Instrumentation & Controls Glossary

• See Articles  -  List of Tags, List of Categories, List of Articles, List of Glossaries, Nomenclature and Symbols

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• Air Receiver Sizing  -  The process of determining the appropriate size and capacity of an air receiver tank in a compressed air system.  An air receiver, also called air storage tank or compressed air tank, is a vessel used to store compressed air and maintain a consistent supply to meet the varying demand of a compressed air system.  The sizing of an air receiver is needed for the efficient and reliable operation of a compressed air system.
• Amplifier  -  A device which draws power from a source other than the input signal and which produces as an output an enlarged reproduction of the essential features of its input.
• Analog  -  An electronic analog signal is a continuous electrical voltage or current waveform that varies smoothly and continuously over time.  In electronic systems, analog signals are used to represent real world quantities, such as sound, temperature, light, or other physical phenomena.  These signals are characterized by an infinite range of possible values within a given range.
• Analog Output  -  Designates a signal coming from a PLC into an instrument or controller.  Depending on the particular instrument, an analog output signal comes in the form of voltage or amperage.  An analog output signal might be used to control the position of a valve or speed of a fan.
• Auto-Zero  -  An automatic internal correction for offsets and/or drift at zero voltage input.
• Automatic Well Test  -  Specified as on instrument bubbles on Piping and Instrumentation Diagram.  It is a process in the oil and gas industry where the performance and productivity of an oil or gas well are evaluated through a series of tests conducted automatically or with minimal human intervention.  These tests are used for assessing the reservoir's characteristics, understanding well behavior, and optimizing production.

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• Background Noise  -  Interfering signals that can cause disturbance affecting a signal that may distort the intended signal.
• Backplane  -  A group of electrical connectors in parallel with each other, so that each pin of each connector is linked to the same relative pin of all the other connectors, forming a computer bus.  It is used as a backbone to connect several printed circuit boards together to make up a complete computer system.
• Bandwidth  -  A symmetrical region around the set point in which proportional control occurs.
• Batch process  -  Any process on which operations are carried out on a limited number of articles, as opposed to continuous process.
• Baud  -  A unit of data transmission speed equal to the number of bits (or signal events) per second; 300 baud = 300 bits per second.  It is the unit of symbol rate, also known as baud rate or modulation rate; the number of distinct symbol changes.  A baud rate, by definition, means the number of times a signal in a communications channel changes state or varies.
• Bipolar  -  The ability of a panel meter to display both positive and negative readings.
• Bus  -  The group of conductors that interconnect individual circuitry in a computer.  Typically, a bus is the expansion vehicle to which I/O or other devices are connected. Examples of PC buses are the ISA and PCI buses

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• Cache  -  High-speed processor memory that buffers commonly used instructions or data to increase processing throughput.
• Capacitance  -  The ability of a system to store electrical charge.  The capacitance of a system depends on its physical characteristics and the arrangement of its components.  In other terms, capacitance quantifies how much charge a capacitor can store per unit of voltage. 
• Car Seal  -  Specified on instrument bubbles on Piping and Instrumentation Diagram.  A car seal is a type of tamper evident seal used in industrial settings to secure various types of equipment, including valves, meters, or other instruments.  These seals are commonly used in industries such as oil and gas, chemical, and manufacturing to ensure the integrity and security of critical processes and equipment.
• Carrier Frequency  -  The transmission of a fixed frequency that has been changed or modified to carry data or information.  The frequency is usually measured in Hertz (cycles per second).
• Center Wavelength  -  The arithmetic center of the passband of a bandpass filter.  It is not necessarily the same as the peak wavelength.
• Channel List  -  A variable length list of channels and their associated gains specifying which analog input channels to convert to digital values.  In continuous A/D acquisition mode, the list wraps around to the first channel after it reaches the end.  The channels need not be in any particular order.
  
