Instrumentation Basics: Measurement Terminology

Instruments are used to measure and control the condition of process streams as they pass through a Plant. Instruments are used to measure and control process variables such as: Temperature; Flow; Level; Pressure; Quality. Automatic instrument control systems are most commonly used to continually monitor these process conditions and correct them, without operator intervention, if there is a deviation from the process value required. The main reason for using automatic controls is that production is achieved more economically and safely. In fact, some of our processes could not be controlled in a stable condition without automatic control systems.  Although I have laid out the basic foundation for measurement and control here, you might check out.


The physical system we are attempting to control or measure. Examples: steam boiler, oil refinery unit, power generation unit.

Process Variable, or PV:

The specific quantity we are measuring in a process. Examples: pressure, level, temperature, flow, electrical conductivity, pH, position, speed, vibration etc.

Setpoint, or SP:

 The value at which we desire the process variable to be maintained at. In other words, the “target” value for the process variable.


Primary Sensing Element, or PSE:

A device directly sensing the process variable and translating that sensed quantity into an analog representation (electrical voltage, current, resistance; mechanical force, motion, etc.). Examples: thermocouple, thermistor, bourdon tube, microphone, potentiometer,
electrochemical cell, accelerometer etc.


 A device converting one standardized instrumentation signal into another standardized instrumentation signal, and/or performing some sort of processing on that signal. Often referred to as a converter and sometimes as a “relay.” Examples: I/P converter (converts 4-20 mA electric signal into 3-15 PSI pneumatic signal), P/I converter (converts 3-15 PSI pneumatic signal into 4-20 mA electric signal).
Note: in general science parlance, a “transducer” is any device converting one form of energyinto another, such as a microphone or a thermocouple. In industrial instrumentation, however, we generally use “primary sensing element” to describe this concept and reserve the word “transducer”

to specifically refer to a conversion device for standardized instrumentation signals.


A device translating the signal produced by a primary sensing element (PSE) into a standardized instrumentation signal such as 3-15 PSI air pressure, 4-20 mA DC electric current, Fieldbus digital signal packet, etc., which may then be conveyed to an indicating device, a controlling
device, or both.

Lower- and Upper-range values (LRV and URV):

the values of process measurement deemed to be 0% and 100% of a transmitter’s calibrated range. For example, if a temperature transmitter is calibrated to measure a range of temperature starting at 300 degrees Celsius and ending at 500 degrees Celsius, its LRV would be 300 oC and its URV would be 500 oC.

Zero and Span:

 alternative descriptions to LRV and URV for the 0% and 100% points of an instrument’s calibrated range. “Zero” refers to the beginning-point of an instrument’s range (equivalent to LRV), while “span” refers to the width of its range (URV − LRV).
For example, if a temperature transmitter is calibrated to measure a range of temperature starting at 300 degrees Celsius and ending at 500 degrees Celsius, its zero would be 300 oC and its span would be 200 oC.


 A device receiving a process variable (PV) signal from a primary sensing element (PSE) or transmitter, comparing that signal to the desired value (called the setpoint) for that process variable, and calculating an appropriate output signal value to be sent to a final control element (FCE) such as an electric motor or control valve.

Automatic mode:

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

Manual mode:

When the controller’s decision-making ability is bypassed to let a human operator directly determine the output signal sent to the final control element.

Final Control Element, or FCE:

 A device receiving the signal output by a controller to directly influence the process.
Examples: variable-speed electric motor, control valve, electric heater.

Manipulated Variable, or MV:

The quantity in a process we adjust or otherwise manipulate in order to influence the process variable (PV). Also used to describe the output signal generated by a controller; i.e. the signal commanding (“manipulating”) the final control element to influence the process.


