Level Measurement

Everyday examples of liquid level measurement devices are the engine oil and gas tank gauges of a car. Another simple device is the level gauge or level glass on a tank or boiler. Measuring and controlling liquid level is essential in a process plant, where a wide variety of liquids are handled in both batch and continuous processes. The accurate measurement of level is important for environmental protection (for example, tank overflow to drains), plant safety, product quality, and inventory control.

Almost all liquid level devices measure by way of the position or height of the liquid above a zero or lowest point, or the hydrostatic pressure or head Method for level measurement can be direct level measurement or indirect or by measuring the head of liquid.

Direct Level Measurement

direct method employs physical properties such as fluid motion and buoyancy, as well as optical, thermal and electrical properties. To measure level directly the position of the interface may be monitored visually through a sight glass, dip stick, or by float device riding on the surface. Sample method are

(1) Direct visual observation of the height by means of sight glass, level gage, or dip stick

(2) A float, which is mechanically linked or electrically connected to an indicator or alarm device

(3) An electrical probe in the liquid

(4) Reflection of sonic waves from the liquid surface or from the bottom

Indirect level measurement

indirect level measurement involves converting measurement of some other quantity, commonly head pressure to a level.

The techniques of this method include:

(1) The buoyant force on a float or displacer, which is partially or completely immersed in liquid

(2) Hydrostatic pressure of the liquid

(3) The amount of radiation passing through the liquid

For detail each type of level measurement transmitter please read per each type in this web.

Terminology in Instrumentation

Instrument engineering has its own terminology. Some of the terms have subtle meanings, and a misunderstanding can lead to a completely mistaken impression of a system’s performance. The following definitions are intended only as an introduction to the use of the terminology. For more complete and precise definitions refer to the ISA standard ANSI/ISA-51.1-1979 (R1993) – Process Instrumentation Terminology.

RANGE

The region between the limits within which a quantity is measured, received, or transmitted, expressed by stating the minimum value (lower range value) and maximum value (upper range value). Every sensor is designed to work over a specified workable range. While an electrical output may be adjusted to suit the application, this is not usually practical with mechanical transducing elements.

The design ranges of these mechanisms are usually fixed, and exceeding them can permanently damage a sensor. Transducing elements must be used over the part of their range in which they provide predictable performance and often truer linearity.

ZERO

A measurement must be made with respect to a known datum. Often, it is convenient to adjust the output of the instrument to zero at the datum. For example, the output of a Celsius thermometer is zero at the freezing point of water; the out value ascribed to some defined point in the measured range.

ZERO DRIFT

One of the problems that occurs with sensors is when the value of the zero signal varies from its set value. This introduces an error into the measurement that may be equal to the amount of variation, or drift, as it is usually termed. All sensors are affected by drift to some extent, which is sometimes specified in terms of short-term and long-term drift. Short-term drift is usually associated with changes in temperature or electronics stabilizing. Long-term drift is usually associated with aging of the transducer or electronic components.

SENSITIVITY

Sensitivity of a sensor is defined as the change in output of the sensor per unit change in the parameter being measured. Sensors may have constant or variable sensitivities, in which cases they are described as having a linear or a nonlinear output, respectively. Clearly, the greater the output signal change for a given input change, the greater the sensitivity of the measuring element. Sensitivity depends on a number of variable factors. The mechanical properties of a transducer may vary with temperature and cause a variation in sensitivity, but often it is the electrical part of the sensor that is responsible for the greatest changes. An amplifier may change its gain because of temperature effects on components or variations in power supplies or even faulty operation.

An example of when sensitivity would be critical is in a blending process that requires a certain mix. The load change that occurs every time an ingredient is added may cause a sharp change in the temperature. The mix could be ruined if the change in temperature were not measured and controlled immediately. High sensitivity of the measuring element increases the chances of a quick response.

RESOLUTION

Resolution is defined as the smallest change that can be detected by a sensor. Although it is evident that sensors using wire-wound potentiometers or digital techniques to provide their electrical output have finite resolutions, no known device has an infinitely small resolution.

RESPONSE

The time taken by a sensor to approach its true output when subjected to a step input is sometimes referred to as its response time. It is common to state that the performance of a sensor has a flat response between specified limits of frequency. This is known as the frequency response, and it indicates that if the sensor is subjected to sinusoidally oscillating input of constant amplitude, the output will faithfully reproduce a signal that is proportional to the input. Fast sensors make it possible for controllers to function in a timely manner. Sensors with large time constants are slow and may degrade the overall operation of the feedback loop.

LINEARITY

A sensor that is described as having a linear transfer function is one whose output is directly proportional to the input over its entire range. This relationship appears as a straight line on a graph of output versus input. In practice, exact linearity is never quite achieved, although most transducers exhibit only small changes of slope over their working range. In such cases, the manufacturer fits a “best” straight line whose error is usually well within the tolerance of the mea- surement. Some sensors, particularly those that use inductive transducing principles, demonstrate considerable changes in the slope of their output versus input  is no change of output. The working range of such a sensor is restricted and must be limited to where the graph is most linear; alternatively, a different factor must be applied to each reading.

HYSTERESIS

Hysteresis becomes apparent when the input to a sensor is applied in a cyclic manner. If the input is increased incrementally to the sensor’s maximum and returned to its zero datum in a similar way, the calibration may be seen to describe two output curves that meet at the maximum. In returning to zero input, the instrument has not returned to its original datum. If the calibration is continued in the negative direction of input, two further curves will be produced that are a mirror image of the previous ones. Further cycling will eventually link these two halves into one complete loop, which will then be repeatable with every cycle. This loop is normally referred to as the sensor’s hysteresis loop, although it also contains any of the other nonlinearity effects that may be present. Consequently, it is usual when specifying a sensor to quote nonlinearity and hysteresis as one parameter.

CALIBRATION

To be meaningful, the measurement of the output of a sensor must be in response to an accurately known input. This process is known as calibration, and the devices that produce the inputs are described as calibration standards. It is usual to provide measurements at a number of points in the working range of the sensor, so a ratio of output to input may be determined from the measured points by calculation.

