs when the pressure at the outlet of the compressor is higher than that produced by the compressor; this causes the flow to reverse momentarily. However, the reduction of flow causes the discharge pressure to drop, and the flow returns. This is referred to as surge. The resulting violent oscillation of gas pressure can cause severe and costly damage to the compressor.

Annubar: A device that uses pitot tubes to measure the gas flow rate within a pipeline.

AGA - American Gas Association: widely used for custody transfer system for calculations of gas density (AGA -8) and gas flow (AGA- 3).Turbine meter (AGA-7 ) Ultrasonic (AGA-9)

American National Standards Institute (ANSI): Standardization and conformity assessment system.

American Petroleum Institute (API): association of oil and gas industry to develop Recommended Practices for the industry.

American Standard Code for Information Interchange (ASCII): The American Standard Code for Information Interchange was established to achieve compatibility between various types of data processing equipment. ASCII, pronounced "ask-key", has become the common code for microcomputer equipment

American Society of Mechanical Engineers (ASME): Professional organization focused on technical; educational and research issues of the engineering and technology community.

American Society for Testing and Materials (ASTM):

ATEX: ATEX is the common name given to the EU directive 94/9/EC, Equipment and Protective Systems intended for use in Potentially Explosive Atmospheres. The word ATEX is derived from the French "ATmospheres EXplosibles".

American Wire Gauge (AWG): AWG Also known as the Brown & Sharpe wire gauge, is a standardized wire gauge system used since 1857 predominantly in the United States and Canada for the diameters of round, solid, nonferrous , nonferrous, electrically conducting wire. The cross-sectional area of each gauge is an important factor for determining its current-carrying capacity.



Baud Rate: The term is named after J.M.E. Baud, the inventor of the Baud telegraph code. Baud rate is serial communications data transmission rate expressed in bits per second.

Bandwidth: Data carrying capacity, the amount of data that can be transmitted in a fixed amount of time. For digital devices, the bandwidth is usually expressed in bits per second (bps) or bytes per second. For analog devices, the bandwidth is expressed in cycles per second, or Hertz (Hz).

Byte: An eight-bit binary number. Also denotes the amount of memory required to store one byte of data. Large amounts of memory are indicated in terms kilobyte (1024), megabytes (1,048,576 bytes), and gigabytes (1,073,741,824 bytes).

Breakdown Voltage: The voltage at which circuit components begin to be damaged.

Bipolar: A signal range that includes both positive and negative values e. g -12 V to +12 V

British thermal unit (BTU): A unit of energy i.e. the amount of energy required to raise the temperature of 1 pound of water 1 degree Fahrenheit when the water is near 39.2 degrees Fahrenheit.

British Approvals Service for Electrical Equipment in Flammable Atmospheres: BASEEFA is the leading internationally recognized certification body for explosion protected equipment, delivering. IECEx, ATEX and DSEAR certification to customers around the globe. UK based testing institute and Notified Body.

Bourdon Tube: Used in pressure recorders; an oval metal tube bent into a C-shaped arc with one closed end. When pressure is applied to the open end the tube tends to straighten which moves the closed end. The pointer is attached to this end.

Barrel (bbl): A volumetric unit of measure for crude oil and petroleum products. One barrel equals 42 US gallons or 35 UK (imperial) gallons, or approximately 159 liters. 6.29 barrels equal one cubic meter and (on average) 7.33 barrels weigh one metric ton (1000 kilograms). One barrel of crude equals 5604 cubic-feet of natural gas.

Barrels of Oil per Day (BOPD): Unit of measurement of crude oil produced by a well or an oil field. The volume of a barrel is equivalent to 42 US gallons (0.16 meters cubed).

Biological Oxygen Demand (BOD): An index of the degree of organic pollution in water. The amount of oxygen used for biochemical oxidation by a unit volume of water at a given temperature and for a given time.

Bus Bar: In electrical power distribution, a conducting bar that carries heavy current to supply several electric circuits.



Cause and Effect (C&E): Typically used to refer to a chart depicting the functions required from a logic system, e.g. Process, FSD, ESD and shutdown.

Cascade Loop: With 2 or more controllers. The output of the "Master" controller is the setpoint for the "Slave" controller.