Accuracy is the conformity of an indicated value to an accepted standard value, or true value. It is usually measured in terms of inaccuracy and expressed as accuracy. It is a number or quantity, which defines the limit that errors will not exceed, when the device is used under reference operating conditions. The units to be used must be stated explicitly. It is preferred that a + and – sign precede the number or quantity. The absence of a sign infers both signs (±). Accuracy can be expressed in a number of forms: Accuracy expressed in terms of the measured variable: Accuracy = ± 1Degree F. Accuracy expressed in percent of span: Accuracy = ± 1/2 % Accuracy expressed in percent of the upper range-value: Accuracy = ±1/2 % of URV


The surrounding or environment in reference to a particular point or object.


A decrease in signal magnitude over a period of time.


The procedure of comparing and determining the performance accuracy is called calibration. To configure a device so that the required output represents (to a defined degree of accuracy) the respective input.
  • The calibration of any measuring instrument is necessary to measure the quantity in terms of standard
  • It is carried out by making adjustments such that the read out device produces zero output for zero
  • The process whereby the magnitude of the output of a measuring instrument is related to the magnitude of the input force driving the instrument (i.e. Adjusting a weight scale to zero when there is nothing on it).
  • The accuracy of the instrument depends on the
  • If the output of the measuring instrument is linear and repeatable, it can be easily calibrated.


Magnification is the process of enlarging something only in appearance, not in physical size so that it is more readable.

Closed loop:

Relates to a control loop where the process variable is used to calculate the controller output. In a closed loop system the control action is independent on desired output.


A device, which operates automatically to regulate the control of a process with a control variable.


It is an undesirable gradual deviation of the instrument output over a period of time that is unrelated to changes in input operating conditions or load.
  • An instrument is said to have no drift if is reproduces the same readings at different times for same variation in measured
  • It is caused by wear and tear, high stress developed at some parts

Elevated zero:

is used when lower range-values is less than zero, Range = -20 to 2000C


This is the ratio of the change of the output to the change in the applied input. Gain is a special case of sensitivity, where the units for the input and output are identical and the gain is unitless.


Generally an undesirable oscillation at or near the required setpoint is called hunting. Hunting typically occurs when the demands on the system performance are high and possibly exceed the system capabilities. The output of the controller can be over controlled due to the resolution of accuracy limitations.


Hysteresis is the difference in the output for given input when the input is increasing and output for same input when input is decreasing. When input of any instrument is slowly varied from zero to full scale and then back to zero, its output varies as shown in the diagram below This is where the accuracy of the device is dependent on the previous value and the direction of variation. Hysteresis causes a device to show an inaccuracy from the correct value, as it is affected by the previous measurement
  • It is caused by friction, slack motion in the bearings and gears, elastic deformation, magnetic and thermal

Input Signal:

Is a signal applied to a device element, or system. The pressure applied to the input connection of a pressure transmitter is an input signal.


Linearity expresses the deviation of the actual reading from a straight line (from linear relation between input and output). If all outputs are in the same proportion to corresponding inputs over a span of values, then input output plot is straight line else it will be non linear (see diagram below) For continuous control applications, the problems arise due to the changes in the rate the output differs from the instrument. The gain of a non-linear device changes as the change in output over input varies. In a closed loop system changes in gain affect the loop dynamics. In such an application, the linearity needs to be assessed. If a problem does exist, then the signal needs to be linearised.

Measured Variable:

Is the physical quantity or condition, which is to be measured. Common measured variables are: Temperature, pressure, rate of flow, level, speed, etc


Measured Signal:

Is the electrical, mechanical, pneumatic, or other variable applied to the input of a device. In a thermocouple, the measured signal is an E.M.F, which is the electrical analogue of the temperature applied to the thermocouple. A measured signal is normally produced by the primary element (sensing element) of an instrument.


Output Signal:

Is a signal delivered by a device, element, or system. The signal (3 to 15 psig, 4 to 20 mA dc, etc) produced at the output connections of a transmitter is an output signal.



An equipment which is precise is not necessarily accurate.

  • Defined as the capability of an instrument to show the same reading when used each time (reproducibility of the instrument).


Defines the delayed and accumulated response of the output for a sudden change in the input.