Such a ratio is described as a calibration factor. The ratio of output to input is not always a constant over the range of a sensor, and the calibration graph may describe a curve. In these instances, a best straight line may be fitted through the points and the errors accepted, or a different calibration factor must be provided for every measurement.

ACCURACY AND PRECISION

When we speak of the accuracy of a measurement we describe the closeness with which the measurement approaches the true value of the variable being measured. Precision is the reproducibility with which repeated measurements of the same variable can be made under identical conditions. In matters of process control, the latter characteristic is more important than accuracy; it is normally more desirable to measure a variable precisely than it is to have a high degree of absolute accuracy. The distinction between these two properties of measurement is best illustrated graphically :

Precision and accuracy

The meaning of accuracy, when quoted as a percentage of full-scale output, is a value of uncertainty that is applied to converted sensor outputs throughout the entire range of measurement. For example, a measurement with an accuracy of +1 percent full scale and with a range of 0–100 units has a value of uncertainty of +1 part in 100 or +1 unit, which applies to every measurement. A measurement of 50 units would be made with a value of uncertainty of +1 unit or +2 percent of the value.

Sensors are designed to be both accurate and precise. A sensor that is accurate but imprecise may come very close to measuring the actual value of the controlled variable, but it will not be consistent in its measurements. A sensor that is precise but inaccurate may not come as close to measuring the actual value of the controlled variable, but its measurements will differ from the actual value by nearly the same amount every time. This consistency makes it possible to compensate for the sensor error.

The accuracy or uncertainty of power supplies, amplifiers, and recorders also contributes to the overall measurement value. Some instrumentation engineers treat all these quantities as a “measuring chain” and do not attempt to break them down, arguing that the accuracy of the measurement can be only the accuracy of the chain as a whole. Practitioners make a distinction between two types of accuracy: static or steady-state accuracy and dynamic accuracy. Static accuracy is the closeness with which the true value of the variable is approached when that true value is constant.

Transducers and Sensors

The role of the sensor in an automatic control system is clearly seen in the traditional functional block diagram (see Figure 1-1).

role of sensor

A transducer is a device that converts one form of energy to another. This conversion may be pressure to movement, electric current to pressure, liquid level to a twisting movement on a shaft, or any number of other combinations. Although the final output of a sensor may be electrical or pneumatic, there may also be one or more intermediate transducing stages.

There are two basic types of sensors: analog, which produces an output proportional to a change in a parameter, and digital, which produces an on/off type of output. Sensors that provide digital outputs (for example, pulses) proportional to changes in the parameter are regarded as digital sensors.

A sensor may also be viewed as an active or a passive transducer. A sensor whose output energy is supplied entirely, or almost entirely, by its input signal is commonly called a passive transducer. The output and input signals may involve energy conversion from one form to another (for example, mechanical to electrical).

An active transducer, on the other hand, has an auxiliary source of power that supplies a major part of the output power, while the input signal supplies only an insignificant portion. Again, there may or may not be a conversion of energy from one form to another (see Table 1-1).

Physical effec used in transducer

P&ID (Piping and Instrumentation Diagram)

What is P&ID? Some people are misunderstanding between P&ID and PID (Proportional Integral and Derivative) for control.

The Piping and instrumentation diagram using to identifyg instruments or devices and their inherent functions, instrumentation systems and functions, and application software functions used for measurement, monitoring, and control, by presenting a designation system that includes identification schemes and graphic symbols.

P&ID also called as PEFS (Process Engineering Flow Schemes) as per in Shell DEP(design Engineering Practices) .

Identification of letters

The instrument symbols used on PEFS shall indicate the functionality of the loop. For DCS and general instrumentation the symbols shall indicate which process condition or property is being measured and controlled and what information is displayed. For safety related function (e.g. IPFs-Instrumented Protective Function) the symbols shall indicate which process condition or property is being measured and what information is displayed.

All components and functions of the loop shall carry the same tag number. The numbering shall be unique at loop level.

Each instrument shall have a tag number of the format:

abc yz

in which:

‘a’            is a three digit number used to identify the process unit.

‘b’           is a measured variable code: one capital letter code identifies the process condition, property measured or initiating variable. Where required, an additional modifier letter is added. (see the 1st letter identification table)

‘c’           is a function code: one or more capital letter codes identify the function of the instrument or loop. (see the 2st letter identification table) is a separation dash, used for clarity

‘y’           is a three digit serial number (i.e. from 001 through 999)

‘z’           is an optional suffix which may be used to make a loop component uniquely identifiable; only to be used if required.

Sample:

letter identifier

Identification Letter

Instrumentation device and function symbols (As Per ISA 5.1 – 2009)

Basic baloon symbols

Instrumentation device or function symbols, miscellaneous

Misc Symbols

Measurement symbols: primary elements and transmittersprimary elements and transmitters

Measurement symbols: primary elements

primary elements

primary elements 2

primary elements 3primary elements 4

Measurement symbols: secondary instruments

secondary instruments 1

Measurement symbols: auxiliary and accessory devices

auxiliary and accessory devices1

Line symbols: instrument to process and equipment connectionsinstrument to process and equipment connections

Line symbols: instrument-to-instrument connections

instrument-to-instrument connectionsinstrument-to-instrument connections2

Final control element symbols

Final control element symbolsFinal control element symbols2

Final control element actuator symbols

Final control element actuator symbols1Final control element actuator symbols2

Self-actuated final control element symbolSelf-actuated final control element symbolSelf-actuated final control element symbol2Self-actuated final control element symbol3

Control valve failure and de-energized position indications

Control valve failure and de-energized position indications

Standard

ANSI/ISA-5.1-2009

Instrument Transmitter

A transmitter carries a signal of the value of the measured variable from the sensor to a controller. Transmitters are necessary because the sensor and the controller are often physically far apart. The transmitter picks up the measurement provided by the sensor, converts it into a standard signal, which can be easily sent and read, and conveys the signal to the controller. Sensors and transmitters are often combined into one device.

