Glossary of Process Control Terminology


Alternating Current
Electrical current that reverses its direction of flow at regular intervals.
AC line frequency
The frequency of the alternating current power line measured in Hertz (Hz), usually 50 or 60Hz.
Closeness between the value indicated by a measuring instrument and the corresponding true value. Sensor accuracy is based on US NIST (NBS) standards.
Accuracy - Calibration Accuracy
Closeness between the value indicated by a measuring instrument and a physical constant or known standard.
Accuracy - Control Accuracy
The ability to maintain a process at the desired setting. This is a function of the entire system, including sensors, controllers, heaters, loads, and inefficiencies.
Accuracy - Indication Accuracy
Closeness between the displayed value and a measured value. Usually expressed as a + or -, a percent of span or number of digits.
Accuracy - Setting Accuracy
Closeness between the value established by an input device, such as a dial, and the desired value. Usually expressed as a percent of span or number of digits.
The response of an output when the process variable is changed. See also direct action, reverse action.
The present value of the controlled variable.
A numerical identifier for a controller when used in computer communications.
Alarm - Deviation Alarm
Warns that a process has exceeded or fallen below a certain range around the set point. Alarms can be referenced at a fixed number of degrees, plus or minus, from a set point.
Alarm - Loop Alarm
Any alarm system that includes high and low process deviation band, dead band, digital inputs, and auxiliary control outputs.
Alarm - Process Alarm
Warns that process values exceed the process alarm setting. A fixed value independent of set point.
Ambient Temperature
The temperature of the encompassing atmosphere, including the environment and air surrounding the equipment in use.
Analog transmission
The transmission of data as a continuous signal, as opposed to an on/off digital signal.
Anti-reset windup
This feature of the PID controller prevents the integral (reset) circuit from operating when the temperature is outside the proportional band, thus stabilizing the system.
Baud Rate
The rate of information transfer in serial communications, measured in bits per second.
An ideal surface that absorbs all incident radiation, regardless of wavelength, the direction of incidence and polarization. It radiates the maximum energy possible for given spectral and temperature conditions. A blackbody has an emissivity of 1.00. See Emissivity.
Bumpless transfer
A situation in which the control output is maintained at the same level while transferring from auto to manual control.
Burst Fire
A power control method that repeatedly turns ON and OFF full AC cycles. Also called zero-cross fire, it switches close to the zero-voltage point of the AC sine wave. Variable-time-base burst fire selectively holds or transits AC cycles to achieve the desired power level.
The comparison of a measuring device (an unknown) against an equal or better standard.
Calibration Offset
An adjustment to eliminate the difference between the indicated value and the actual value.
Cascade Control
Control in which the output of one controller is the setpoint for another.
A manufacturer's mark that demonstrates compliance with European Union (EU) laws governing products sold in Europe.
Compliant with the essential requirements of European directives pertaining to safety and/or electromagnetic compatibility.
Formerly known as Centigrade. A temperature scale in which water freezes at 0C and boils at 100C at standard atmosperic pressure. The formula for conversion to Fahrenheit scale is: degF = (1.8*degC) + 32.
Center Wavelength
Center wavelength is the arithmetic center of the passband of a bandpass filter. It is not necessarily the same as the peak wavelength.
The rapid On-Off cycling of an electromechanical relay or mercury displacement relay due to insufficient controller bandwidth. It is commonly caused by excessive gain, little hysteresis, and short cycle time.
Closed loop control
A control system in which all adjustments necessary to maintain the system occur automatically through a feedback signal from the sensor.
Cold Junction
Connection point between thermocouple metals and the electronic instrument.
Cold Junction Compensation
Electronic means to compensate for the effective temperature at the cold junction.
Common Mode Rejection (CMR)
The ability of an electronic device to eliminate the effect of AC or DC noise between signal and ground. Normally expressed in dB at DC to 60Hz. See also Normal Mode Rejection.
Control cycle
The rate at which the output signal is updated.