Calibration: The process of adjusting an instrument so that its reading can be correlated to the actual values being measured.

Capacitance: The capability of a device to store electric charge. The unit is the farad, which expresses the ratio of stored charge in coulombs to the impressed potential difference in volts.

Control systems engineering: It is the engineering discipline that applies control theory to design systems with desired behaviors. The practice uses sensors to measure the output performance of the device being controlled (often a final element) and those measurements can be used to give feedback to the input actuators that can make corrections toward desired performance.

Control valves: The control valve is a final control element, manipulates a flowing fluid, such as gas, steam, water, or chemical compounds, to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point. Widely used in continuous process industries i.e. refinery, chemical, oil sector, paper, power and steel etc.

Coefficient of Valve (CV): A measurement of valve capacity. The number of gallons per minute of room temperature water that will flow through the valve with a pressure drop of 1 psi across the valve.

Cavitations: A condition of liquid flow where a liquid vaporizes and the vapors bubbles subsequently collapse. Vapor bubbles can surface damage to control valves seat and plug or pumps etc.

Coaxial Cable: Coaxial cable was invented in 1929 and first used commercially in 1941. AT&T established its first cross-continental coaxial transmission system in 1940. Depending on the carrier technology used and other factors, twisted pair copper wire and optical fiber are alternatives to coaxial cable. Cable consisting of a central wire surrounded by an electrical insulator which in turn is surrounded by a shield conductor Provides a low capacitance and inductance cable most suited to carry high frequency current.

Compensating Cable: Thermo-electric properties of cables similar to the thermocouple. Thermocouple extension and compensating cables are designed for interconnection between thermocouple probes and control instrumentation.

Cold Junction Compensation: In Thermocouple, when making temperature measurements, if the current produced by this other junction (cold junction) is ignored, an unknown error is created. For accurate measurement automatic cold junction compensation is accomplished by sensing the terminal temperature (cold junction) with an RTD. The RTD is in circuit which produces a current equal but opposite to that produced by the cold junction. Thus the current change from the cold junction plus that from the compensation circuit cancel one another. Once this is accomplished then it can be assumed that the current reading on the voltmeter represents the current produced by the measuring junction only.

Central Processing Unit (CPU)



Differential Pressure: Differential pressure is the difference in pressure between two separate points. A differential pressure can be measured between two points on independent systems or between two different points on the same system.

Derivative: With derivative action, the controller output is proportional to the rate of change of the measurement or error. The controller output is calculated by the rate of change of the measurement with time. CONTROLLER OUTPUT = DERIVATIVE (dm/dt), Where m is the measurement at time t. Some manufacturers use the term rate or pre-act instead of derivative. Derivative, rate, and pre-act are the same thing. DERIVATIVE = RATE = PRE ACT. Derivative action can compensate for a changing measurement. Thus derivative takes action to inhibit more rapid changes of the measurement than proportional action. When a load or set-point change occurs, the derivative action causes the controller gain to move the "wrong" way when the measurement gets near the set-point. Derivative is often used to avoid overshoot.

Distributed Control System (DCS): A control system with multiple processors, I/O subsystems connected by a network for communication and monitoring of process plant.

DDE: Dynamic Data Exchange, a standard software method for communicating between applications under Microsoft Windows. Created by Microsoft, DDE is being replaced by OPC (OLE for process control).

Damping: The suppression of oscillation. Achieved using viscosity of a fluid for viscous damping, and the induced current in electrical conductors for magnetic damping.

Dead band : The range through which an input can be changed without causing an observable response.

Dead Time: Dead time is the delay from when a controller output (CO) signal is issued until when the measured process variable (PV) first begins to respond. The presence of dead time, Ө p, is never a good thing in a control loop.

Drift: An uncontrolled slow change in some operating characteristic of a piece of equipment or, an electronic circuit or component, change in the input-output relationship over a period of time.

Decibel (dB): A decibel (dB) is a unit of measurement of voltage or other intensity. Decibels are one tenth of a bel. dBA is the noise power calculated in dB. 0 dBA is equal to 3.16 picowatts (-85 dBm).

Dielectric: A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material as they do in a conductor, but only slightly shift from their average equilibrium positions causing dielectric polarization.