The two most common types of transmission used in industry are pneumatic and electronic. A pneumatic transmitter converts the value of the measurement into an air pressure signal that is sent through tubing to the controller. The connecting tubing carries the transmitted pressure to a receiver, which is a component located in the controller housing. The tubing is almost always one-quarter inch in outside diameter and may be copper, aluminum, or plastic. The receiver is simply a pressuregage element, and the transmitted air pressure is converted into the movement of a bellows or diaphragm (that is, pressure is transduced into a position or force that is used by the controller).

As distances increase, the speed of response of pneumatic transmission systems becomes a problem, and alternative solutions, including electronic transmission, are necessary

.Featured imageSample of Pneumatic transmitter

An electronic transmitter converts the measurement into an electric signal, usually a voltage or a current, then transmits the signal by wire or radio linkage. Electronic signals can be transmitted over very long distances while still providing a virtually instantaneous response; therefore, their dynamics do not become serious problems in process control applications. Other types of transmission sometimes used are hydraulic, telemetering, and optical.

electronoc transmitter

Hydraulic. The transmitter converts the value of the measurement into an equivalent value of fluid pressure and sends the signal through tubing. Hydraulic transmitters can be more accurate than pneumatic ones because, unlike air, a liquid does not compress. However, hydraulic transmitters are temperature sensitive, and a small leak will destroy the transmission.

hodraulyc transmitter

Telemetering. The transmitter converts the value of the measured variable into certain frequencies or amplitudes of radio signals and sends the signal by radio linkage. An example of a telemetered transmitter is a microwave transmitter. A microwave transmitter converts the value of the measured variable to microwave radio signals. These transmitters require no wiring or cabling but are subject to interference.

wireless trasmitter

Optical. The transmitter converts the value of the measured variable to light frequencies and sends the signal through optical fibers. Optical transmitters are not subject to electrical noise or interference. Signals are usually transmitted within standard ranges.

For electronic transmitters the most common standard is 4–20 mA DC. The most common standard pneumatic signal is 3–15 psig (20–100 kPa).

The information transmitted by the transmitter has to cover the entire range of information on the measured variable. For example, if the range of a process temperature is 100–500°F and the output signal range of the transmitter is 4–20 mA, the transmitter is calibrated so that 4 mA corresponds to 100°F, and 20 mA corresponds to 500°F.