Control mode
The output form or type of control action used by a controller to control temperature process, i.e. on/off, time proportioning, PI, PID, or manual.
Canadian Standards Administration
Center wavelength is the arithmetic center of the passband of a bandpass filter. It is not necessarily the same as the peak wavelength.
Cycle time
The time, usually expressed in seconds, for a controller to complete one on/off cycle.
Direct Current. Electrical current that flows in only one direction from a source.
dB (Decibel)
Twenty times the log to the base 10 of the ratio of two voltages. Every 20 dBs correspond to a voltage ratio of 10; every 10 dBs to a voltage ratio of 3.162.
Dead band
A region selected around the setpoint where proportional control is withheld. This is usually between the heating and cooling proportional bands.
An output shift in the thermocouple so that it no longer conforms to established standards. The shift is caused by the altering of alloys in the thermocouple conductors.
The increments in a temperature scale, or the increments of rotation of a dial. The location of a reference point in electric or phase in a cycle, in mechanical or electrical cyclic scales. (One cycle is equal to 360 degrees).
The rate of change of a process variable. Also referred to as "rate."
Deutsche Industrial Norm (DIN)
A German agency that sets engineering and dimensional standards that now has worldwide recognition.
The difference between the value of the controlled variable and the value at which it is being controlled.
Differential Control
A control algorithm where the set point represents a desired difference between two processes. The control then manipulates the second process and holds it at the set value relative to the first.
See Deutsche Industrial Norm
Direct Action
An output control action in which an increase in the process variable causes an increase in the output. Cooling applications usually use direct action.
A change in reading or value that occurs over long periods. Changes in ambient temperature, component aging, contamination, humidity and line voltage may contribute to drift.
Dual Element Sensor
A sensor with two independent sensing elements. Usually used to measure temperature gradients or provide redundancy in a single point sensor assembly.
Electromagnetic Compatibility (EMC)
The ability of equipment or a system to function as designed in its electromagnetic environment without introducing intolerable electromagnetic disturbances to that environment, or being affected by electromagnetic disturbances in it.
Electromagnetic Interference (EMI)
Electrical and magnetic noise imposed on a system. There are many possible causes, such as switching AC power on inside the sine wave. EMI can interfere with the operation of controls and other devices.
The ratio of radiation emitted from a surface compared to radiation emitted from a blackbody at the same temperature.
A programmable ON/OFF output signal. Events can control peripheral equipment or processes, or act as an input for another control or control loop.
Explosion-Proof Enclosure
An enclosure designed to withstand an explosion of gases inside, to isolate sparks inside from explosive or flammable substance outside, and to maintain an external temperature that will not ignite surrounding flammable gases or liquids.
Extension Wire
dashed_border_bottom">See Thermocouple Extension Wire
External Transmitter Power Supply
A DC voltage source that powers external devices.
A process by which a material is melted and allowed to flow through a die to create a desired shape.
The temperature scale that sets the freezing point of water at 32ºF and its boiling point at 212ºF at standard atmospheric pressure. The formula for conversion to Celsius is: ºC = 5/9 (ºF - 32ºF).
Filter - Digital Filter (dF)
A filter that slows the response of a system when inputs change unrealistically or too fast. Equivalent to a standard resistor-capacitor (RC) filter.
The motion of liquids or gases in response to a force.
See FM approved
FM approved
An instrument that meets a set of specifications established by the Factory Mutual Research Corporation.
Form A
A single-pole, single-throw relay that uses only the normally open (NO) and common contacts. These contacts close when the relay coil is energized. They open when power is removed from the coil.
Form A or C
An electromechanical relay capable of Form A or C function, selected by a jumper wire.
Form B
A single-pole, single-throw relay that uses only the normally closed (NC) and common contacts. These contacts open when the relay coil is energized. They close when power is removed from the coil.
Form C
A single-pole, double-throw relay that uses the normally open (NO), Form A or Form B contact.
The number of cycles over a specified period of time, usually measured in cycles per second. Also referred to as Hertz (Hz). The reciprocal is called the period.