Dielectric Constant: A number that expresses the degree of non-conductivity of different substances

Dew Point: Warm gas (e.g. air) can hold more water vapour than a cool gas. If the gas is cooled sufficiently it will reach a temperature where the gas becomes saturated and condensation will form. At this temperature the gas is said to have reached its dew point. Further cooling will allow further moisture to condense.

Double Block and Bleed: Double Block and Bleed consists of the closure of two block valves in series with an intermediate bleed valve used to provide isolation of process from maintenance personnel .Can be constructed from individual valves or in a single purpose built unit.

Duplex Sensor: A dual element RTD or thermocouple usually isolated from each other and in the same housing or sheath.



Error: Uncertainty in measurement is "error." The difference between the measured signal and the true value.

End of Line Resistor: Name given to resistors added to a circuit for line monitoring i.e. end of line resistors. Fire and gas detection equipment used EOR

Equal Percentage: An equal percentage valve trim is machined so that an equal increment of stem travel produces an equal change in flow through the valve

Emergency Shutdown System (ESD): An automatic system for shutting down plant according to cause and effect or sequence logic to prevent escalation of a potentially hazardous situation

European Committee for Electro technical Standardization (CENELEC): CENELEC, the European Committee for Electro technical Standardization, was created in 1973 because of the merger of two previous European organizations: CENELECELCOM and CENELECEL. Nowadays, CENELEC is a non-profit technical organization set up under Belgian law and composed of the National Electro technical Committees of 31 European countries. In addition, 12 National Committees from neighboring countries are participating in CENELEC work with an Affiliate status.

Exia : The protection concept is used to assess and test electronic circuits that are low power devices. There are limitations on voltage, current, capacitance and inductance such that the available energy at a sparking device is below the minimum ignition energy of the potentially explosive atmosphere



Field BUS: A communications protocol used to connect process instrumentation and control systems,

Fire and Gas (F&G): The system that detects fire, smoke, gas and thermal by means sensing devices and command to automatic system for shutting down plant or suppression according to cause and effect or sequence logic to prevent escalation of a potentially hazardous situation.

Flameproof: By definition as used in the “Specification for flameproof enclosures of electrical apparatus (Second Revision) - IS:2148-81, is, “An enclosure for electrical machinery or apparatus that will withstand, when the covers or other access doors are properly secured, an internal explosion of the flammable gas or vapor which may enter or which may originate inside the enclosure, without suffering damage and without communicating the internal Flammarion (or explosion) to the external flammable gas or vapor in which it is designed to be used, through any joints or other structural openings in the enclosure.

Fire Resistant: A fire-resistance rating typically means the duration for which a passive fire protection system can withstand a standard fire. A fire resistance cable to function in a fire ensuring the integrity of vital circuits such as essential and emergency systems.

Factory Acceptance Test (FAT): A test of equipment carried out at supplier's factory premises prior to shipment of equipment to site, usually witnessed by Third Party Inspector or end user.

Flash Point: "Flashpoint" is a temperature at which a fluid will burn and keep burning. Flash point assumes these are a spark or some other source of ignition which contains enough heat to warm the fluid to start the fire. Flash Point Temperature is the lowest temperature at which a liquid releases sufficient vapour that can be ignited by an energy source.



Gain: Gain is a term most commonly associated with amplifiers. In process control PID loop, Gain (of the controller) is another way of expressing the "P" part of the PID controller. GAIN = 100/ (Proportional Band). The more gain a controller has the faster the loop response and more oscillatory the process.

Gain Margin: The difference in the logarithms of the amplitude ratios at the frequency where the combined phase angle is 180 degrees lag is the GAIN MARGIN.

Ground: The electrical neutral line having the same potential as the surrounding earth; the negative side of a direct current power system; the reference point for an electrical system.

Gauge Pressure: These measurements above atmosphere pressure are referred to as “gauge pressure” and are indicated as PSIG. Example of this would be that 100 PSIG equals 114.7 PSIA. Unless otherwise indicated or defined, the reference to PSI is usually taken to be PSIG.



Highway Addressable Remote Transducer (HART): Open protocol that supports two way digital communications for process measurement and control devices.

Human machine Interface (HMI): The space where interaction between humans and machines occurs.