IEC Standards List

IEC 60027 Letter symbols to be used in electrical technology
IEC 60028 International standard of resistance for copper
IEC 60034 Rotating electrical machinery
IEC 60038 IEC Standard Voltages
IEC 60041 Field acceptance tests to determine the hydraulic performance of hydraulic turbines, storage pumps and pump-turbines
IEC 60044 Instrument transformers
IEC 60045 Steam turbines
IEC 60050 International Electrotechnical Vocabulary
IEC 60051 Recommendation for direct acting indicating analogue electric measuring instruments and their accessories
IEC 60055 Paper-insulated metal-sheathed cables for rated voltages up to 18/30 kV
IEC 60060 High-voltage test techniques
IEC 60062 Marking codes for resistors and capacitors
IEC 60063 Preferred number series for resistors and capacitors
IEC 60061 Lamp caps and holders together with gauges for the control of interchangeability and safety
IEC 60064 Tungsten filament type GLS (General Lighting Solutions) bulbs
IEC 60065 Audio, video and similar electronic apparatus – Safety requirements
IEC 60068 Environmental Testing
IEC 60071 Insulation Co-ordination
IEC 60073 Basic Safety principles for man-machine interface, marking and identification – coding principles for indicators and actuators
IEC 60076 Power transformers
IEC 60077 Railway applications – Electric equipment for rolling stock
IEC 60079 Explosive Atmospheres
IEC 60083 Plugs and socket-outlets for domestic and similar general use standardized in member countries of IEC
IEC 60085 Electrical insulation
IEC 60086 Primary batteries
IEC 60092 Electrical installations on ships
IEC 60094 Magnetic tape sound recording and reproducing systems
IEC 60095 Lead-acid starter batteries
IEC 60096 Radio-frequency cables
IEC 60098 Rumble measurement on Vinyl Disc Turntables
IEC 60099 Surge arresters
IEC 60119 The Electrical Performance of Semiconductor Rectifiers (Metal Rectifiers)
IEC 60134 Absolute maximum and design ratings of tube and semiconductor devices
IEC 60137 Bushings for alternating voltages above 1000V
IEC 60146 Semiconductor Converters
IEC 60169 Radio-frequency connectors
IEC 60183 Guide to the selection of high voltage cables
IEC 60193 Hydraulic turbines, storage pumps and pump-turbines – Model acceptance tests
IEC 60204 Safety of machinery
IEC 60214 On-load tap changers
IEC 60228 Conductors of insulated cables
IEC 60233 Tests on Hollow Insulators for use in Electrical Equipment
IEC 60238 Edison screw Lampholders
IEC 60239 Graphite electrodes for electric arc furnaces – Dimensions and designation
IEC 60245 Rubber-Insulated Cables
IEC 60254 Lead-acid traction batteries
IEC 60255 Electrical Relays
IEC 60268 Sound system equipment
IEC 60269 Low voltage fuses
IEC 60270 High-Voltage Test Techniques – Partial Discharge Measurements
IEC 60273 Characteristics of indoor and outdoor post insulators for systems with nominal voltages greater than 1000V
IEC 60287 Calculation of permissible current in cables at steady state rating
IEC 60296 Mineral Insulating oils for transformers & switchgear
IEC 60297 Dimensions of mechanical structures of the 482.6 mm (19 in) series
IEC 60298 high voltage switchgear in metallic enclosure
IEC 60308 Hydraulic turbines – Testing of control systems
IEC 60309 Plugs, socket-outlets and couplers for industrial purposes
IEC 60317 Specifications for particular types of winding wires
IEC 60320 Appliance couplers for household and similar general purposes
IEC 60331 Tests for Electric Cables under Fire Conditions
IEC 60332 Flame Retardant vs Fire Rate Cables
IEC 60335 Safety of electrical household appliances
IEC 60364 Electrical installations of buildings
IEC 60397 Test methods for batch furnaces with metallic heating resistors
IEC 60398 Installations for electroheating and electromagnetic processing – General test methods
IEC 60417 Graphical symbols for use on equipment
IEC 60439 Low voltage switchgear and controlgear assemblies
IEC 60445 Basic and safety principles for man-machine interface
IEC 60446 Wiring colours
IEC 60457 Method of sampling insulating liquids
IEC 60479 Effects of current on human beings and livestock
IEC 60502 Power cables with extruded insulation and their accessories for rated voltages from 1kV up to 30kV
IEC 60519 Safety in installations for electroheating and electromagnetic processing
IEC 60529 Degrees of protection provided by enclosures (IP Code)
IEC 60545 Guide for commissioning, operation and maintenance of hydraulic turbines
IEC 60546 Controllers with analogue signals for use in industrial-process control systems
IEC 60571 Electronic equipment used on rail vehicles
IEC 60574 Audio-visual, video and television equipment and systems
IEC 60598 Luminaires
IEC 60559 Binary floating-point arithmetic for microprocessor systems
IEC 60601 Medical Electrical Equipment
IEC 60603 Connectors for frequencies below 3 MHz for use with printed boards
IEC 60609 Hydraulic turbines, storage pumps and pump-turbines – Cavitation pitting evaluation
IEC 60617 Graphical symbols for diagrams
IEC 60651 Sound level meters
IEC 60662 High-pressure sodium lamp – Performance specifications
IEC 60664 Insulation coordination for equipment within low-voltage systems
IEC 60669 Switches for household and similar fixed-electrical installations
IEC 60676 Industrial electroheating equipment – Test methods for direct arc furnaces
IEC 60680 Test methods of plasma equipment for electroheat and electrochemical applications
IEC 60683 Test methods for submerged arc furnaces
IEC 60688 Electrical measuring transducers for converting AC electrical quantities to analogue or digital signals
IEC 60694 Common Specifications For High-Voltage Switchgear and Controlgear Standards
IEC 60703 Test methods for electroheating installations with electron guns
IEC 60715 Dimensions of low-voltage switchgear and controlgear. Standardised mounting on rails for mechanical support of electrical devices in switchgear and controlgear installations.
IEC 60721 Classification of environmental conditions
IEC 60726 Dry type power transformers
IEC 60728 Cable networks for television signals, sound signals and interactive services
IEC 60730 Class B certification requirements for appliances.
IEC 60747 Semiconductor devices; Part 1: General
