Instrumentation and Control Symbols

Here we look at common instrument symbols used in various types of technical diagrams that are used to document instrument systems.

Instrument Bubbles

Line Types

Note that, the single backlash signifying discrete or binary type has been removed from ISA standard. Regular Pneumatic and Electrical Line Symbols may represent either continuous or discrete states. 

Process/Instrument Line Connections

Process Valve Types

Valve Actuator Types

Valve Failure Mode

Liquid Level Measurement Devices

Flow Measurement Devices  (Flowing from left to right)

Process Equipment Symbols

Functional Diagrams Symbols

Single-line electrical diagram

Fluid Power Diagram Symbols

You can also read: 

• Basics of Instrumentation and Control Systems
• Process Flow Diagrams

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Functional Diagrams

Functional Diagrams form part of diagrams used In Instrumentation. In our previous articles, we looked at Process Flow Diagrams, Process & Instrument Diagrams as well as Loop Diagrams.

In this article, we look at a unique form of technical diagram for describing functions comprising a control system (e.g. PID Controllers, Rate Limiters, Manual Loaders). The diagrams used to document control strategies are termed as functional diagrams. Note that, functional diagrams focus on the flow of information within a control system rather than on the process piping or instrument interconnections i.e. wires, tubes etc. The general flow of a functional diagram is top-to-bottom, with the process sensing instrument (transmitter) located at the top and the final control element (valve or variable-speed motor) located at the bottom.

Functional  Diagrams are all about the algorithms used to control decisions, so no attempt is made to have the symbols arranged to  correspond with actual equipment layout. 

Let's consider a functional diagram shown below:

The above functional diagram shows a flow transmitter (FT) sending a process variable signal to a PID Controller, which then sends a manipulated variable to a flow control valve (FCV).

A cascaded control system, where the output of one controller acts as the set-point for another controller to follow, appears in functional diagram as shown below:

In the above cascaded control system, the primary controller senses the level in a vessel, commanding the secondary (flow) controller to maintain the necessary amount of flow either in or out of the vessel as needed to maintain level at some point.

You can also read: How Loop Diagrams are used In Industrial Control

Functional diagrams may show varying degrees of detail about the control strategies they document e.g. you may see the auto/manual controls represented as separate entities in a functional diagram, apart from the basic PID controller function. In the following Functional Diagram, a transfer block (T) and two manual adjustment blocks (A) providing a human operator with the ability to separately adjust the controller’s set point and output (manipulated variables) and to transfer between automatic and manual modes:

Rectangular blocks such as the Δ, P, I and D shown in the diagram below represent automatic functions.  Diamond-shaped blocks such as A and T blocks represent manual functions which must be set by a human operator. The Functional diagram also shows the presence of set point tracking in the controller algorithm, a feature that forces the set point value to equal the process variable value any time the controller is in manual mode.

A solid line in a functional diagram represent analog (continuously variable) signals such as process variable, set point, and manipulated variable. Dashed lines represent discrete (on/off) signal paths, in this case the auto/manual state of the controller commanding the PID algorithms to get its set point either from the operator’s input (A) or from the process variable input (the flow transmitter: FT).

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Loop Diagrams

Having looked at the PFD and P & ID Diagrams in our previous posts, we can now discuss the loop diagrams also called the loop sheets. Let's consider the diagram of a compressor surge system shown below:

From the loop diagram above we can see more additional instruments that weren’t shown in the PFD and  P & ID: We have 2 transmitters, a controller, and a valve. We also have 2 signal transducers. The first transducer  42a modifies the flow transmitter signal before it goes into the controller. and the second transducer 42b converts the electronic 4-20mA signal into a pneumatic 3-15 PSI air pressure signal. Each instrument  ''bubble''  in a loop diagram represents an individual device, with its own terminals for connecting wires.

The dashed lines represent individual copper wires instead of whole cables. Terminal blocks where these wires connect to are represented by squares with numbers in them. Cable numbers, wire colors, junction block numbers, panel identification and even grounding points are all shown in the loop diagrams.  You can also notice from this loop diagram, the action on each instrument. You will see a box and arrow (pointing either up or down) next to each instrument bubble. An ” up” arrow (↑) represents a direct-acting instrument: one whose output signal increases as the input stimulus increases. A down arrow (↓) represents a reverse-acting instrument, one whose output signal decreases as the input stimulus increases.