Hunting: An undesirable oscillation which continues for some time after an external stimulus has disappeared.

Hysteresis: Similar to dead band.

Hastelloy: A widely used nickel-molybdenum-chromium alloy. Offers excellent resistance to wet chlorine, hypochlorite bleach, ferric chloride and nitric acid.

High Integrity Protective System (HIPS): High Integrity Protection System is an automatic safety system with a SIL level of 3 or more.

Hot Job: The use of any flame or electric arc or the use of any equipment likely to cause heat or flame or spark, welding and drilling are hot jobs.

Health and Safety Executive (HSE): A independent regulatory authority responsible for the regulation for work- related to health, safety and illness across Great Britain's workplaces.

Hydrotest: A pressure testing of vassals, pipes lines, tubing usually performed by pressurizing the internal volume with water. The pressure during the test must be maintained to be at least 125% MAOP.

Hazards: Hazards are any operation that could possibly cause a release of toxic, flammable or explosive chemicals (including oil and gas) or any action that could result in injury to personnel or harm to the environment.



Integral Action: The "I" part of the PID controller. With integral action, the controller output is proportional to the amount and duration of the error signal. If there is more integral action, the controller output will change more when error is present. “I” eliminate the offset.

Ingress Protection (IP): In the IEC (International Electro technical Commission) 60529 Standards document, it specifies an international classification system for the sealing effectiveness of enclosures of electrical equipment against the intrusion into the equipment of foreign bodies (i.e., tools, dust, fingers) and moisture. This classification system utilizes the letters "IP" ("Ingress Protection") followed by two digits. (An "X" is used for one of the digits if there is only one class of protection; i.e., IP X4 which addresses moisture resistance only).

Intrinsic Safety (IS): Method of explosion prevention by limiting the energy stored in electrical circuits (electronics device made of passive or active electronics component)

Impedance: Opposition to the flow of ac current; the equivalent of resistance in dc circuits. Expressed in units called ohms. The impedance of an ac circuit is one ohm if a potential difference of one volt creates a current flow of one ampere within it.

Institute of Electrical and Electronics Engineers (IEEE): US based non- profit technical professional association


Linearity: A measure of the departure from a straight-line response in the relationship between two quantities, where the change in one quantity is directly proportional to a change in the other quantity, expressed as maximum percent.

Lead Length Consideration: Connecting leads can affect the accuracy of small current transformers, DC shunts and low voltage analog voltmeters. Use leads supplied with device or lead size specified.

Local Area Network (LAN): Ethernet is an example of a standard LAN. Allowing easy interconnection of devices through data communications network. Lag Time: Lag time is also called a capacity element or a first order process. Lag time is the amount of time after the dead time that the process variable takes to move of its final value after a step change. Load Upset: An upset to the process.



Microcomputer: A dedicated set of integrated circuits mounted on a single circuit card. It comprises a central processing unit (CPU, or microprocessor), memory circuits, and input/output (interface) circuits. Input circuits accept signals from external sources (and internal sources such as a clock) and convert them to corresponding digital voltages for CPU processing. The CPU performs logical and computational operations on these signals, as directed by a control program held in memory, based on conditions at the controlled process or instrument. CPU outputs are command signals which are sent to appropriate locations in the controlled system to accomplish the objectives of the control program.

Microprocessor: The small central processing unit (CPU) that performs the logic operations in a microcomputer system. Decodes instructions from the stored program, performs arithmetic and logic operations, generates timing signals, and produces commands for external use in process or instrument control.

Maximum Allowable Working Pressure (MAWP): The maximum pressure allowed in a piece of equipment e.g. vessel or pipe at its designated temperature.

Motor Control Centre (MCC): Motor control centers are simply physical groupings of combination starters in one assembly. A combination starter is a single enclosure containing the motor starter, fuses or circuit breaker, and a device for disconnecting power. Other devices associated with the motor, such as pushbuttons and indicator lights may also be included.

MODBUS: This Protocol is a messaging structure developed by Modicon in 1979. It is used to establish master-slave/client-server communication between intelligent devices.

Manometer: Manometers measure a pressure difference by balancing the weight of a fluid column between the two pressures of interest. Large pressure differences are measured with heavy fluids, such as mercury (e.g. 760 mm Hg = 1 atmosphere).

