Terms are frequently used in Instrumentation

  • Accuracy:

The difference between the reading of an instrument and the true value of what is being measured, expressed as percentage of full instrument scale.

  • Automatic Control:

The process of using the difference between the actual value and desired value of a variable to take corrective action without human intervention.

  • Automatic Control System:

Any combination of automatic controllers connected in closed loops with one or more processes.

  • Automatic Controller:

A device that measures the value of a variable and operates to correct or limit deviation from a selected reference. It includes means for both measurement and control.

  • Automation:

In mass production manufacturing, the technique of using self-directing, and self-correcting machines instead of human labor and control.

  • Batch Process:

Applies where a given amount of material is processed in one time of operation to produce what is required.

  • Calibration:

The procedure laid down for determining, correcting, or cross-checking the absolute values corresponding to graduations on a measuring instruments.

  • Cascade Control:

In an automatic control system, the resetting of the control point of a secondary controller by the output of a primary controller.

  • Closed Loop:

A combination of control units in which the process variable is measured and compared with the desired value (set-point). If the measured value differs from the desired value, a corrective signal is sent to the final control element to bring the controlled variable to the proper desired value.

  • Computer:

A machine carrying out mathematical calculations automatically.

  • Continuous Process:

Applies to a process where raw material treated by flowing continuously through a series of operation for getting the desired final product.

  • Control agent:

The process energy or material which is manipulated to hold the controlled medium at its desired value. In heating water with steam, the steam is the control agent.

  • Control Elements:

The portions of the feedback control system which is required to produce the manipulated variable from the actual signal.

  • Control Point:

The value of the controlled variable which, under any fixed set of conditions, an automatic controller operates to maintain.

  • Control System:

An assemblage of control apparatus coordinated to execute a planned set of control functions.

  • Controlled medium:

The process, energy or material which is to be adjusted to a definite value.

  • Desired value:

The value of the controlled variable which it is desired to maintain.

  • Deviation:

The difference between the instantaneous value of the controlled variable and the set-point.

  • Digital computer:

A type of computer that makes the calculations using digits rather than a continuous signals.

  • Drift:

Gradual departure of instrument output from the correct value.

  • Electric Controller:

A device or a group of devices which serves to govern the electric power delivered to the apparatus to which it is connected.

  • Error:

In automatic control terminology, the difference between the actual controlled variable and the set-point. The margin by which an automatic controller misses its target.

  • Feedback:

A part of a closed loop system which brings back information about a given condition for comparison with the desired condition.

  • Feed-back Controller:

A mechanism which measures the value of the controlled variable, accepts the value of the command, and, as a result of comparison, manipulates the controlled system in order to maintain an established relationship between the controlled variable and the command.

  • Feed-back control system:

A control system which tends to maintain a prescribed relationship of one system variable to another by comparing functions of these variables and using the difference as a mean of control.

  • Feed-back elements:

The portion of the feed-back control system which establishes the relationship between the primary feed-back and the controlled variable. 

  • Feed-back signal:

A signal that is returned to the input of the system and compared with the reference signal to obtain an actuating signal which returns the controlled variable to the desired value.

  • Final Control Element:

Unit of a control loop such as valve which manipulates the control agent.

  • Full Scale:

The maximum value of the rate or range of the Instrument.

  • Graphic Panel:

A control panel which pictorially displays and traces the relative position and function of  measuring and control equipment. Graphic panels can represent a total plant operation.

  • Hysteresis:

The total difference between the response of a unit or system to an increase in signal and the response to a decreasing signal.

  • Industrial Control:

The methods and means of governing the performance of a device, apparatus, equipment,   or system used in industry.

  • Input:

Incoming signal to a control unit or system.

  • Instrument:

Used broadly to denote a device incorporating measuring, indicating, recording, controlling, and / or operating abilities.

  • Instrumentation:

The instruments that are sued in a process system, usually including the control valves. Also refers to the science of applying instruments to manufacturing processes.

  • Linearity:

The degree to which the calibration curve of a device matches a straight line. The linearity error is generally greatest departure from the best straight line that can be drawn through the measured calibration points.

  • Manipulated variable:

The quantity of condition of the control agent which is varied by the automatic controller so as to affect the value of the measured (controlled) variable. In heating water with steam, the flow of steam is the manipulated variable.

  • Manual Controller:

A controller having all its basic functions performed by devices that are operated by hands

  • On-Off Control Action:

When a final control element is moved from one of the two fixed positions to the other with a very small change of controlled variable.

  • Open Loop:

A system in which no comparison is made between the actual value and the desired value of a process variable. Only recording, indicating or alarming functions are available.

  • Primary Element:

The portion of the measuring means which first either utilizes or transforms the energy from the controlled medium to produce an effect in response to change in the value of the controlled variable. The effect produced by the primary element may be change in pressure, force, temperature or electrical potential.

  • Primary Measuring Element:

A device or an instrument which measures a variable. Primary measuring elements are used to convert a measurement to a signal for transmission to a controller, a recorder, or an indicator. Also known as detector, sensor or sensing element.

  • Process:

Comprises the collective functions performed in and by the equipment in which a variable is to be controlled.

  • Program Control:

A control system in which the set-point is automatically varied during definite time intervals in order to make the process variable vary according to some prescribed manner.

  • Proportional Action:

This action of a controller produces an output signal proportional to the magnitude of the input signal. In a control system proportional action produces a value correction proportional to the deviation of the controlled variable from the set-point.

  • Proportional Band:

The amount of deviation of a controlled variable from the set-point required to move the final control element through the full range. This is expressed in percentage.

  • Integral Action:

Integral action is a mode of control action in which the value of the manipulated variable is changed at a rate proportional to the deviation.

  • Derivative Action:

Derivative action is a mode of control action in which the magnitude of the manipulated variable is proportional to the rate of change of deviation.

  • Range:

The difference between the maximum and the minimum values of a physical output over which an instrument is designed to operate.

  • Rangeability:

Describes the relationship between the range and the minimum quantity that can be measured.

  • Ratio Control:

Maintains the magnitude of a controlled variable a fixed ratio to another variable.

  • Receiver

Receiver in instrument terminology is one which receives the signal from transmitter. After that it does the required processing before displaying as indication, recording, alarm or control.

  • Reproducibility:

The ability of an instrument to duplicate with exactness measurements of a given value. Usually expressed as a percentage of span of the instrument.

  • Secondary element:

Secondary element merely converts the condition produced by the primary element into condition useful to function the instrument.

  • Sensing Element:

The part of a transmitter / transducer mechanism which is in contact with the medium being measured and which responds to changes in the medium.

  • Sensitivity:

The Ratio of change of output signal to change of Input signal.

The least signal input capable of causing an output signal having desired characterstics.

  • Set-Point:

The position to which the control point setting mechanism is set, which is same as the desired value of the controlled variable.

  • Span:

The difference between the top and the bottom scale values of an instrument. On instrument starting at zero, the span is equal to the range.

  • Stability:

Freedom from undesirable deviation, a measure of the controllability of a electro-motive force.

  • System Engineering:

Control engineering in which a process and all the elements affecting a process and all the possibilities for introducing automatic controls are considered during the design and installation of processing equipments

  • Transmitter:

Transmitter is one which transmits the signal from primary element to other component of the control system.

  • Transducer:

An electro-mechanical device which converts one form of signal energy to the other form of signal energy. Ex. I/P converter changes electrical current signal to Pneumatic air signal.

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