IEC 60748 Semiconductor devices – integrated circuits
IEC 60760 Flat, quick-connect terminations (merged into IEC 61210:2010-08)
IEC 60774 VHS/S-VHS video tape cassette system
IEC 60793 Optical fibres
IEC 60779 Test methods for electroslag remelting furnaces
IEC 60801 EMI and RFI Immunity
IEC 60805 Guide for commissioning, operation and maintenance of storage pumps and of pump-turbines operating as pumps
IEC 60809 Filament lamps for road vehicles – Dimensional, electrical and luminous requirements
IEC 60812 International Standard on Fault Mode and Effects Analysis
IEC 60815 Selection and dimensioning of high-voltage insulators intended for use in polluted conditions
IEC 60825 Laser safety
IEC 60826 Design criteria of overhead transmission lines
IEC 60849 Sound Systems for Emergency Purposes
IEC 60865 Short Circuit Current: Calculation of Effects
IEC 60870 Telecontrol equipment and systems
IEC 60874 Connectors for optical fibres
IEC 60884 Plugs and socket-outlets for household and similar purposes
IEC 60898 Electrical accessories. Circuit breakers for overcurrent protection for household and similar installations.
IEC 60904 Photovoltaic Devices (Part 1-10).
IEC 60906 IEC system of plugs and socket-outlets for household and similar purposes
IEC 60908 Compact disk digital audio system
IEC 60909 Short-circuit currents in three-phase a.c. systems – Part 0: Calculation of currents
IEC 60921 Ballasts for tubular fluorescent lamps – Performance requirements
IEC 60929 AC-supplied electronic ballasts for tubular fluorescent lamps – Performance requirements
IEC 60942 Electroacoustics – Sound calibrators
IEC 60945 Maritime Navigation and Radiocommunication Equipment and Systems – General Requirements – Methods of Testing and Required Test Results
IEC 60947 Standards for low-voltage switchgear and controlgear
IEC 60950 Safety of information technology equipment
IEC 60994 Guide for field measurement of vibrations and pulsations in hydraulic machines (turbines, storage pumps and pump-turbines)
IEC 61000 Electromagnetic compatibility (EMC)
IEC 61008 Residual current operated circuit-breakers without integral overcurrent protection for household and similar uses (RCCBs)
IEC 61009 Residual current operated circuit breakers with integral overcurrent protection for household and similar uses (RCBO’s)
IEC 61010 Safety requirements for electrical equipment for measurement, control and laboratory use
IEC 61024 Protection of structures against lightning
IEC 61025 Fault tree analysis
IEC 61030 Domestic Digital Bus – a standard for a low-speed multi-master serial communication bus for home automation applications.
IEC 61043 Sound intensity meters with pairs of microphones
IEC 61058 Switches for Appliances
IEC 61084 Cable trunking and ducting systems for electrical installations
IEC 61097 Global maritime distress and safety system (GMDSS)
IEC 61116 Electromechanical equipment guide for small hydroelectric installations
IEC 61131 Programmable Logic Controllers
IEC 61149 Safety of mobile radios
IEC 61156 Multicore and symmetrical pair/qud cables for digital communications
IEC 61158 Industrial communication networks – Fieldbus specifications
IEC 61162 Maritime navigation and radiocommunication equipment and systems, Digital Systems
IEC 61164 Reliability growth – Statistical test and estimation methods
IEC 61174 Maritime Navigation and Radio Communications, Electronic Chart Display and Information System (ECDIS)
IEC 61194 Characteristic parameters of stand-alone photovoltaic (PV) systems
IEC 61210 Connecting devices – Flat quick-connect terminations for electrical copper conductors – Safety requirements
IEC 61211 Insulators of ceramic material or glass for overhead lines with a nominal voltage greater than 1 000 V – Impulse puncture testing in air
IEC 61215 Crystalline silicon terrestrial photovoltaic (PV) modules – Design qualification and type approval
IEC 61226 Nuclear power plants – Instrumentation and control important to safety – Classification of instrumentation and control functions
IEC 61238 Compression and mechanical connectors for power cables for rated voltages up to 30 kV
IEC 61241 Electrical apparatus for use in the presence of combustible dust
IEC 61277 Terrestrial photovoltaic (PV) power generating systems – General and guide
IEC 61280 Field testing method for measuring single mode fibre optic cable
IEC 61286 Character set with electrotechnical symbols
IEC 61307 Industrial microwave heating installations – Test methods for the determination of power output
IEC 61308 High-frequency dielectric heating installations – Test methods for the determination of power output
IEC 61326 Electrical equipment for measurement, control and laboratory use – EMC requirements
IEC 61334 Distribution automation using distribution line carrier systems – a standard for low-speed reliable power line communications by electricity meters, water meters and SCADA [1]
IEC 61345 UV test for photovoltaic (PV) modules
IEC 61346 Industrial systems, installations and equipment and industrial products – Structuring principles and reference designations
IEC 61347 Lamp Controlgear
IEC 61355 Classification and designation of documents for plants, systems and equipment
IEC 61362 Guide to specification of hydraulic turbine control systems
IEC 61363 Electrical installations of ships and mobile and fixed offshore units
IEC 61364 Nomenclature for hydroelectric powerplant machinery
IEC 61366 Hydraulic turbines, storage pumps and pump-turbines – Tendering Documents
IEC 61378 Converter Transformers
IEC 61400 Wind turbines
IEC 61439 Low-Voltage switchgear and controlgear assemblies
IEC 61467 Insulators for overhead lines – Insulator strings and sets for lines with a nominal voltage greater than 1 000 V – AC power arc tests
IEC 61499 Function blocks
IEC 61508 Functional safety of electrical/electronic/programmable electronic safety-related systems
IEC 61511 Functional safety – safety instrumented systems for the process industry sector
IEC 61513 Functional safety – safety instrumented systems for the Nuclear Industries
IEC 61523 Delay and Power Calculation Standards
IEC 61537 Cable management – Cable tray systems and cable ladder systems