Monel: A nickel-copper alloy which combines high strength with high ductility - these usually being a trade off in metals selection. Also offers excellent general corrosion resistance. Monel is usually used in H2S gas or seawater applications.

Mineral Insulated: A term used to express a swaged or drawn thermocouple construction where a mineral oxide insulation, usually magnesium oxide or aluminum oxide, is densely compacted in a sheath for rugged bendable high temperature, sensor construction.

Mean Time between Failures (MTBF): it is a basic measure of system’s reliability, the mean time between failures. Higher the MTBF number higher the system reliability.

Mean Time To Repair (MTTR): The mean elapsed time from the occurrence of an Incident to the restoration of service.

Multiplexer (MUX): A switching device that sequentially connects multiple inputs or outputs in order to process several signal channels with a single A/D or D/A converter

Molecular Weight (MW): The weight of a molecule is the sum of the weights of the atoms of which it is made.



North American Corrosion Engineers (NACE): World’s largest organization dedicated to the study of corrosion NACE International is the global voice of the corrosion control and prevention industry. National Pipe Thread (NPT): National Pipe Thread, tapered, the fitting that is brazed or welded onto the RTD or thermocouple sheath, standard sizes are from 1/8 inch to 1 inch.

NEMA-7: A standard from the National Electrical Manufacturers Association, which defines explosion-proof enclosures for use in locations classified as Class I, Groups A, B, C, or D, as specified in the National Electrical Code.

NEMA-12: A standard from the National Electrical Manufacturers Association, which defines enclosures with protection against dirt, dust, splashes by non-corrosive liquids and salt spray.

Non Destructive Examination (NDE): Nondestructive examination methods of inspection make it possible to verify compliance to the standards on an ongoing basis by examining the surface and subsurface of the weld and surrounding base material. Five basic methods are commonly used to examine finished welds: visual, liquid penetrate, magnetic particle, ultrasonic and radiographic (X-ray).

Non Destructive Testing (NDT): ND -Tests are performed in a manner that does not affect the future usefulness of the object or material. In other words, NDT allows parts and material to be inspected and measured without damaging them. Because it allows inspection without interfering with a product's final use,

National Electrical Manufacturers Association (NEMA): The Association of Electrical Equipment and Medical Imaging Manufacturers which sets standards relating to electrically Powered instrumentation.

National Fire Prevention Association (NEPA): The mission of the international nonprofit NFPA, established in 1896, is to reduce the worldwide burden of fire and other hazards on the quality of life by providing and advocating consensus codes and standards, research, training, and education.

Noise: Any undesirable electrical signal from both external and internal sources. The sources include ac power lines; motors; electrical storms; radio transmitters and electrical components.

Non-linearity: The deviation from the best fit straight line that passes through zero.

Nitrogen Oxides (NOx): The term used to describe the sum of nitric oxide (NO); nitric dioxide (NO2) and other oxides of nitrogen which play a major role in the formation of ozone.

Normal Pressure and Temperature (NPT): NTP is used in many thermodynamic calculations and tabulations and is defined as 20 degrees Celsius and 1 atmosphere of pressure.



Ohm's Law: Ohms law deals with the relationship between voltage and current in an ideal conductor. This relationship states that: The potential difference (voltage) across an ideal conductor is proportional to the current through it. The constant of proportionality is called the "resistance", R., Ohm's Law is given by: V = I R Where, V is the potential difference between two points which include a resistance R. I is the current flowing through the resistance. For biological work, it is often preferable to use the conductance, g = 1/R; In this form Ohm's Law is: I = g V Material that obeys Ohm's Law is called "ohmic" or "linear" because the potential difference across it varies linearly with the current.

ON –Off Control: The simplest control action is ON/OFF. This type of control is inexpensive, but not accurate enough in most process and machine control applications. ON/OFF control almost always means overshoot and resultant system cycling. A dead band is usually required around the set point to prevent relay chatter at set point. ON/OFF control has no provision for adjusting to the time constants of a particular system

Offset: In proportional control, any change in system output is corrected for by an appropriate change in controller output. Unfortunately, the operation of a proportional controller leads to process deviation known as OFFSET or DROOP.