Is the region between the limits within which a quantity is measured, received or transmitted, expressed by stating the lower and upper range-value. -20 to + 2000C; 20 to 1500C; 4mA to 20Ma

Range of Span:

Defined as the range of reading between minimum value and maximum value for  the measurement of an instrument. The range of span of an instrument which has a reading range of –100°C to 100 °C is 200 °C.

Rangeability or turndown:

Ratio of the maximum adjustable span / the minimum adjustable span for a given Instrument, R = 100 bars / 10 bars = 10


Readability refers to the ease with which the readings of a measuring instrument can be read.
  • Fine and widely spaced graduation lines improve the
  • To make the micrometers more readable they are provided with venier scale or magnifying

Relative Error:

Ratio between the absolute error and the true value of the quantity to be measured. Expressed in percent: x = (ΔX/X) x 100


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


Ability of an instrument to give identical indications or responses for repeated applications of the same value of the quantity measured under the same conditions of used. Good repeatability does not guarantee accuracy. It is the ability of the measuring instrument to repeat the same results for the measurements for the same quantity, when the measurements are carried out
  • by the same observer,
  • with the same instrument,
  • under the same conditions,
  • without any change in location,
  • without change in the method of measurement,
  • the measurements are carried out in short intervals of


  • When the input is slowly increased from some non-zero value, it is observed that the output does not change at all until a certain increment is exceeded; this increment is called
  • It is the min. change in measured variable which produces an effective response of the


The frequency of oscillation is maintained due to the natural dynamics of the system.


When the output of a device is expressed as a function of time (due to an applied input) the time taken to respond can provide critical information about the suitability of the device. A slow responding device may not be suitable for an application. This typically applies to continuous control applications where the response of the device becomes a dynamic response characteristic of the overall control loop. However in critical alarming applications where devices are used for point measurement, the response may be just as important. The diagram below shows response of the system to a step input.


Is the ratio of the change in transducer output to the corresponding change in the measured value, i.e. sensitivity = (change of output signal) / (change of input signal). For example: A pressure-to-current converter could have a sensitivity of 0.1 mA / mbar.
  • Sensitivity may be defined as the rate of displacement of the indicating device of an instrument, with respect to the measured
  • Sensitivity of thermometer means that it is the length of increase of the liquid per degree rise in temperature. More sensitive means more noticeable
  • In other words, sensitivity of an instrument is the ratio of scale spacing to the scale division value. For example, if on a dial indicator, the scale spacing is 1 mm and the scale division value is 0.01 mm then sensitivity is 100. It is also called as amplification factor or gearing
Sensitivity (K) = Δθο / Δθi Δθο : change in output; Δθi : change in input Example 1: The resistance value of a Platinum Resistance Thermometer changes when the temperature increases. Therefore, the unit of sensitivity for this equipment is Ohm/°C.


Used in closed loop control, the set point is the ideal process variable. It is represented in the units of the process variable and is used by the controller to determine the output to the process.


Is the algebraic difference between the upper and lower range-values.Range: -20 to 2000C, Span is 2200C; Range: 20 to 1500C, Span is 1300C.
  • Input span:
  • Output span

Span Adjustment:

The difference between the maximum and minimum range values. When provided in an instrument, this changes the slope of the input-output curve.

Steady state:

Used in closed loop control where the process no longer oscillates or changes and settles at some defined value.

Suppressed zero:

is used when lower range-values is greater than zero ,Range = 20 to 1500C


The min. value below which no output change can be detected when the input of an instrument is increased gradually from zero is called the threshold of the instrument. Threshold may be caused by backlash

Time constant:

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


  • Closely related to accuracy of equipment where the accuracy of an equipment is sometimes referred to in the form of tolerance
  • Defined as the maximum error expected in an


An element or device that converts information from one form (usually physical, such as temperature or pressure) and converts it to another ( (usually electrical, such as volts or millivolts or resistance change). A transducer can be considered to comprise a sensor at the front end (at the process) and a transmitter.


A sudden change in a variable, which is neither a controlled response, nor long lasting.


A device that converts one form of energy to another. Usually from mechanical to electrical for the purpose of signal integrity for transmission over longer distances and for suitability with control equipment.