All the instruments in this loop are direct-acting with the exception of the pressure differential transmitter PDT-42. Here the down arrow tell us that the transmitter will output a full-range signal (20 mA) when it senses zero differential pressure, and a 0% signal (4 mA) when it sensing a full 200 PSI differential. Excessive pressure drop across the compressor is considered dangerous because it may lead to the compressor surging. For this reason, the controller will naturally take action to prevent surge by commanding the anti-surge control valve to open, because it, “thinks” the compressor is about to surge i.e. the transmitter is intentionally calibrated to be reverse-acting such that any break in the signal wiring will naturally bring the system to its safest condition.

You can also read: 

• Process and Instrument Diagrams
• Basics of Instrumentation and Control Systems

The only diagram that can be more detailed than a loop diagram is the electronic schematic diagram for an individual instrument but then it shows details for that particular instrument alone, thus the loop diagram is the most detailed form of diagram for any control system as a whole and it must contain details omitted by PFDs and P & IDs.            

Process and Instrument Diagrams (P & IDs)

In the previous post, we looked at Process Flow Diagrams (PFDs),  where we indicated that a PFD represents a big picture of the entire process. In this post, we look at the Process and Instrument Diagrams (P & IDs), where we will try to get more details that weren’t shown in the PFD. Let's consider the compressor control system diagram below:

From the above, we can see that there is more instrumentation associated with the compressor than just a flow transmitter. We have the differential pressure transmitter (PDT), a flow indicating controller (FIC), and a recycle control valve (FV42), that allows some of the vapor coming out of the compressor discharge line to go back around the compressor suction line. Also, we have a pair of temperature transmitters (TT41 & TT43) reporting suction and discharge line temperatures to an indicating recorder.

Additional details emerge in the P & ID above, the flow transmitter, flow controller, pressure transmitter and flow valve all bear a common number 42. This common, ”loop number” indicates these four instruments are all part of the same control system. An instrument with any other loop number is part of a different control system, measuring and/or controlling some other function in the process like the two temperature transmitters and their respective recorders, bearing the loop numbers 41 and 43.

You can also read: Process Flow Diagrams

The other information we can derive from the P & ID above, are the different instrument ” bubbles” used. Some of the bubbles are just open circles, while others have lines going through the middle as shown below:

Each of these symbols have meaning according to the ISA ( Instrumentation, Systems and Automation Society).

The type of “bubble” used for each instrument tells us something about its location. The rectangular box enclosing box enclosing the temperature recorders (TIR 41 and TIR 43) shows they are part of the same physical instrument i.e. this indicates that there is really only one temperature recorder instrument, and that it plots both suction and discharge temperatures (most likely on the same trend graph). This suggests that each bubble may not necessarily represent a discrete, physical instrument, but rather an instrument function that may reside in a multi-functional device.

The P & ID shows more details than PFD, but we cannot see other details like the cable types, wire numbers, terminal blocks, junction boxes, instrument calibration ranges, failure modes, power sources etc. To examine this level of details, we need to look at the loop diagram.

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Process Flow Diagrams

Instrumentation has its own standardized way of making descriptive diagrams. In this article, we are going to look at the types of diagrams commonly used in industrial instrumentation like the Process Flow Diagrams (PFDs). Essentially we have the following types of instrumentation diagrams:

• Process Flow Diagrams (PFDs)
• Process and Instrument Diagrams (P & IDs)
• Loop Diagrams (Loop Sheets)
• Functional Diagrams

At the highest level, an instrument technician is interested in the interconnections of process vessels, pipes and flow paths of process fluids therefore he/she would be more likely go for the Process Flow Diagram (PFD), that represent the big picture of the entire process.

At the lowest level, the instrument technician will be more interested in the interconnections of individual instruments including all the wire numbers, terminal numbers, cable types, instrument calibration ranges etc. The proper form of diagram for this level of fine detail is a loop diagram.