Overshoot: this is the magnitude by which the controlled variable 'swings' past the setpoint. 5/10% overshoot is normally acceptable for most loops.

OPC: OLE for Process Control (OPC), which stands for Object Linking and Embedding (OLE) for Process Control, is the original name for standards specification developed in 1996 by an industrial automation industry task force. The standard specifies the communication of real-time plant data between control devices from different manufacturers.

Opto-Isolator: An isolation device that provides an electrical barrier between related circuits.

Over Range: In digital meters, a reading that exceeds full-scale (but is less than an overload) that does not require switching to a high range, i.e., for 31/2 digit DPM 0-999 is full-scale rating. 1000-1999 is over range. 2000 and up is overload.



Piping and Instrument Diagram (P&ID: Piping and Instrumentation Diagrams or simply P&IDs are the “schematics” used in the field of instrumentation and control (Automation)The P&ID is used to by field techs, engineers, and operators to better understand the process and how the instrumentation is inter connected

Process Flow Diagram (PFD): A Process Flow Diagram - PFD - (or System Flow Diagram - SFD) shows the relationships between the major components in the system. PFD also tabulate process design values for the components in different operating modes, typical minimum, normal and maximum. A PFD does not show minor components, piping systems, piping ratings and designations.

Programmable Logic Controller (PLC): Programmable controllers are often defined as miniature industrial computers that contain hardware and software used to perform control functions are primarily based on equipment and alarm status.

Power Management System (PMS): is automation architecture designed to provide full control over the complete power network of industrial plants, allowing reliable and flexible energy management and supervision. It ensures the maximum electrical power availability of the plant, under all circumstances, in order to avoid electrical power failures resulting in production losses. The PMS system manages in coherence the regulation functions of Power Generation (load sharing between the running generators) and the logic functions of Power Distribution (Load Shedding, Automatic Transfer...) in case of overload or sudden loss of generating capacity.

Plant Design and Management System (PDMS): Software package used for plant design and creation of 3 dimensional models

Proportional Integral Derivative (PID): The PID algorithm is the most popular feedback controller used within the process industries. It is a robust easily understood algorithm that can provide excellent control performance despite the varied dynamic characteristics of process plant.

Port: A communications connection on an electronic or computer based device.

Primary Element: An element that converts a measured variable other form suitable for measurement.

Profibus: Vendor-independent open protocol. That becomes a part of international Standard IEC: 61158 in 2000.Increasingly used in process control.

Pressure Safety Valve (PSV): A spring loaded pressure relief valve actuated by the static pressure upstream of the valve and characterized by rapid opening.



Redundancy: Redundancy is a common approach to improve the reliability and availability of a system. Adding redundancy increases the cost and complexity of a system design and with the high reliability of modern electrical and mechanical components.

Range: The region between the limits within which a quantity is measured; received or transmitted expressed by stating lower and upper range values.

Repeatability: A quantities measure of the variability associated in obtaining successive repeat results from the same instrument in the same location under similar conditions.

Reproducibility: The closeness of agreement among repeated measurements of an output for the same value of input made under the same operating conditions over a period of time.

Resolution: The smallest change in input which produces a detectable change in output i.e. the smallest increment of change that can be detected by a measurement system.

Response Time: An output expressed as a function of time. Common to use T90, i.e. time to achieve 90% of output.

Raised Face (RF): The mating faces of a flange where in the matching face the gasket is inserted.

Ring Type Joint (RTJ): The mating face of a flange where the mating face has a groove into which the metallic ring is inserted.

Radio Frequency Interference (RFI): Noise induced upon signal wires by ambient radio-frequency electromagnetic radiation with the effect of obscuring an instrument signal.

Relative Humidity (RH): An expression of the level of water vapour present compared to the maximum amount of water vapour that a gas could carry at that temperature. It is expressed in percentage terms.

Resistance Temperature Detector (RTD): Resistance Temperature Detectors Temperature sensing element temperature measurement proportional to changes in resistance.

Remote Telemetry Unit (RTU): A device that collects data from data acquisition equipment and sends them to the main system over a wired or wireless network. It also includes Modbus TCP support for easy connectivity to programmable logic controllers (PLCs)/ (SCADA).