Range of values within which the true value lies with a specified probability Uncertainty of +/-1 % at 95 % confidence means the instrument will give the user a range of +/-1 % for 95 readings out of 100.


Generally, this is some quantity of the system or process. The two main types of variables that exist in the system are the measured variable and the controlled variable. The measured variable is the measured quantity and is also referred to as the process variable as it measures process information. The controlled variable is the controller output which controls the process.


This is the periodic motion (mechanical) or oscillation of an object.

Zero adjustment:

The zero in an instrument is the output provided when no, or zero input is applied. The zero adjustment produces a parallel shift in the input-output curve.


  • Human natural observation capabilities are generally not designed for process conditions.
  • Instruments must have desired capabilities to match process
  • Process Control has the role of a decision makers (Like brain)
Sensors feel the condition and originate the signal followed by modification and amplification for effective display /transmission or control objectives.


  • Typical components of instrument
    • A Sensor: (measures a physical quantity and converts it into a signal)
    • A Modifier (Change the type of signal)
    • A Display unit (transmitting arrangement )


  • A measure of how close repeated trials are to each
  • The closeness of repeated
  • Precision is the repeatability of the measuring process. It refers to the group of measurements for the same characteristics taken under identical
  • It indicated to what extent the identically performed measurements agree with each
  • If the instrument is not precise it will give different results for the same dimension when measured again and

Active leg
A regulator and a nitrogen blanket used to prevent vapor condensation in the vapor space. A top transmitter is required because the regulator is not a perfect controller.

Address (HART®)
Units using HART protocol may be multi-dropped to communicate digitally with a control system. The HART address range is 0 to 15.

Address (MODBUS®)
Units using MODBUS to communicate with host system may have addresses ranging from 1 to 247.

The transmitting and receiving part of a radar level gauge that directs the radar signal toward the material being measured. Different types of antennas are available to fit different applications. Examples are: cone, parabolic and still-pipe antennas

The abbreviation for American Petroleum Institute. API is a trade association representing American OilandGas industry. API has led the development of petroleum and petrochemical equipment and operating standards for more than 75 years. For Level measurement the Manual of Petroleum Measurement Standard is applicable.

The abbreviation for Average Sensing Unit—an accessory to a custody transfer level measurement system. The ASU receives multiple temperature sensor input readings, averages the appropriate temperature readings (from submerged sensors), and sends the average temperature input to the level device.

The reduction in the level of a quantity, such as the intensity of a wave, over an interval of a variable, such as the distance from a source.

Average area
The average area of a tank is used for calculating density and is expressed as volume per level (example: bbls/ft, liters/meter, gallons/ft, etc.)

Balanced remote seal system

A situation where the remote seal system (diaphragm seal, capillary and fill fluid) is the same on both the high and the low pressure side of a transmitter.


An indication of specific gravity in degrees. Baumé is calibrated in accordance with, or according to, either of two arbitrary hydrometer scales for liquids lighter than water, or for liquids heavier than water.


The determination of sugar concentration by density measurement.


The coating of process material, from liquids or solids, on the sensor. The build-up may be wet, dry, conductive or non-conductive.


An electrical term referring to the property of an electric nonconductor that permits the storage of energy as result of electric displacement when opposite surfaces of the nonconductor are

maintained at a difference of potential. The unit of capacitance is the Farad.

Capacitance probe

An electrically isolated rod inserted into a vessel to measure level or interface level. It consists of a probe and transmitter pre-calibrated by the manufacturer. As the level or interface changes, a corresponding change in capacitance changes between the rod

and the vessel, or between the rod and the ground. Measured capacitance is proportional to liquid level

The information needed for proper calibration includes: the dielectric constant of the product, the angle of probe insertion, the length of inactive section, and the length of the probe.

Capacitance transmitter

A device consisting of a set electronics for the capacitance probe that outputs a calibrated signal proportional to level or interface level.

Capacitive span

The amount of capacitance increase sensed by a capacitance probe, with a specific probe and a specific level change.



