Some General Instrumentation Terms and Measurements

Some General Instrumentation Terms and Measurements


The physical system we are trying to control or measure. Examples: water filtration system, molten metal casting system, steam boiler, oil refining unit, power generation unit.

Process variable, or PV

A specific quantity we measure in a process. Examples: pressure, level, temperature, current, electrical conductivity, pH, position, motion, vibration.

Setpoint, or SP

The value at which we want the process variable to be maintained. and is also known as the target value of the process variable.

Primary Sensing Element, or PSE

A device that directly senses a process variable and translates that sensed quantity into an analog representation (electrical voltage, current, resistance; mechanical force, velocity, etc.). Examples: thermocouple, thermistor, Borden tube, microphone, potentiometer, electrochemical cell, accelerometer.


A device that converts one standardized signal to another standardized instrumentation signal and/or performs some form of processing on that signal. An example of a transducer I/P converter converts a 4-20 mA electric signal to a 3-15 PSI pneumatic signal and a P/I converter converts a 3-15 PSI pneumatic signal to a 4-20 mA electric signal.
Note: In common science terms, a "transducer" is a device that converts one form of energy into another, such as a microphone or thermocouple. In industrial instrumentation, however, we typically use "primary sensing element" to describe this concept and reserve the term "transducer" to refer specifically to the conversion device for standardized instrument signals.


A device that translates the signal produced by the primary sensing element (PSE) into a standardized instrument signal such as 3-15 PSI air pressure, 4-20 mA DC current, fieldbus digital signal packets, etc. . sent to the indicating device, the control device, or both.
Lower- and Upper-Range Values, abbreviated LRV and URV respectively: process measurement values ​​considered to be 0% and 100% of the transmitter's calibrated range. For example, if a temperature transmitter is calibrated to measure a temperature range starting at 300 degrees Celsius and ending at 500 degrees Celsius, its LRV will be 300 degree celsius and its URV will be 500 degree celsius.

Zero and Span

Alternate descriptions of LRV and URV for the 0% and 100% points of the instrument's calibrated range. Basically zero refers to the starting point of the instrument range and is equivalent to LRV, while span is the width of the instrument range and is equivalent to URV or LRV. For example, if a temperature transmitter is calibrated to measure a temperature range starting at 300 degrees Celsius and ending at 500 degrees Celsius, its zero will be 300 degrees Celsius and its span will be 200 degrees Celsius.


A device that receives a process variable (PV) signal from a primary sensing element (PSE) or transmitter, compares that signal to a desired value (called a setpoint) of that process variable, and calculates the appropriate output signal value. Sent.
A controller can be a physical device or soft logic built into a PLC/DCS system. Mainly we use PLC/DCS systems soft logic controller where the actual input and output devices are connected to this soft controller.

Final Control Element, or FCE

A device that directly influences the process by receiving a signal output from the controller. And example of FCE is control valve, electric motor or heater etc.
Manipulated variable, or MV: A quantity in a process that we adjust or otherwise manipulate to influence the process variable (PV). Also used to describe the output signal generated by a controller; That is, the final control element influencing the signal commanding ("manipulating") process.

Automatic mode

When the controller generates an output signal based on the relationship of the process variable (PV) to the setpoint (SP).

Manual Mode

When the decision-making ability of the controller is bypassed to allow the human operator to directly determine the output signal sent to the final control element.

Measurement variable

Common instruments include measuring variables such as temperature, pressure, flow rate, level, velocity, vibration, etc.


Sensitivity is the ratio of the change in transducer output to the corresponding change in measured value which is the sensitivity and is expressed as sensitivity = change in output signal / change in input signal.


Accuracy is the conformity of an indicated value with an accepted standard value or true value. And it is measured in terms of inaccuracy and expressed as accuracy.
Accuracy is a number or quantity that defines a limit that the error will not exceed when using the device under reference operating conditions

Complete error

The algebraic difference between the signal and the true value of the quantity being measured. And the absolute error is denoted by = signal - true value.

relative error

The ratio between the absolute error and the true value of the quantity being measured.

closed loop

Corresponds to the control loop where the process variable is used to calculate the controller output. Basically a closed loop system means that the control action is independent of the desired output.


It is the ratio of the change in output to which the change is applied. Gain is a special case of sensitivity, where the input and output have the same units and the gain has no unit.


The probability that a device will perform within its specifications for certain operations or for a certain period of time.


Equality of one measurement over time to another measurement where the operating conditions change over time but the input is restored.

Span adjustment

The span adjustment is the difference between the maximum and minimum range value. This input changes the slope of the output curve when provided to the instrument.

steady state

Closed loop control uses a steady state where the process no longer oscillates or changes and stabilizes at some defined value

time constant

The time constant of a first order system is defined as the time required for the output to reach 63.2% of the total change when the input is subjected to a step change.


A variable is some quantity of a system or process. And basically there are two types of variables i.e. measured variable and controlled variable. A measured variable is a measured quantity and is also referred to as a process variable as it measures process information. A control variable is a controller output that controls the process.


Vibration is the moving motion of an object.

Zero adjustment

Zero is the given output when zero or zero input is applied. A zero adjustment produces a parallel shift in the input output curve.