Supervisory Control and Data Acquisition (SCADA): A hierarchy level of DCS, a PC based control and acquisition software package that allows operator control of a process from a PC.

Sensitivity: The minimum change in a physical variable to which an instrument can respond.

Sensor: An element or device that detects a variable by receiving information in the form of one quantity and converting it to information in the form of that or another quantity.

Set point: A variable - expressed in the same units as the measurement - which sets either the desired target for a controller or the condition at which alarms or safety interlocks are to be energized.

Safety Integrity Level (SIL): SILs are measures of the safety risk of a given process. it is stratified into four discrete levels of safety. Each level represents an order of magnitude of risk reduction. The higher the SIL level, lower the failure.

Solenoid Valve (SV): A solenoid valve is a combination of two basic functional units: 1- A solenoid (electromagnet) with its core. 2- A valve body containing one or more orifices. Flow through an orifice is shut off or allowed by the movement of the core when the solenoid is energized or de-energized.

Synchronous Motor: An AC motor whose speed is exactly proportional to the frequency of the applied alternating voltage.

Span: The algebraic difference between the upper and lower range values expressed in the same units as the range.

Span Adjustment: The ability to adjust the gain of a process usually Transmitters so that a specified display span in engineering units corresponds to a specified signal span.

Stability: The ability of an instrument or sensor to maintain a consistent output when a constant input is applied.

Steady State: A characteristic of a condition e.g. value; rate; periodicity or amplitude exhibiting only negligible change over an arbitrary long period of time.

Signal Conditioner: A circuit module which offsets attenuates, amplifies, linearizes and/or filters the signal for input to the A/D converter.

Surge: Surge is a condition that occurs on compressors when the amount of gas they are trying to compress is insufficient for the speed of the compressor and the turbine blades lose their forward thrust, causing a reverse movement in the shaft. This condition can have catastrophic effects on the machine, so compressor manufacturers include anti-surge valves that recycle gas from the discharge to the suction when a low flow is detected. Usually these valves are designed to be only open on startup or under reduced rates.

Site Acceptance Test (SAT): A test of equipment carried out at site during installation and of equipment prior to commissioning

Servo: A device used to provide control of a desired operation through the use of feedback



Transmission Control Protocol/Internet Protocol (TCP/IP): The suite of communications protocols used to connect devices on the internet or similar network.

Triple Modular Redundant (TMR): Redundancy is a common approach to improve the reliability and availability of a system. Triple Modular Redundancy (TMR) uses three functionally equivalent units to provide redundant backup. This approach is very common in turbine and compressor control applications where the cost of failure is extremely high.

Transducer: A transducer is an electronic device that receives information in the form of one quantity and converts it to information in the same or another quantity or form.

Two Wire Transmitter: A specially constructed R vs. 1 signal conditioner in which the output loop contains the power source; minimizes wires to control room by use of only two (2) wires.

Trim: The internal elements of a valve are collectively referred to as a valve's trim, The trim typically includes a disk, seat, stem, and sleeves needed to guide the stem. A valve’s performance is determined by the disk and seat interface and the relation of the disk position to the seat.

TUV: Notified Body based in Germany and Austria.

Thermowell: The pressure vessel into which an RTD or thermocouple is inserted for easy removal and/or replacement purposes.

Time Constant: The tem required for a sensor to respond to 63% of its total resistance change resulting from a newly impressed temperature. Five time constants are required to attain 99% of the total change. Values given are in a well stirred oil bath. Time constant in still air is approximately ten times longer.



Universal Serial Bus (USB): The Universal Serial Bus (USB) standard was originally developed in 1995, to minimize the number of ports in the back of the PC. An external bus standard for connecting as many as 127 peripheral devices to computers with data transfer rates up to 12 Mbps. USB featured a maximum bandwidth of 1.5Mbit for low speed devices such as mouse and keyboards, and a maximum bandwidth of 12Mbit for higher speed devices such as web cams, printers, scanners and external CD-RW drives.

Unit Control Panel (UCP): UCP encompass a variety of control functions for the safe operation of machinery, Field devices are often wired and terminated on a DIN rack mounted terminal strip. Digital and analog inputs are internally wired to the appropriate input/output modules. The I/O is communicated to the PLC via Control-Net, Ethernet, or other appropriate communication network protocol.