• Circular Buffer  -  When reading data from the buffer, always all data available in the buffer are fetched.  With the maximum possible buffer size the transfer may take some time.  Reduce the buffer size if only a few data shall be fetched.
• Cold Junction  -  Connection point between thermocouple metals and the electronic instrument.
• Closed Loop (Feedback Loop)  -  A signal path which includes a forward path, a feedback path and a summing point, and forms of closed circuit.
• Control Action  -  The nature of the change of the output affected by the input of a controller or a controlling system.
• Control Mode  -  A specific type of control action such as proportional, integral or derivative.
• CPS  -  Cycles per second; the rate or number of periodic events in one second, expressed in Hertz (Hz).
• CPU  -  Central processing unit.  The part of the computer that contains the circuits that control and perform the execution of computer instructions.
• Current  -  The flow of electric charge in a circuit or a conductor.  The current flow is caused by the movement of electrons, which are negatively charged particles, through a conductor such as a wire.  The rate of flow of electric charge (current) is typically determined by the voltage (potential difference) applied across the conductor and the resistance of the conductor.
• Current Drive Capability  -  The amount of current a digital or analog output channel is capable of sourcing or sinking while still operating within voltage range specifications.
• Current Proportioning  -  An output form of a temperature controller which provides a current proportional to the amount of control required.
• Cutoff Frequency  -  Also known as corner frequency or break frequency, is a boundary in a system’s frequency response where energy flowing through a system begins to reduce rather than pass through.
• Cycle Time  -  The time, usually expressed in seconds, for a controller to complete one on/off cycle.

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• Data Length  -  Indicates the set data format.
• Data Logger  -  A stand alone data acquisition device that acquires data and stores it in local memory from which it may then be downloaded to a computer for subsequent analysis and display.
• Data Recorder  -  Another name for a data logger, though data recorders are typically higher performance and offer higher sample rates than a typical data logger.
• Deviation  -  Any departure from a desired or expected value or path.
• Digital Output  -  An output signal which represents the size of an input in the form of a series of discrete quantities.
• Differential Input  -  An analog input consisting of two terminals, both of which are isolated from computer ground, whose difference is measured.
• Differential Pressure Flowmeter  -  There are many different types of instruments for measuring pressure.  The functions of these instruments are to monitor and control pressure in a process stream.
• Differential Set Pressure  -  The pressure difference between the set pressure and constant back pressure.
• Discrete Input  -  A type of signal or data that can only take on a limited set of distinct values.  Discrete inputs are often binary, meaning they can have only two possible states: either a TRUE or FALSE, 1 or 0, or ON of OFF as an input signal.
• Discrete Output  -  A type of signal or data that can only take on a limited set of distinct values.  Similar to discrete inputs, discrete outputs are often binary, meaning they can have only two possible states: either a TRUE or FALSE, 1 or 0, or ON of OFF as an input signal.  Discrete outputs are commonly used in digital systems where information is processed and communicated in a digital format.

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• Electric Current  -  The rate of flow of electricity in a circuit, measured in amperes.
• Element  -  A component of a device or system.
• Error Signal  -  In a closed loop, the signal resulting from subtracting a particular return signal from its corresponding input signal.
• Ethernet  -  A family of computer networking technologies for local area networks (LANs) first introduced in 1980.  Systems that use Ethernet for communications divide a stream of data into individual packets called frames.  Each frame contains source and destination addresses and error-checking data so that damaged data can be detected and re-transmitted.
• Excitation Voltage  -  The nominal voltage required for excitation of a circuit.  Excitation is the process of generating a magnetic field by means of an electric current.
• External Trigger  -  A voltage pulse from an external source that trig- gers an event such as A/D conversion.

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• Fail Position  -  The term used to describe how an actuator reacts when there is a loss of power, loss of signal or similar event.  There are three different fail scenarios, each has its own consequences and safety effects.
• Feedback Element  -  Those elements in the controlling system which change the feedback signal in response to the directly controlled variable.
• Feedback Signal  -  That return signal which results from a measurement of the directly controlled variable.
• Filter  -  A device that allows certain parts of a signal to pass through while blocking others.  In data acquisition systems, the most common type of filter used is a low pass, anti-aliasing filter.
• Final Controlling Element  -  That forward controlling element which directly changes the value of the manipulated variable.
• Firewall  -  A device or set of devices designed to permit or deny network transmissions based upon a set of rules and is frequently used to protect networks from unauthorized access while permitting legitimate communications to pass.
  