The abbreviation for Configuration Data Sheet. A detailed survey form that identifies all the data needed to fully configure a device.


Closed tank

A tank that is not open to atmospheric pressure. It may be either pressurized or evacuated of pressure.


Concentric shield

A circular plate surrounding a probe. It is a sometimes used with capacitance probes to provide a second capacitance plate. This is done when:

the measurement is made in nonmetallic tanks, in metal tanks without straight walls, or when the

distance between the capacitance plates needs to be minimized.


Condensate pot

In DP applications, the condensate pot is located at the bottom of a dry leg. It is used for collecting any moisture that may appear in the dry leg due to variances of temperature in the dry leg. Typically, a

drain valve or spout is located on the condensate pot to remove any accumulated moisture from the dry leg.


Conversely, in wet leg applications, the condensate pot is located at the top of wet leg where it is used to hold a constant fluid height. This is especially useful in steam applications where the system sees large pressure variations that could cause the wet leg to lose fluid.



The ability of a material to transmit the flow of electricity. Usually measured in micromhos. It is the reciprocal of resistance.



The process of downloading information into a device that determines how it operates.

Configuration includes a completed customer configuration data sheet (CDS) and the CDS data that has been downloaded into the device.



The basic metric unit of electrical capacitance measurement.  

Float gauge

The traditional technique for measuring level. A buoyant float resting on a liquid surface is mechanically connected to a gauge head by a perforated tape. A spring motor in the gauge head keeps the tape taut. The tape perforations are counted and related to a liquid level.  

Floating roof

A roof on an upright cylinder tank that rests on the product stored inside the tank. The roof height follows any changes in product level. This prevents the product inside the tank from easily evaporating. The floating roof has legs (or pins) that prevent the floating roof from resting on the floor of the tank when the tank is empty.  

Flow rate

The flow rate is positive when pumping into the tank and negative when pumping out. The flow rate is calculated as: (volume of tank 1 – volume of tank 2) / the calculation period. Flow rate can also be calculated using level or mass.  

Flow rate calculation period

The time between flow rate calculation updates.  


The abbreviation for Frequency Modulated Continuous Wave. In radar level terminology, it refers to a continuous radio sine wave that is generated with a changing (modulated) frequency providing a highly accurate and stable level measurement.

Pin height

The height at which the floating roof is totally supported by the pins, rather than the product.  


One-trillionth farad (10-12 farad). Common unit used in calculation and discussion of capacitance values occurring in level applications.  

Point indication

Yes/No indication of attained level.  

Point level

The term associated with detection of the presence or absence of material at a specific level height.  

Pressure transmitter

A device commonly used for measuring level in process applications. It uses the principle that the measured hydrostatic pressure is equal to the height of the fluid multiplied by its specific gravity.  

Probe end projection (PEP)

A method of inferring level with guided wave radar where the end of the probe is used in combination with the media’s dielectric constant to calculate the location of the surface. It is used in low dielectric media where the end of the probe is detectable below the surface and the actual surface is too weak to reflect the signal reliably.  

Process level

The term associated with continuous level measurements in the process industries. Generally, the accuracy requirements of this measurement are not as stringent as with inventory level. See also inventory level  

Product temperature

The temperature of the material being measured. It may be a manually entered constant if no temperature sensor is installed on the tank. If a sensor is used, it is automatically used for compensation purposes. Temperature compensation is needed for standard volume and density calculations. It is also needed for compensation of signal travel time for ultrasonic transmitters when the vapor space temperature is different from ambient conditions.


The part of a radar transmitter that receives return signals reflected from the surface of a material or obstructions within a tank.


Usually the point on a tank where the level is equal to zero. All sensor mounting heights are measured from the reference point.

Reference temperature

The temperature at which actual density and actual volume are referenced to calculate standard density and standard volume. This reference temperature is required for all applications where standard density and volume calculations are required.


The degree to which an object can reflect light, images, sound waves, or microwaves back to an instrument.