IEC 61544 Panel-Mounted equipment – Electrical measuring instruments – Dimensions for panel mounting
IEC 61557 Equipment for measuring electrical safety in low-voltage distribution systems
IEC 61558 Safety of power transformers, power supplies, reactors and similar products
IEC 61588 Precision clock synchronization protocol for networked measurement and control systems
IEC 61603 Infrared transmission of audio or video signals
IEC 61642 Industrial a.c. Networks Affected by Harmonics – Application of Filters and Shunt Capacitors
IEC 61643 Surge protective devices connected to low-voltage power distribution systems
IEC 61646 Thin-film terrestrial photovoltaic (PV) modules – Design qualification and type approval
IEC 61672 Electroacoustics – Sound level meters
IEC 61683 Procedure to Measure Efficiency
IEC 61690 Electronic design interchange format, EDIF
IEC 61701 Salt mist corrosion testing of photovoltaic (PV) modules
IEC 61723 Safety Guidelines for grid connected photovoltaic Systems mounted on the building
IEC 61724 Photovoltaic system performance monitoring – Guidelines for measurement
IEC 61727 Photovoltaic (PV) systems – Characteristics of the utility interface
IEC 61730 Photovoltaic modules
IEC 61753 Fibre optic interconnecting devices and passive components performance standard
IEC 61784 Industrial communication networks – Profiles
IEC 61800 Adjustable speed electrical power drive systems
IEC 61803 Determination of power losses in high-voltage direct current (HVDC) converter stations
IEC 61829 Crystalline silicon photovoltaic (PV) array – On-site measurement of I-V characteristics
IEC 61846 Ultrasonics – Pressure pulse lithotripters – Characteristics of fields
IEC 61850 Communication Networks and Systems in Substations
IEC 61883 Consumer audio/video equipment – Digital interface
IEC 61892 Mobile and fixed offshore units, electrical installations
IEC 61922 High-frequency induction heating installations – Test methods for the determination of power output of the generator
IEC 61966 Multimedia systems — Colour measurement
IEC 61968 Application integration at electric utilities – System interfaces for distribution management
IEC 61970 Application integration at electric utilities – Energy management system application program interface (EMS-API)
IEC 61992 Railway applications – Fixed installations – DC switchgear
IEC 61993 Maritime navigation and radiocommunication equipment and systems.
IEC 62006 Hydraulic machines – Acceptance tests of small hydroelectric installations
IEC 62040 Uninterruptible power systems
IEC 62041 EMC requirements for power transformers, power supplies, reactors and similar products
IEC 62052 Electricity metering equipment (AC) General requirements, tests and test conditions
IEC 62056 DLM/COSEM communication protocol for reading utility meters
IEC 62061 Safety of machinery: Functional safety of electrical, electronic and programmable electronic control systems
IEC 62076:2006, Industrial electroheating installations – Test methods for induction channel and induction crucible furnaces
IEC 62087 Methods of measurement for the power consumption of audio, video and related equipment
IEC 62097 Hydraulic machines, radial and axial – Performance conversion method from model to prototype
IEC 62107 Super Video Compact Disc
IEC 62108 Concentrator photovoltaic (CPV) modules and assemblies – Design qualification and type approval
IEC 62138 Nuclear Power Plants – Instrumentation and control important for safety – Software aspects for computer-based systems performing category B or C functions
IEC/TR 62157 Cylindrical machined carbon electrodes – Nominal dimensions
IEC 62196 Plugs and sockets for charging electric vehicles
IEC 62256 Hydraulic turbines, storage pumps and pump-turbines – Rehabilitation and performance improvement
IEC 62262 Degrees of protection provided by enclosures for electrical equipment against external mechanical impacts (IK code)
IEC 62264 Enterprise-control system integration
IEC 62265 Advanced Library Format (ALF) describing Integrated Circuit (IC) technology, cells and blocks
IEC 62270 Hydroelectric power plant automation – Guide for computer-based control
IEC 62271 Standards for high-voltage switchgear and controlgear
IEC 62278 Railway applications – Specification and demonstration of reliability, availability, maintainability and safety (RAMS)
IEC 62282 Fuel cell technologies
IEC 62301 Household electrical appliances – Measurement of standby power
IEC 62304 Medical Device Software – Software Life Cycle Processes
IEC 62305 Protection Against Lightning
IEC 62325 Standards related to energy market models & communications
IEC 62351 Power System Control and Associated Communications – Data and Communication Security
IEC 62353 Medical electrical equipment – Recurrent test and test after repair of medical electrical equipment
IEC/TR 62357 Power system control and associated communications – Reference architecture for object models, services and protocols
IEC 62365 Digital audio – Digital input-output interfacing – Transmission of digital audio over asynchronous transfer mode (ATM) networks
IEC 62366 Medical devices – Application of usability engineering to medical devices
IEC 62379 Common control interface for networked digital audio and video products
IEC 62386 Digital addressable lighting interface
IEC 62388 Maritime Navigation and Radio Communications, Shipborne Radar
IEC 62395 Electrical resistance trace heating systems for industrial and commercial applications
IEC 62443 Industrial communication networks – Network and system security (DRAFT)
IEC 62455 Internet protocol (IP) and transport stream (TS) based service access
IEC 62464 Magnetic resonance equipment for medical imaging
IEC 62471 Photobiological safety of lamps and lamp systems
IEC 62474 Material declaration for products of and for the electrotechnical industry
IEC 62481 Digital Living Network Alliance (DLNA) home networked device interoperability guidelines
IEC 62502 Analysis techniques for dependability – Event tree analysis (ETA)
IEC 62531 Property Specification Language (PSL)
IEC 62680 Universal Serial Bus (USB) interfaces for data and power
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Magnetic Flowmeters