• Flow Quality Indicating Transmitter  -  An instrument that is used for measuring the amount of a liquid and a gas in two pase flow.
• Flow Switch  -  Provides a discrete input to the PLC.  The flow switch is typically used as a method of control in tanks, vessels, pumps and other types of process equipment.
• Flowmeter  -  An instrument used to measure the flow rate of gasses and liquids.
• Forward Controlling Element  -  Those elements in the controlling system which change a variable in responce to the actuating error signal.
• Frequency Output  -  An output in the form of frequency which varies as a function of the applied input.
• Full Bridge Circuits  -  Not often used compared to a half bridge or quarter bridge circuit, but is the optimal configuration for strain gages.  It provides the highest sensitivity, fewest error, produces highest output, and noise being a non-factor.  A full bridge contains four strain gages, all mounted on the test member (two on top to measure tension and two on the bottom to measure compression).

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• Gain Accuracy  -  A measure of deviation of the gain of an amplifier from the ideal gain.
• Gauge Factor  -  A measure of the ratio of the relative change of resistance to the relative change in length of a piezoresistive strain gage.
• Ground Loop  -  Many measurements are made between an input signal and ground.  However, ground is not an absolute reference point and current flowing in wires between various ground connections can cause the potential at different “ground” points in the system to be at different potentials.  These differences then manifest themselves as errors in the measurement.
• Grounded Junction  -  A form of construction of a thermocouple probe where the hot or measuring junction is in electrical contact with the sheath material so that the sheath and thermocouple will have the same electrical potential.

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• Half Bridge  -  Two active elements or strain gages.
• Half Bridge Circuit  -  A bridge circuit where only two strain gages are used on a test member compared to the four used in a full bridge.  Two discrete resistor create the bridge.  This configuration is typically used due to physical constraints and obstacles not allowing the use of four strain gages.
• Hall Effect Sensor  -  Is a transducer that varies its output voltage in response to a magnetic field.  They are used for proximity switching, positioning, speed detection, and current sensing applications.  Hall sensors are normally used to time the speed of wheels and shafts.
• High Limiting Control Action  -  Control action which the output never excedes a predetermined high limit value.

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• Input Impedance  -  The resistance measured across the excitation terminals of a transducer.
• Input Signal  -  A signal applied to a device, element or ststem.
• Instrument Air  -  It is compressed air that is specifically used for various industrial instrumentation and control purposes.  In industrial settings, especially in sectors like petrochemical, chemical, and power plants, there are numerous instruments and control devices that rely on compressed air to operate effectively.
• Instrument Tag  -  The first letter indicates the process.  The second letter or two letters indicates the function.  Sometimes a third letter is used to indicates additional measured variables.  These letters identify the instrumentation process and are displayed in a instrument bubble consisting of two parts.  The top half of the bubble represents the process and the bottom half the tag number.
• Instrumentation & Controls Engineering Standards  -
• Insulation Resistance  -  The resistance measured between two insulated points on a transducer when a specific DC voltage is applied at room temperature.
• Integral Control  -  A control action that eliminates the steady-state offset inherent in proportional control.
• Interface  -  The means by which two systems or devices are connected and interact with each other.
• Interrupt  -  A computer signal indicating that the CPU should suspend its current task to service a designated activity.

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• Junction  -  The point in a thermocouple where the two dissimilar metals are joined.

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• Level Gauge  -  A device used to measure the level of fluids (liquids or solids) in a container or tank.  It provides continuous or periodic information about the level of the substance being measured.
• Level Switch  -  A device used to detect the presence or absence of a liquid or solid substance at a specific level within a container or tank.  The level switch is typically used as a method of control in tanks and vessels.  Level switches are crucial in various industrial and commercial applications to monitor and control processes involving liquids or solids.
• Level Transmitter Interface  -  The capability of a level transmitter to accurately measure and transmit the interface level between two immiscible liquids within a tank or vessel.  This interface level measurement is crucial in industries where the separation of different liquids or phases is critical for process control, quality monitoring, and safety.
• Load  -  The electrical demand of a process expressed as power (watts), current (amps) or resistance (ohms).
• Load Impedance  -  The impedance presented to the output terminals of a transducer by the associated external circuitry.
• Loop Resistance  -  The total resistance of a thermocouple circuit caused by the resistance of the thermocouple wire.  Usually used in reference to analog pyrometers which have typical loop resistance requirements of 10 ohms.
• Low Limiting Control Action  -  Control action which the output is never less than a predetermined low limit value.
• Lower Range Limit  -  The lowest quantity that a device can be adjusted to measure.