An electrical line with the same electrical potential as the surrounding earth. Electrical systems are usually grounded to protect people and equipment from shocks due to malfunctions. Also called safety ground.
Ground Loop
A condition created when two or more paths for electricity are created in a ground line, or when one or more paths are created in a shield. Ground loops can create undesirable noise.
Ground Potential
The electrical potential of the earth. A circuit, terminal or chassis is said to be at ground potential when it is used as a reference point for other potentials in the system.
Grounded thermocouple
A thermocouple constructed in such a way that the measuring junction is an integral part of the sheath material. Electrical isolation is not provided in this type of construction.
Heat Sink
Any object that conducts and dissipates heat away from an object in contact with it. Also a finned piece of metal, usually aluminum, that is used to dissipate heat generated by electrical and electronic devices.
Hertz (Hz)
Frequency, measured in cycles per second.
A band of change in process variable around the setpoint required to re-energize the control or alarm output.
Input/output--either analog or discrete.
Intenational Electromechanical Conference
The total opposition of a circuit to the flow of alternating current. It includes resistance and reactance, and is measured in ohms.
Process variable information that is supplied to the instrument.
Input Scaling
The ability to scale input readings (readings in percent of full scale) to the engineering units of the process variable.
Input Type
The signal type that is connected to an input, such as thermocouple, RTD, linear or process.
Instrument Society of America (ISA)
An engineering society that defines and maintains standards for scientific and technical measuring devices.
Insulation resistance
The resistance measured between two insulated points on a transducer when a specific DC voltage is applied at room temperature.
The function in a PI controller that adjusts the process variable to the setpoint after system stabilization.
IP Codes (Ingreso Transmission)
International classification system for the sealing effectivenes of enclosures of electrical equipment against foreign bodies.
IPTS-48, -68
International Practical Temperature Scales of 1948 and 1968. These have been superseded by ITS-90. See ITS-90.
See Instrument Society of America
The electrical separation between circuits from high voltage circuitry.
International Temperature Scale of 1990. The standard scale made of fixed points that closely approximate thermodynamic temperatures. All temperatures between the fixed points are derived by interpolation using the assigned interpolation instrument. Adopted in late 1993, this scale replaces both IPTS-48 and -68.
See Joint Industrial Standards.
Joint Industrial Standards (JIS)
A Japanese agency that establishes and maintains standards for equipment and components. Also known as JISC (Japanese Industrial Standards Committee), its function is similar to Germany's Deutsche Industrial Norm (DIN).
The point where two dissimilar metal conductors join to form a thermocouple.
Junction - Cold Junction
Connection point between thermocouple metals and the electronic instrument. See reference junction.
Junction - Exposed Junction
A type of thermocouple probe in which the hot, or measuring, junction protrudes beyond the sheath material and is fully exposed to the substance being measured. It usually gives the fastest response time. No electrical isolation provided.
Junction - Grounded Junction
Type of thermocouple probe in which the hot, or measuring junction, is an integral part of the sheath material. No electrical isolation is provided.
Junction - Isolated Junction
A form of thermocouple probe construction in which the measuring junction is fully enclosed in a protective sheath and electrically isolated from it. Commonly called an ungrounded junction.
Junction - Measuring Junction
The thermocouple junction that is affixed to or inserted into the material being measured. Also called hot junction.
Junction - Reference Junction
The junction in a thermocouple circuit held at a stable, known temperature (cold junction). Standard reference temperature is 32ºF (0ºC).
Junction - Thermocouple Junction
The point where the two dissimilar metal conductors join. In a typical thermocouple circuit, there is a measuring junction and a reference junction. See measuring junction and reference junction.
Junction - Ungrounded Junction
See Isolated Junction
Kelvin (K)
An absolute temperature scale. Zero Kelvin is absolute zero. No degree symbol (deg) is used with the Kelvin scale. (0C = 273.15K, 100C = 373.15K)
Kilo (k)
A prefix meaning thousand.