Magnetic Flowmeters
A magnetic flowmeter is a very common type of velocity style flowmeter that can be found in use in most industries, including mining, pulp and paper and water treatment and electrical generations, amongst others.
Theory
Magnetic flowmeters or magmeters have been available since the mid 1950’s and are ideally suited for measuring liquids which are clean or contain particles (slurries).  Magmeters are sized from (1/10 inch to 96 inch and can measure flow from 0.01 gallons per minute to 500,000 gallons per minute.
Magmeters are obstruction less, contain no moving parts to wear out and provide practically no pressure drop.
The basic construction consists of a flow tube complete with a non-conductive, non-magnetic liner.  Coils to produce a magnetic field surround the flowtube.  Two electrodes protrude into the flowtube to pickup the very small induced voltage.  Electronic components are used to excite the coils and amplify the voltage developed at the electrodes.  The wetted parts (contact the process), which consists of the liner and electrodes are selected for compatibility with the process with respect to corrosion and/or contamination. (Figure 1)
Magnetic Flowmeters
A magnetic flowmeter is a very common type of velocity style flowmeter that can be found in use in most industries, including mining, pulp and paper and water treatment and electrical generations, amongst others.
Theory
Magnetic flowmeters or magmeters have been available since the mid 1950’s and are ideally suited for measuring liquids which are clean or contain particles (slurries).  Magmeters are sized from (1/10 inch to 96 inch and can measure flow from 0.01 gallons per minute to 500,000 gallons per minute.
Magmeters are obstruction less, contain no moving parts to wear out and provide practically no pressure drop.
The basic construction consists of a flow tube complete with a non-conductive, non-magnetic liner.  Coils to produce a magnetic field surround the flowtube.  Two electrodes protrude into the flowtube to pickup the very small induced voltage.  Electronic components are used to excite the coils and amplify the voltage developed at the electrodes.  The wetted parts (contact the process), which consists of the liner and electrodes are selected for compatibility with the process with respect to corrosion and/or contamination. (Figure 1)
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Figure 1: Mag-Flo Meter
The small voltage developed at the electrodes is proportional and linear with respect to flow through the pipe.  Flow measurement with a magnetic flowmeter is independent of temperature, pressure, density or viscosity of the liquid.  Accuracy over a wide range (typically 10 to 1) has evolved from 1% of full scale reading to 1% of rate as standard.
Applications of magnetic flowmeters vary from corrosive chemicals to sanitary requirements such as milk and other food products.
Faraday’s Law
The operation of a magnetic flowmeter is based on Faraday’s Law which states that a voltage developed in a conductor is proportional to the strength of the magnetic field (B),  the length of the conductor (L) and the velocity (V) of the conductor as it moves through the magnetic field.  This relationship can be shown in the following formula.
E = B x L x V
In the conventional magnetic flowmeter (figure 2) the magnetic field is created by a  set of coils mounted outside of the non-magnetic, non-conductive flow tube.  These coils are excited by either an AC voltage or a DC pulse depending upon the type of meter used (more detail provided later in this unit).   In the magnetic flow meter the actual fluid becomes the moving conductor (must be conductive) therefore the path length (L) is set by the distance between the pick-up electrodes.  These probes are mounted on opposite sides of the pipe and protrude through the pipe and liner making the distance between them almost equal to the flow tube diameter.  The velocity of the conductive fluid through the pipe becomes the ‘V’ term in Faraday’s equation.
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Since the magnetic field (B) and flow tube diameter (L) are known and the induced voltage (E) can be measured by electronic circuitry, the velocity (V) of the fluid can be calculated.  Magnetic flowmeters are consequently a type of velocity measuring flow meter.   Using the continuity equation and knowing the cross-sectional area of the flow tube,  this velocity can then be used to calculate the flow rate through the magnetic flowmeter.
Continuity Flow Equation:  Q = V x A
Where:
Q – volumetric flow rate
V – average velocity of liquid
A – cross-sectional area of the pipe
It should be noted that most manufacturers have established the maximum flow  velocity of 10 m/sec (32.8 ft/sec) and minimum velocity of 1 m/sec (3.28 ft/sec) for optimum meter operation.  Abrasive slurries velocities are limited to 2 m/sec (6.5 ft/sec) to effectively increase service life of the liner and electrodes.  Sludge and grease bearing liquids should be measured at velocities greater than 2 m/sec (6.5 ft/sec) in order to reduce coating of the liner and electrodes.
Parts of a Magnetic Flowmeter
The major components of a magnetic flowmeter are the:
Flow tube and liner
Magnetic coils
Electrodes
Electronic (transmitter) housing
Although the process fluid may not be considered an actual part of the magnetic flow meter it is still the first parameter to consider when designing a flow system using a magmeter.
The process fluid has to be conductive as has been shown from Faraday’s Law – just how conductive depends upon the requirements specified by the manufacturer.  The standard minimum is typically 1 micromhos/cm (mhos/cm) to 5 mhos/cm.
Note that the standard (imperial) unit of conductivity is the ‘mho’ which is just the word ohm written backwards.
Conductivity is thus defined as the ability to conduct current.
The SI unit of conductivity is microsiemens/cm (S/cm), which is numerically equal to mho/cm.
Typical conductivities are:
Sodium hydroxide 80000 S/cm
Sulphuric acid 107000 S/cm
Water 10 S/cm
Ethyl alcohol 0.44 S/cm
Fuel oil 0.000001 S/cm
As conductivity levels drop below 5 S/cm electrical noise becomes a problem.  As velocity increases the amount of noise tends to increase, thus low conductivity liquid applications should have magmeters sized to a maximum flow of about 3 ft/sec (0.914 m/sec).
Magnetic flowmeters require the process fluid to have a minimum conductivity with no maximum.   Since gases have a low conductivity and thus are not measured, magmeters are generally referred to as liquid flowmeters.
The magmeter flow tube has a liner to electrically isolate the electrodes from the meter body and pipe.   The liner can be made from a variety of materials, the most common are Teflon, polyurethane, and rubber.  Since the liner is in contract with the process fluid (a wetted part) it must be selected to not deteriorate in the given application.  Polyurethane and rubber are mainly used in abrasive services whereas Teflon is used in sanitary, highly corrosive, and high temperature applications.
The coils of the magmeter create the magnetic field.  The two general types of coils are the AC excited and the DC excited which will be discussed later in this unit.
The electrodes are physically mounted  in the flow tube such that they are at right angles to both the magnetic field and the axis of the pipe.  Like the liner, the electrodes are also a wetted part and therefore must be made of a material that is compatible with the process.  Some common materials are stainless steel, Hastelloy, Monel.
One of the common problems associated with a magmeter is electrode coating since the electrodes are in contact with the process.   An insulating type coating on the electrodes can create a significant shift in the magmeter zero and span.   Several different methods have and are being used to prevent or reduce electrode coating.   The electrodes can be manually brushed,  however the pipeline has to be shutdown during this procedure.   A momentary  AC voltage can be applied to the electrodes to ‘burn-off’ the insulating coating however there is a loss of flow signal during this time.  Electrodes can be rounded and designed to  protrude slightly into the process to create a turbulence which scrubs the tip and thus are classified as a self-cleaning electrodes.  Some electrodes can be removed from the magmeter for cleaning and some of these without stopping the flow.  These electrodes must be carefully re-installed as the distance between the probes is one of the determining factors in determine the quantity of  induced voltage.   Ultrasonic generators can also be to clean the electrodes without interrupting the process.  The more modern approached to electrode insulating coating is to ignore it through the use of a pulsed DC excitation systems which will be explained late.
The transmitter component of the magmeter converts the small voltage induced at the electrodes to a useable output signal.   Traditionally the output of the transmitter is an analog  4 -20 mA signal although more modern magmeter transmitters are microprocessor based and have a digital, frequency output.   Microprocessor based transmitters can automatically measure bi-directional flow;  can be configured for totalizing or scaling,  can display different units of flow, alarm points can be set,  bi-directional communication established, and many more features.
Magnetic Field Excitation
There are two common methods used to create the magnetic field in the magmeter
AC excitation
Pulsed DC excitation
AC excitation was the first to be used however the advantages of the pulsed DC type have resulted in its more predominate use.