Process and Instrument diagrams (P & IDs) lie somewhere in the middle between process diagrams and loop diagrams. A P & ID shows the layout of all relevant process vessels, pipes and machinery, but with instruments superimposed on the diagram showing what gets measured and what gets controlled. You are able to view the flow of the process as well as the flow of information between instruments measuring and controlling the process.

Functional Diagrams are used to document the strategy of a control system. In a functional diagram, emphasis is placed on the algorithms used to control a process, as opposed to piping, wiring, or instrument connections.

An instrument technician has the responsibility of reading the different diagrams when troubleshooting a complex control system. First you begin with a PFD or P&ID to get an overview of the process to see how the major components interact. After identifying which instrument or loop you need to investigate, you go to the appropriate loop diagram to see the interconnection details of that instrument system so that you know where to connect your test equipment and what signals you expect to find when you do so.

You can also read: 

• Introduction to Industrial Automation
• The Basics of Industrial Instrumentation Systems

Process Flow Diagrams

To help understand better process flow diagrams, we are going to examine the diagrams of a compressor control system. In this process, we assume that water is being evaporated from a process solution under partial vacuum that is being provided by the compressor. The compressor then transports the vapors to a knockout drum where some of them condense into liquid form. As a typical PFD, this diagram shows the major interconnections of process vessels and equipment. But it omits details such as instrument signal lines and auxiliary instruments.

From the diagram above you might find it hard to determine which control system if any, controls the compressor. All that the PFD shows relating directly to the compressor is a flow transmitter (FT) on the suction line. This level of uncertainty is acceptable for a PFD, because its purpose is merely to show the general flow of the process itself and very little details on Control Instrumentation.

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Advantages and Disadvantages of Pneumatic Instruments

Although we commonly use current to relay information, some critical process measurements use compressed air  to transmit information from one point to another, an example of this can be a petroleum refinery.  Pneumatic instruments find use in some applications that won’t work well with say 4-20 mA current signals due to safety concerns. Pneumatic Instruments still find wide application in industry, although it is increasingly rare to encounter completely pneumatic control loops. 

One of the most common applications for pneumatic control system components is control valve actuation. Not only is compressed air used to create the actuation force in many control valve mechanisms, it is still often the signal medium employed to command the valve’s position. In most cases this pneumatic signal originates from a device called an I/P transducer or current-to-pressure converter, taking a 4-20 mA control signal from the output of an electronic controller and translating that information as a pneumatic 3-15 PSI signal to the control valve positioner or the actuator.

Below is an example of Pressure Transmitter being applied in Pneumatic instrumentation:  

Let's now look at the advantages and disadvantages of Pneumatic Instruments.

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Disadvantages of Pneumatic Instruments Include:

• Sensitivity to vibration, changes in temperature and mounting position which may affect the calibration accuracy to a far greater degree than electronic instruments.
• Compressed air is an expensive utility which is much more expensive per equivalent watt-hour than electricity which makes the operational cost of pneumatic instruments far greater than electronic. The installed cost of pneumatic instruments can be quite high as well given the need for special material i.e. Stainless steel, Copper, or Tough plastic tubes to carry air and pneumatic signals to distant locations.
• The volume of air tubes used to convey pneumatic signals over distances act as a low-pass filter, naturally damping the instrument’s response and thereby reducing its ability to respond quickly to changing process conditions.

So with the above disadvantages, why are the pneumatic instruments still in use today?

The main reason may be due to legacy hence facilities using these pneumatic instruments  and have them in good work conditions, won’t see the need to replace them since in most cases the cost of labor to remove old tubing, install new conduit and configure new (expensive) electronic instruments is often not worth the benefits.

You can also read: Pneumatic Signal Transmission

Advantages of pneumatic Instruments include:

• Intrinsic Safety of pneumatic field instruments. Instruments that do not run on electricity cannot generate electrical sparks. This is of utmost importance in classified industrial environments where explosive gases, liquids, dusts and powders exist.
• Pneumatic instruments are also self-purging. The continual bleeding of compressed air from vent ports in pneumatic relays and nozzles acts as a natural clean-air purge for the inside of the instrument, preventing the intrusion of dust and vapor from the outside with a slight positive pressure inside of the instrument case. Pneumatic instruments mounted inside larger enclosures with other devices tend to protect them all by providing a positive-pressure air purge for the entire enclosure.
•   Some pneumatic instruments can also function in high-temperature and high-radiation environments that would damage electronic instruments.
•  Pneumatic instruments can also operate on compressed gases besides air. This is an advantage in remote natural gas installations, where the natural gas itself is sometimes uses as a source of pneumatic ”power” for instruments. So long as there is compressed natural gas in the pipeline to measure and to control, the instruments will operate. No air compressor or electrical power source is needed in these installations. All you need is a good filtering equipment to prevent contaminants in the natural gas (Dirt, liquids, Debris) from causing problems within the sensitive instrument mechanisms.

Variable Area Flowmeters

Variable area flowmeters typically feature a vertically positioned measuring cone through which the medium flows from bottom to top, lifting against the weight. We also have the horizontal and inverted (top to bottom) versions that are used where the installation structure doesn't allow for vertical version. 

We have two main types of variable area flowmeters:

• Float type (Rotameter)
• Tapered plug type

Float Type

The float is inside a tapered tube as shown below:

The fluid flows through the annular gap around the edge of the float. The friction causes a pressure drop over the float and the pressure forces the float upwards. Because the tube is tapered, the restriction is decreased as the float moves up. Eventually a level is reached where the restriction is just right to produce a pressure force that counteracts the weight of the float. The level of the float indicates the flow rate. If the flow changes, the float moves up or down to find a new balance position.

In case dangerous fluids are used, protection is needed against the tube fracturing. The tube may be made of non-magnetic metal. The float has a magnet on it, as it moves up and down; the magnet moves a follower and pointer on the outside. The position of the float may be measured electrically by building a movement transducer into the float.

You can also read: 

• The Ultimate Guide to Industrial Flow Instruments
• How orifice plates are used in Flow Measurement

Tapered Plug Type

This type has a tapered plug aligned inside a hole or orifice.

A spring holds it in place. The flow is restricted as it passes through the gap and a force is produced which moves the plug. Because it is tapered, the restriction changes and the plug takes a position where the pressure force just balances the spring force. The movement of the plug is transmitted with a magnet to an indicator on the outside.

Industrial Applications of Variable Area Flowmeters

Variable flowmeters are used in various industrial flow measurement applications, e.g.

• Continuous gas and liquid measurement
• Measurement of non-conductive media
• Compressor monitoring
• Industrial Burner controlling
• Dry-run protection of pumps

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The working Principle of Thermocouples

When two wires with dissimilar electrical properties are joined at both ends and one junction is made hot and the other cold, a small current is produced proportional to the difference in the temperature.

For example, we have the cold end joined at a sensor millivolt meter, and the hot junction forming the sensor end as shown below:

Peltier showed that the heat is absorbed at the hot end and rejected at the cold end. Thompson showed that part of the e.m.f. is due to temperature gradient in the wire as well as the temperature difference between the junctions.  Most of thermocouple metals produce a relationship between the two temperatures and the e.m.f. as follows:

The α and β are constants for the type of thermocouple. The relationship is nearly linear over the operating range. The actual characteristics and suitable operating temperatures depend upon the metals used in the wires. The various types are designated in international and national standards. Typical linear operating ranges are shown for standard types. Note, it is important for thermocouples to be standard so that the e.m.f. will always represent the same temperature.

Thermocouples come in several forms, they may be wires insulated from each other with plastic or glass fibre materials. For high temperature work, the wire pairs are put inside a tube with mineral insulation. For industrial uses, the sensor comes in a metal enclosure such as a stainless steel. 

You can also read: Temperature transducers as applied in Industrial Instrumentation

Example of typical thermocouple industrial probes is shown below:

Instrument Errors and Calibration

In any measurement system, the instruments in use are prone to errors due to aging or manufacturing tolerances. These instrument errors must be stated before taking any measurement. Some of the common terms used when describing the performance of an instrument are:

• Accuracy – The accuracy of an instrument is often stated as % of the range or full-scale deflection. e.g. A Pressure gauge with a range 0 to 500 kpa and an accuracy of plus or minus 2% full-scale deflection, could have an error of plus or minus 10 kpa. When the gauge is indicating 10 kpa the correct reading could be anywhere between 0 and 20 kpa and the actual error in the reading could be 100 %. When the gauge indicates 500 kpa the error could be 2 % of the indicated reading.
• Range – The range of an instrument is usually regarded as the difference between the maximum and minimum reading, e.g.  a thermometer that has a scale from 20 to 100 ° C has a range of 80 °C. This is also called the Full scale deflection (f.s.d.).
• Repeatability – If an accurate signal is applied and removed repeatedly to the system and it is found that the indicated reading is different each time, the instrument has poor repeatability. This is often caused by friction or some other erratic faulty in the system.
• Stability – Instability will likely occur in instruments involving electronic processing with a high degree of amplification. Causes of this could be environmental factors such as temperature and vibration, e.g. a rise in temperature may cause a transistor to increase the flow in current which in turn makes it hotter and so the effect grows and the displayed reading DRIFTS. In extreme cases, the displayed value may jump about caused by for example a poor electrical connection affected by vibration.
• Time lag error – This occurs when an instrument takes time for a change in the input to show up on the indicated output. This time may be very small or very large depending upon the system. If the indicated output is incorrect because it has not yet responded to the change, then we have time lag error. When a signal changes a lot and quite quickly e.g. a speedometer, the person reading the dial would have a great difficulty determining the correct value as the dial may be still going up when in reality the signal is going down again.
• Drift – This occurs when the input to the system is constant but the output tends to change slowly. For example when switched on, the system may drift due to the temperature change as it warms up.
• Reliability – An instrument in most cases will have a predicted life span. The more reliable it is, the less chance it has of going wrong during its expected life span. The reliability is hence a probability ranging from zero ( it will definitely fail) to 1.0 ( it will definitely not tail).

INSTRUMENT CALIBRATION

Most instruments have a built-in mechanism for making adjustments. These are:

• The range adjustment
• The zero adjustment

In order to Calibrate  an instrument an accurate gauge is required. This is likely to be a Secondary standard. Instruments calibrated as a secondary standard have themselves been calibrated against a primary standard.

Procedure

An input representing the minimum gauge setting should be applied. The output should be adjusted to be correct. Next the maximum signal is applied. The range is then adjusted to give the required output. This is repeated until the gauge is correct at the minimum and the maximum values.

You can also read: 

• The Basics of Instrumentation and Control Systems
• Basic Industrial Instrumentation Terms

Calibration Errors

Range and zero Error – After obtaining correct zero and range for the instrument, a calibration graph should be produced. This involves plotting the indicated reading against the correct reading from the standard gauge. This should be done in about 10 steps with increasing signals and then with reducing signals. Several forms of error could show up. If zero or range is still incorrect the error will appear as shown figure 1 and 2 below:

Figure 1

Figure 2

Hysteresis and Nonlinear Errors – Hysteresis is produced when the displayed values are too small for increasing signals and too large for decreasing signals. This is commonly caused in mechanical instruments by loose gears, linkages and friction. It occurs widely with equipment involving magnetization and demagnetization.  The calibration may be correct at the maximum and minimum values of the range but the graph joining them may not be a straight line ( when it ought to be). This is a nonlinear error. The instrument may have some adjustments for this and it may be possible to make it correct at mid range as shown in figure 3 and 4. 

Figure 3

Figure 4

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Instrumentation Sensors and Transducers

A basic instrumentation system consists of 3 elements:

• Sensor or Input Device
• Signal Processor
• Receiver or output device

A block diagram of a basic instrumentation system is shown below:

Most modern analogue instruments work on the following standard signal ranges:

• Electric 4-20 mA 
• Pneumatic 0.2 to 1.0 bar which is equivalent to 3 -15 PSI

The old electrical equipment use 0 to 10 v. Pneumatic signals are commonly used in process industries for safety especially when there is a risk of fire or explosion.

The advantage of having a standard range or using digital signals is that all equipment may be purchased ready calibrated. For analogue systems the minimum signal (Temperature, Speed, Force, Pressure etc.) is represented by 4 mA or 0.2 bar or 3 PSI. and the maximum signal is represented by 20 mA or 1.0 bar or 15 PSI.