KiloWatt (kW)
Unit of electrical power equal to 1000 watts or 3412 Btus per hour when the power factor equals 1.0.
Kilowatt Hour (kWh)
Unit of electrical energy, or work, expended by one kilowatt in one hour. Also expressed as 1000 watt hours.
The delay between the output of a signal and the response of the instrument to which the signal is sent.
A low resistance path in materials that are normally insulators.
See Light Emitting Diode.
Light Emitting Diode (LED)
A a solid state electronic device that glows when electric current passes through it.
Limit or Limit Controller
A highly reliable, discrete safety device (redundant to the primary controller) that monitors and limits the temperature of the process, or a point in the process. When temperature exceeds or falls below the limit set point, the limit controller interrupts power through the load circuit. A limit controller can protect equipment and people when it is correctly installed with its own power supply, power lines, switch and sensor.
Linearization, Square Root
The extraction of a linear signal from a nonlinear signal corresponding to the measured flow from a flow transmitter. Also called square root extraction.
Liquid Crystal Display (LCD)
A type of digital display made of a material that changes reflectance or transmittance when an electrical field is applied to it.
The electrical demand of a process, expressed in power (watts), current (amps) or resistance (ohms). The item or substance that is to be heated or cooled.
A device that transmits a set point signal to other controlling devices called remotes.
Maximum Load Impedance
The largest load that the output device can operate. Usually specified in ohms.
Maximum output impedance
The largest load that the output device can drive while providing the full range of output.
See Relay - Mercury Displacement
Mercury Displacement Relay (MDR)
See Relay - Mercury Displacement
One one millionth of a volt
One thousandth of an inch, or 0.001 inches in decimal form.
Milliampere (mA)
One thousandth of an ampere.
Millivolt (mV)
One one thousandth of a volt.
National Bureau of Standards (NBS)
Now called the National Institute of Standards Technology (NIST).
National Electrical Code (NEC)
A set of specifications devised for the safe application and use of electric power and devices in the United States.
National Electrical Manufacturers Association (NEMA)
A United States association that establishes specifications and ratings for electrical components and apparatuses. Conformance by manufacturers is voluntary.
National Institute of Standards and Technology (NIST)
A United States government agency responsible for establishing scientific and technical standards. Formerly the National Bureau of Standards.
National Pipe Thread (NPT)
The taper pipe thread standard used in North America.
See National Bureau of Standards.
See National Electrical Code.
A standard from the National Electrical Manufacturers Association.
General purpose indoor.
Drip proof indoor.
Dust tight, raintight, & sleet-resistant (ice-resistant).
Water tight & dust tight--indoor and outdoor
Water tight, dust tight, and corrosion proof--indoor and outdoor.
See National Institute of Standards and Technology.
Unwanted electrical signals that usually produce signal interference in sensors and sensor circuits. See electromagnetic interference (EMI).
Normal Mode Rejection (NMR)
The ability of an electrical device to filter out noise superimposed on the signal and applied across the SIG HI to SIG LO input terminals. Normally expressed in dB at 50/60 Hz.
The difference in temperature between the setpoint and the actual process temperature. Also referred to as a "droop."
Ohm's Law
Current in a circuit is directly proportional to the voltage, and inversely proportional to resistance; stated as: E = IR Where: I = Amps, I = E/R, E = Volts, R = E/I, R= Ohms The complete Ohm's Law also includes the relationships of watts to amps, volts and ohms.
On/Off control
A method of temperature control in which the controller acts as a switch, turning the final control element either "on" or "off," depending on the value of the setpoint.
Open loop control
A control system with no feedback signal from the sensor.
Optical Isolation
Two electronic networks that are connected through an LED (light emitting diode) and a photoelectric receiver. There is no electrical continuity between the two networks.
Occupational Safety and Health Act. Also the Occupational Safety and Health Administration, the United States governmental agency that establishes and enforces safety standards in the workplace.
Control signal action in response to the difference between set point and process variable.