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• Manual Reset  -  The adjustment on a proportioning controller which shifts the proportioning band in relationship to the set point to eliminate droop or offset errors.
• Maximun Operating Temperature  -  The maximum temperature at which an instrument or sensor can be safely operated.
• Maximum Power Rating  -  The maximum power in watts that a device can safely handle.
• Meter  -  A device that provides a measurement for the flow change in a gas or liquid.
• Motherboard  -  The pc board of a computer that contains the bus lines and edge connectors to accommodate other boards in the system.  In a microcomputer, the motherboard contains the microprocessor and connectors for expansion boards.
• Mounting Error  -  The error resultant from installing the transducer, both electrical and mechanical.
• Multiplex  -  A technique which allows different input or output signals to use the same lines at different times, controlled by an external signal.
• Multiplexer  -  Also known as a MUX, is a device that selects one of several analog or digital input signals and forwards the selected input in a single line.  Multiplexers are mainly used to increase the amount of data that can be sent over the network within a certain amount of time and bandwidth.  A multiplexer is also known as a data selector.

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• Oil Water Detector  -  An instrument that measures the percentage of oil or water in a process stream.  It is a device used to detect the interface between oil and water in various industrial applications, particularly in oil and gas production, wastewater treatment, environmental monitoring, and marine industries.
• Open Circuit  -  The lack of electrical contact in any part of the measuring circuit.  An open circuit is usually characterized by rapid large jumps in displayed potential, followed by an off-scale reading.
• Open Loop  -  A signal path without feedback.
• Operating System  -  A collection of programs that controls the overall operation of a computer and performs such tasks as assigning places in memory to programs and data, processing interrupts, scheduling jobs and controlling the overall input/output of the system.
• Output  -  The electrical signal which is produced by an applied input to the transducer.
• Output Impedance  -  The resistance as measured on the output terminals of a pressure transducer.
• Output Signal  -  A signal delivered by a device, element or system.
• Overload  -  An overload occurs when the voltage of an incoming signal is higher than the voltage range the sensor or data acquisition device is designed to handle.  This can damage the device, or cause inaccurate readings.  To reduce the risk of overload, many data acquisition systems include protection circuits which will disconnect the sensor or device from the signal if it becomes overloaded.

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• Port  -  A communications connection on a computer or a remote controller.
• Position Switch  -  Provides a discrete input to the PLC.  The position switch is used to confirm the position of a critical component in the process, often a valve.
• Pressure Differential Transmitter  -  Measures the difference between a high pressure input and a low pressure input.  This can be used to measure process properties such as flow, density, level, viscosity and temperature.
• Pressure Indicating Transmitter  -, abbreviated as PT and PIT, is an instrument for measuring, controlling and indicating positive, negative or differential pressures of different fluids.
• Pressure Indicator  -  It is a field instrument that requires little to no maintenance.
• Pressure Switch  -  A discrete input to the PLC.  The pressure switch is typically used as a method of control in tanks, vessels and over all process control.

Q

• Quantization Error  -  The inherent uncertainty in digitizing an analog value due to the finite resolution of the conversion process.
• Quarter Bridge Circuit  -  This version of a bridge circuit only uses one strain gage and three bridge completion resistors.  This configuration has the smallest output with alot of noise potential.

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• Repeatability  -  The ability of the switch to actuate repeatedly at the desired set point within sensor tolerance.Reset Point  -  After the pressure has reached set point and operated the electrical switch, it must return to the reset point before the electrical switch returns to its original position.
• Resource Locking  -  A technique whereby a device is signaled not to use its local memory while the memory is in use from the bus.
• Return Signal  -  In a closed loop, the signal resulting from a particular input signal, and transmitter by the loop and to be subtracted from the input signal.
• Resistance Temperature Detectors  -  A sensor used to measure temperature by correlating the resistance of the element with temperature.
• Reverse acting Controller  -  A controller in which the absolute value of the output signal decreases, as the absolute value of the input inceeases.