The physical quantities commonly measured include:

• Flow rate
• Temperature
• Pressure
• Level
• Mass or Weight
• Density
• Speed
• Strain
• Movement, Velocity and Acceleration
• Acidity/Alkalinity

Sensors may operate simple on or off switches to detect the following:

• Objects (proximity switch)
• Hot or Cold (Thermostat)
• Empty or full (level switch)
• Pressure high or low (Pressure switch)

The block diagram of a sensor as shown below:

You can also read: Introduction to Industrial Instrumentation

We have different types of sensors used in instrumentation:

• Temperature transducers e.g. Thermocouple, Resistance Temperature detectors etc. 
• Pressure transducers e.g. Bourdon Tube
• Speed transducers e.g. Tachometer, Magnetic pickup & optic types
• Flowmeters e.g. Positive Displacement meters, Differential Pressure Flowmeters, Turbine etc. 
• Force sensors: mechanical, hydraulic, electric strain gauge
• Position sensors: Resistive, Optical, Inductive
• Depth gauges: Ultrasonic, Pressure Gauge, Electronic level gauge
• Strain gauges

From the above, we learn that any instrumentation system whether basic or complex, has an input/sensor, a signal processing unit and lastly an output or receiver.

Introduction to Industrial Automation

Automation is a technique that can be used to reduce costs or improve the quality of products being manufactured. Automation can increase manufacturing speed while at the same time reduce the cost. Automation can also produce products with the same consistent good quality.

Note that Automation is not a straightforward solution to a financial problem however it is a valuable tool that can be used to improve product quality. With improved product quality, you will have lower costs. Producing inexpensive, high quality products is a good policy for any company.

Automation Control/Process Control

Automated processes can be controlled by humans operators, by computers or by a combination of the two. If a human operator is available to monitor and control a manufacturing process, then open loop control system may be acceptable. If a manufacturing process is automated, then it requires a closed loop control.

Fig 1

Fig 2

Fig 1 and 2 above shows examples of open loop and closed loop control. One major difference is the presence of the sensor in the closed loop control system. The motor speed controller uses the feedback it receives from this sensor to verify that the speed is correct, and drives the actuator harder or softer until the correct speed is achieved. In the open loop control system, the operator uses his/her built-in sensors (eyes, ears etc.) and adjusts the actuator (via dials, switches etc.) until the output is correct. Since the operator provides the sensors and the intelligent control functions, these elements do not need to be built into an open loop manufacturing system.

Human operators are more inconsistent than properly programmed computers. Computerized controls, however can also make mistakes, when programmed to do so. Programming a computer to control a complex process is very difficulty. The recent development of affordable digital computers has made automation control possible.

Process control usually implies that the product is produced in a continuous stream. Automation control usually implies a sequence of mechanical steps. A camshaft is an automation controller because it mechanically sequences the steps in the operation of an internal combustion engine. Manufacturing processes are often sequenced by special digital computers known as programmable logic controllers (PLCs), which can detect and can switch electrical signals on and off. Digital computers are ideally suited for automation control type tasks because they consists of circuits each of which can only be either on or off.

Process control is now accomplished using digital computers. Digital controllers may be built into cases with dials and displays making them look like analogue ones. PLCs can also be programmed to operate as analog process controllers. They offer features which allow them to measure and change analog values. Robots and NC equipment use digital computers and a mixture of analog and digital circuit components to control “continuous” variables such as position and speed.
Digital computer control gives us soft automation meaning that they can be reprogrammed easily. Digital computers are also cheap, powerful, fast and compact. They offer many advantages to the automation user. A single digital controller can control several manufacturing processes. The designer only needs to ensure the computer can monitor and control all processes quickly enough and has some excess capacity for future changes.

You can also read: Introduction to Industrial Instrumentation

Soft automation systems can be programmed to detect and to adapt to changes in the work environment or to changes in demand. For example an NC lathe can modify its own speed if it detects a sudden change in the hardness of a raw material being cut. It may also change its own programming in response to a signal from another automated machine requesting a modification in a machined dimension.

Bottom line
Industrial automation is continuously improving, the developments in computers especially with regard to speed has led to more efficient automation systems. As we move into the future, we will see more and better automation systems that have greater performances than current ones.

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