Upper Flammable Limit (UFL): The concentration of flammable gas in air at atmospheric pressure above which combustion will not occur. The figure is expressed as a percentage by volume.

Underwriters Laboratories (UL): North American based independent not-for-profit product-safety testing and certification organization.


Variable Frequency Drive (VFD): VFD to a motor-driven system can offer potential energy savings in a system in which the loads vary with time. The operating speed of a motor connected to a VFD is varied by changing the frequency of the motor supply voltage. This allows continuous process speed control.

Viscosity: Viscosity is an internal property of a fluid that offers resistance to flow a measure of a fluid's resistance to flow.

Variable Speed Drive (VSD): A system either electrical or mechanical - that allows a motor to be controlled to run at different speeds.

Vacuum: This is the amount of pressure below atmospheric pressure 0 PSIG or 14.7 PSIA. It increases in value as the vacuum increases (pressure decreases) toward “Absolute Zero” or no pressure. Vacuum values are usually expressed in inches of mercury vacuum ("Hg VAC), inches of water vacuum ("H20 VAC) or millimeter of mercury vacuum (mmHg VAC). A common method of expression of vacuum is to refer to it as as 770 mmHg.



Wheatstone Bridge: A full resistance two-wire bridge with RTD, power source, zero and sensitivity adjustments balanced at some reference temperature. Heating or cooling causes a resistance change and discrete circuit unbalance, which indicates the temperature. Various RTD bridge network connections use 2-, 3-, or 4-wire hook-up arrangements, depending on the accuracy required in the temperature measurement. These are designed to balance out lead wire resistance between the sensor and the bridge.

Watt: The measure of real power. It is the power expended when on ampere of direct current flows through a resistance of one ohm. WATTS = EI x PF

Wide Area Network (WAN): The term Wide Area Network (WAN) usually refers to a network which covers a large geographical area, and use communications circuits to connect the intermediate nodes. Transmission rates are typically 2 Mbps, 34 Mbps, 45 Mbps, 155 Mbps, and 625 Mbps. A network usually constructed with serial lines extending over distances greater than one 1Km.

WWW: The term WWW refers to the World Wide Web or simply the Web. The World Wide Web consists of all the public Web sites connected to the Internet worldwide, including the client devices (such as computers and cell phones) that access Web content. The WWW is just one of many applications of the Internet and computer networks. The World Web is based on these technologies: HTML - Hypertext Markup Language/HTTP - Hypertext Transfer Protocol/Web servers and Web browsers.



Zero Adjustment: Adjustment of a measuring instrument so that it indicates zero in unloaded condition- it means that a measuring device is adjusted down to zero before a measurement is done. The adjustment range is normally expressed in counts.

Zero Drift: The maximum deviation from zero reading of a meter having zero input over a given range. Expressed as a percent of full scale reading.

Zero Suppression: Suppose a DPT is mounted at the bottom of the tank through an impulse pipeline, then even if the tank is empty the impulse pipeline will have some water in it. This water will exert pressure on the HP side of the DPT and we will get some output even if the tank is empty, so the output needs to be suppressed to the zero value.

Zero Elevation: A range in which the zero value of the measured variable exceeds the lower range value. A DPT is mounted at the bottom of the closed tank and to the LP the impulse tube which is connected is filled with a reference liquid. Then the pressure exerted on LP>HP. Hence the output will be negative .So it has to be elevated to reach the zero level.

Z-Transformer: Z-transform, like the Laplace transform, is an indispensable mathematical tool for the design, analysis and monitoring of systems. The z-transform is the discrete-time counter-part of the Laplace transform and a generalization of the Fourier transform of a sampled signal. Like Laplace transform the z-transform allows insight into the transient behavior, the steady state behavior, and the stability of discrete-time systems. A working knowledge of the z-transform is essential to the study of digital filters and systems

Zigbee: ZigBee fills yet another industry niche. It is a PAN technology based on the IEEE 802.15.4 standard. Unlike Bluetooth or wireless USB devices. One ZigBee network can contain more than 65,000 nodes (active devices).