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• Self Operating Controller  -  A controller in which all the energy to operate the final controlling element is derived from the controlled system, throuth the sensing element.
• Sensing Element  -  The portion of a device directly responsive to the value of the measured quantity.
• Sensor  -  A device or element that is used to measure physical quantities and convert them into signals that can be interpreted, displayed, or recorded.  These sensors play a critical role in various fields, including industrial automation, aerospace, healthcare, environmental monitoring, and more.
• Setpoint  -  An input variablewhich sets the desired value of the controlled variable.
• Setting Time  -  The time required, following initiation of a specified stimulus to a system, for the output to enter and remain within a specified narrow band centered on its steady state value.
• Shunt  -  A calibrated low resistance connected in parallel with the input terminals of an ammeter in order to enable measurement of higher currents. It can be internal or external.
• Shunt Resistor  -  A low resistance connection between two points in an electric circuit that forms an alternative path for a portion of the current.  Shunts allow meters to produce accurate readings in a much wider range.  A shunt resistor is used to measure AC or DC electrical currents by the voltage drop the currents create across the resistance.  By inserting a current shunt into a circuit whose current you want to measure, you can find the current by measuring the voltage drop across the shunt.
• Signal Source  -  Any device that produces a signal that can be used to measure a physical quantity.  Common signal sources in data acquisition include sensors, signal generators, and transducers.  The signal produced by the source is then converted into a digital signal and used as input for a data acquisition system.
• Signal to Noisr Ratio  -  Ratio of signal amplitude to noise amplitude.
• Span  -  The algebraic differences between the upper and lower range values.
• Static Gain  -  The value of the gain approached as a limit as frequency approaches zero.
• Steam Boiler Controls  -  These are a list of some of the controls that should be considered when putting together controls for a steam generator.  This list is meant to be a starting place for the process design and to offer topics for discussion.  
• Step Response  -  The response of an instrument when subjected to an instantaneous change in input from one steady state value to another.
• System Noise  -  A measure of the amount of noise seen by an analog circuit or an ADC when the analog inputs are grounded.

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• Temperature Controller  -  An instrument designed to minimize operator input or involvement.  A temperature controller has an input that is connected to a temperature source such as an RTD or other type of temperature element.
• Temperature Differential Switch  -  Contains the same functionality of a temperature switch with the exception that it is looking for the difference of temperature rather than a single input.
• Temperature Recorder  -  A device that records the temperature of a process.  This could be a data logger or a temperature transmitter capable of storing data.
• Temperature Switch  -  Provides alarm, shutdown and control for many types of processes.  Switch that activates when a certain temperature condition is met.
• Termination  -  A load connected to the end of a transmission line.  To avoid signal reflections, it must match the characteristic impedance of the line.
• Thermistor  -  A semiconductor sensor that exhibits a repeatable change in electrical resistance as a function of temperature.  Most thermistors exhibit a negative temperature coefficient.
• Thermopile  -  A number of thermocouples connected in series, arranged so that alternate junctions are the referenced temperature and at the measured temperature to increase the output for a given temperature difference between the measuring and reference junctions.
• Thermowell  -  Used in temperature measurement and provide isolation from the temperature sensor and the process fluid.
• Time Response  -  The response of an instrument when subjected to an instantaneous change in input from one steady state value to another.
• Torque Sensor  -  Also known as a torque transducer or torquemeter, is a device that is used to measure and record the torque on a rotating system such as engine, transmission, etc.  Static torque is relatively easy to measure while dynamic torque can be more difficult (Requires transfer of some effect, electric or magnetic, from the shaft being measured to a static system).  These sensors use strain gages applied to a rotating shaft or axle.
• Transducer  -  An element or device which receives information in the form of one physical quantity and converts it to information in the form of the same or other physical quantity.
• Transmitter  -  A transducer which responds to a measured variable by means of a sensing element, and converts it to a standardized transmission signal which is a function only of the measurement.
• Tribocharging  -  Tribocharging is a contact electrification process that enables buildup of static electricity due to touching or rubbing of surfaces in specific combinations of two dissimilar materials.

U

• Upper Range Limit  -  The highest quantity that a device can be adjusted to measure.

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• Variable Data  -  Numerical information that can be changed during application operation.  It includes timer and counter accumulated values, thumbwheel settings, and arithmetic results.
• Velocity Flowmeter  -
• Volt  -  A unit of electrical pressure.  One volt is the amount of pressure that will cause one ampere of current in one ohm of resistance.
• Voltage  -  The value of electrical potential difference across a conductor expressed in volts.
  
• Voltage-to-Frequency Converter  -  A device that converts analog input voltage into a sequence of digital pulses at a frequency proportion-al to the analog input voltage.

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• Watt Density  -  The watts emanating from each square inch of heated surface area of a heater.

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