Remote seal

Remote seal systems act as a pressure transport medium between the process and the sensor of the pressure transmitter. Remote seals are used: when it is necessary to isolate the transmitter from the process due to high process temperature, for corrosive or viscous products, for applications that require the use of sanitary connections, to avoid contamination between batches, or when there is a need to make density or interface measurements.

Remote seal system

A combination of the remote diaphragm seal, capillary, and fill fluid attached to a transmitter. A seal system may be one high pressure, low pressure, or both sides of a pressure transmitter.


The abbreviation for Resistance Temperature Detector. A device that predictably changes its resistance when a change in temperature is detected.


Sensor trim

The digital trim function that allows adjustment of the digital process variable reading to a precise pressure input. Zero trim and full trim are the two sensor trim functions.

Tank tag

The instrument number.  

Tank type

The tank shape that best describes the tank. This is needed for correctly calculating a formula based strapping table. Some examples of different tank types are: upright cylinder, horizontal cylinder, sphere, upright bullet and horizontal bullet.  


The decrease in the strength of a signal between two points or between two frequencies.  

Temperature range

The minimum and maximum range of temperatures encountered by the product during operation.  

Temperature sensor

A device that measures the temperature of the product inside the tank. Typically this is an RTD, although the bottom pressure transmitter with local temperature sensor could also be used. See also multi-spot temperature sensor  

Thermal expansion

The amount of expansion of material due to changes in temperature. Usually measured in mm/m/° C.  

Thermo well

The housing for a temperature sensing device protruding into a tank. It allows an RTD to be replaced even when the tank is in use (full).  


Used with top-down technologies as a filtering method. Different amplitude thresholds are used in order to filter out unwanted signals in order to pick up the correct pulses.  

Time domain reflectometry (TDR)

See Guided Wave Radar  


The elapsed time from the instant a particle leaves a source to the instant it reaches a detector.  

Traditional volume

The volume calculated from a strapping table and level measurement.


A device that receives information in the form of one quantity and converts it to information in the form of the same or another quantity.  

Transfer type

The method by which the product will be transferred to and from the tank—either gross volume, standard volume, level, or mass.  

Transition zone

A short distance from either the reference peak or the end of the probe where the signal peak and reference or end of probe peak may be merged. Measurements in this area may not meet accuracy or linearity specifications. The length of the transition zones is dependent on probe type and dielectric constant of the measured media.  


A transducer that responds to a measured variable by means of a sensing element and converts it to a standardized transmission signal.  

Trim near zone

Software functionality that minimizes the Upper Transition Zone and ensures that measurement performance in the near zone is maintained.  

Vapor compensation

A calculation used to correct for properties of the vapor.

Vapor pressure

Pressure of the vapor in the vapor space of a pressurized tank.

Vent hole

The hole near the top of the stilling well that allows air release as fluid rises in the tube. Ensures that the level within the tube is equal to the vessel level at all times.

Vented tank

A tank that is covered but not pressurized. A vent located on top of the tank releases any pressure buildup.


The resistance that a gaseous or liquid system offers to flow when it is subjected to a shear stress. Also known as flow resistance.


The amount of space occupied as measured in cubic units (as in., ft., and meters).

Volume (measured)

The volume measured at product temperature.

Volume (roof)

The volume of product displaced by the weight of the floating roof.

Volume (standard)

The volume of a product at a reference

Water bottom level

The level of water at the bottom of a tank.

Water bottom volume

The volume of water at the bottom of a tank.

Wave guide

The entire path, in a radar gauge, through which the radar signal travels. It includes the coaxial-to-wave guide adapter, the housing- to-flange adapter, and the mounting flange.

Wave length

A measure, expressed in meters, of the length of each complete electromagnetic wave.

Wet leg

A filled reference connection between the vapor space area of a tank and the low pressure side of the transmitter. Wet legs are used when condensation is common and dry legs are impractical. The pressure exerted by the weight of the fluid in the wet leg must be compensated for in the calibration of the pressure transmitter.

Wetted parts

The portion of an instrument exposed to a process.


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