An AC voltage with its sinusoidal nature is always changing.  When an AC voltage excites the coils of a magmeter two signals are induced at the electrodes – one created by the velocity of the fluid  (Faraday’s Law), the other by the changing magnetic field itself.  The second signal is called noise and must be reduced or eliminated for an accurate flow measurement.
The amount of noise can be determined by stopping the liquid flow.  Once stopped, the only signal the transmitter would see is the noise due to the changing magnetic field.   This ‘noise’ signal could then be zeroed-out in the transmitter.
The non-practicality of periodically stopping the flow to re-zero the meter,  the large size of the AC coils compared to DC coils,  and the electrode coating problems of AC magmeters have lead to the decline of the AC magmeter.
It should be noted however that an advantage of the AC excited meter is its fast speed of response as the coils are continuously energized although with advances in pulsed DC technology even this may not be an advantage for long.
A pulsed DC excited magmeter purposely has the coils de-energized for a period of time and energized for a period of time.   During the ‘on-time’ the voltage supplied to the coils is constant and therefore the voltage signal induced at the electrodes and seen by the transmitter is due only to the fluid velocity.  There is no noise due to changing excitation current.
It should be noted that it is also possible for noise to be generated from sources external to the magmeter.  These may be due to other magnetic fields from inductive loads (motors) or voltage build-up at the electrodes dues to stray electrical current on the pipe or even electrochemical reactions within the process liquid itself.  These could create a voltage noise at the electrodes that is not a result of the flow rate.  These external noise signals can be automatically (electronically in the transmitter) zeroed-out in a pulsed DC magmeter once every cycle when the coils are de-energized or turned off (figure 3).
This is not possible in the AC excited magmeter because the coils are never really turned off.
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The advantages of a pulsed DC magmeter include lower power consumption and automatic re-zeroing due to its ‘turn-off’ time.  The coils of an DC meter are smaller and therefore have less weight than an comparable AC magmeter.   A limitation of a pulsed DC meter is its speed of response again due to its off time.  During this off-time there is no flow reading and for very fast changing flow conditions this may result in a less accurate average flow rate reading.  The more modern pulsed DC magmeters have an adjustable on/off time that allows flexibility for a given flow situation and thereby reduces flow errors.
Installation
The importance of installation of a magmeter cannot be overemphasized.   The direction of flow, the orientation of the meter for given process, bolting, and grounding has a significant impact on the operation of the flowmeter.
A magmeter can measure flow in either direction – the only difference is in the polarity of the voltage induced at the electrodes.    Unless the transmitter is designed to detect this change, reverse flow will give the meter a ‘zero flow’ indication.  For this reason magmeters have a flow direction stamped on the housing.
Magnetic flowmeters are designed to measure liquids in full pipes thus the preferred meter orientation is vertical with the flow upward through the meter.  Sloping upward or horizontal piping is acceptable however piping must be designed to ensure the flow tube is always full (see figure 4)
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The minimum straight run pipe lengths for a typical magmeter is significantly less than for orifice plates – one resource states 20D upstream and 5D downstream for an orifice plate and 10D upstream and 3D downstream for a magmeter.   The manufacturer’s information should be consulted for actual values for a given application.
Grounding and Ground Rings
Proper process grounding is one of the most important installation details, as it ensures that the flowtube and process fluid are at the same potential and only the induced flow signal is therefore measured.
This is necessary to provide a stable reference for the measuring electronics, which is usually an ungrounded differential amplifier. In addition, stray electrical currents can be induced in metal pipelines by capacitive or inductive coupling from other electrical sources.  These electrical currents can create voltage zero shifts at the magmeter electrodes.  Proper bonding of the magmeter to either side of the pipeline connection allows these stray currents to pass by the meter rather than go through it.
By connecting the magmeter flowtube, the fluid, and the reference for the amplifier to a stable, noise free reference point, the user is ensured of getting the best performance from their magnetic flowmeter
Where necessary magnetic flow meters are provided with 2 grounding rings of a conductive material recommended for the intended service. These grounding rings although designed primarily to provide proper grounding on installations with insulated piping, also serve to protect and shield the flowtube liner edge from process abrasion.
Most manufacturers include grounding ring kits if installing on insulated or plastic pipe and a detailed explanation of how to properly ground their magmeter (see figure 5). However inexperienced construction personnel have been known to “forget” to install these components, so be aware.
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One of the most common faults of initial magmeter failure is improper tightening of the flange bolts.   The liner of the magmeter can be easily damaged beyond repair by over-torquing the flange bolts.   Manufacturer’s literature should be consulted for the proper torque requirements for their particular meter.
Application
Magnetic flowmeters measure the volumetric flow rate of conductive liquids.  Since they are velocity flowmeters their signal output is linear and directly proportional to the flow rate.
Meters range in size from small (0.1 inches, 2 mm) to large (96 inches, 243 cm) and with flow rates from 0.01 USGPM (37 mL/min) to 500, 000 USGPM (1.89 ML/min).   Generally, magmeters have a relatively wide rangeability (10:1) and inaccuracies of 1% of full scale to 1% of rate.
Magmeters are virtually obstruction less and therefore have no pressure drop.  They are widely used in the pulp and paper industry and in mining applications.  They are not used in the oil industry as hydrocarbons are not conductive.
Advantages and Limitations
Although the features of the magmeter have been discussed throughout this document they are outlined here.
Advantages
No moving parts
Obstruction less, no pressure drop
Low power requirements, especially for pulsed DC excited meters
Wide application for corrosive and slurry process because of liner
Can measure low and high flow rates, good rangeability
Can measure bi-directional flow
Relatively high accuracy
Limitations
Can only measure conductive liquids, no hydrocarbons or gases
Relatively heavy, especially AC excited meters
Caution must be used in installing the meter (grounding, torquing, full pipe)
Relatively expensive
Electrodes can become coated
Velocity measurement is limited to a maximum of 30 ft/sec (10m/sec) in practice.
Calibration
A magnetic flowmeter system has two major components – the magmeter body that is installed in the pipeline and the transmitter, which may be mounted remotely from the body.
Each of these has its own calibration techniques and can be done independently however better accuracy is achieved if they are done together as a system.
Before a flowmeter leaves the factory it is calibrated with its own unique K Factor.
Passing a volume of liquid through the meter performs this true hydraulic calibration.  This specific volume is measured using a weigh tank or a master meter such as a turbine meter.  This accurate volume is then compared to the frequency count (number of pulses).  From several tests the number of pulses per unit volume (K Factor) is determined.  Each magnetic flowmeter is then stamped with its own unique K Factor.  If these calibrations are performed in a certified facility then a calibration certificate traceable to NIST (National Institute of Standards and Technology) can be obtained.
As ‘real’ flows are difficult to attain in the field, only the transmitter part of the magmeter is field calibrated.  A field calibrator (usually manufacturer specific) is used to simulate the input electrical output from the electrodes and to test and troubleshoot the electronic circuitry of the transmitter.
Conclusion
With the many advantages of a magnetic flowmeter, it is a widely used flow meter in many industries other than petroleum.  This unit described the basic parts of a magmeter:  the flow tube and liner, the electrodes, the coils, and the transmitter.  The basic principle of operation of the magmeter shows an application of Faraday’s Law.   Some of the operational considerations of a magmeter were mentioned such as the coating of the electrodes and ways to reduce it.   The importance of proper installation practices was emphasized.  The methods of generating the magnetic field through AC and pulsed DC exciting were described.
Data relating to possible applications of the magmeter along with its advantages and limitations were outlined.  A brief explanation of the calibration involved with magmeters concluded the unit.