Output Type
The form of control output, such as time proportioning, distributed zero crossing, serial digital-to-analog converter or analog. Also the description of the electrical hardware that makes up the output, such as relay, voltage pulse, or analog.
The amount by which a process variable exceeds the setpoint before stabilizing. The opposite condition is called "undershoot."
The time-based relationship between alternating current cycles and a fixed reference point. In electricity, it is usually expressed in angular degrees. It describes the relationships of voltage and current of two or more alternating waveforms.
Phase-Angle Firing
A mode of power control in silicon controlled rectifiers (SCRs). Phase-angle firing varies the point at which the SCR switches voltage inside the AC Sine wave.
PID (Proportional, Integral, Derivative) control
A three-mode control action in which the controller has proportioning, integral (reset), and derivative (rate) action. Proportional action dampens the system response; integral corrects the offset; and derivative prevents over- and undershoot.
The closeness of a setting, indication, calibration, or control of a measurement device to the actual value of the quantity being measured. Usually expressed as a percentage of full scale.
Proportioning band
The area around setpoint where the control output is neither fully on nor fully off for the entire time cycle.
A programmed rate of rise or decline in the setpoint, and hence, the process variable.
The area between two limits in which a quantity or value is measured. It is usually described in terms of lower and upper limits.
Rate time
See Derivative.
Reference junction
The junction in a thermocouple circuit which is held at a stable known temperature (cold junction). The standard reference temperature is 0ºC (32ºF). However, other temperatures can be used.
Relative Humidity (RH)
The moisture content of air, in relation to the maximum it can contain at that same pressure and temperature.
Relay - Mechanical
An electromagnetic device that completes or interrupts a circuit by physically moving electrical contacts into contact with each other.
Relay - Mercury Displacement (MDR)
A power switching device in which mercury, displaced by a plunger, completes the electric circuit across contacts.
Relay - Solid-State (SSR)
A solid-state switching device which completes or interrupts a circuit electrically with no moving parts.
A device that accepts a signal from a master controller.
Remote setpoint
A feature that allows setpoint setting from a remote location using an analog signal.
The precision of a measured value. Increased resolution does not necessarily mean increased accuracy.
See Analog Transmission
A European Union directive aimed at restricting the use of certain hazardous substances commonly found in electrical and electronic equipment. A product that is RoHS compliant has none (or very little) of these hazardous substances. For more information on RoHS, please click here.
Resistance Temperature Detector.
Sampling rate
The rate at which input data is polled for information.
SCR (Silicon Controlled Rectifier)
A solid-state device that controls AC voltages to a load within one cycle, by zero-cross or phase-angle firing.
The desired value of a controlled variable.
To Maintain the process variable at a desired value.
Temperature Calibration Point
A temperature at which the output of a sensor is compared against a standard.
Temperature Limit Switch
Factory Mutual (FM) Standard 3545. See Limit Controller.
Temperature, Ambient
The temperature of the air or other medium that surrounds the components of a thermal system.
A temperature-sensitive passive semiconductor which exhibits a large change in electrical resistance when subjected to a small change in temperature, usually with a negative temperature coefficient.
The junction of two dissimilar metals which has a voltage output proportional to the difference in temperature between the hot junction and the lead wires connected to the sensing device (cold junction).
Thermocouple Break Protection
The ability of a control to detect a break in the thermocouple circuit and take a predetermined action.
Thermocouple Extension Wire
A pair of wires connecting a thermocouple sensor to its reference junction or instrumentation. The electromotive force (EMF) characteristics of the extension wire must be similar to the EMF characteristics of the thermocouple.
A primary device that translates a process measurement into a current/voltage signal for long-distance transmission.
A solid-state device used to switch alternating currents.
Underwriters Laboratories, Inc. An independent laboratory that establishes standards for commercial and industrial products.
The amount by which a process variable falls below the setpoint before stabilizing.
Ungrounded thermocouple
A thermocouple construction in which the measuring junction is enclosed in a protective layer and electrically isolated from the sheath.
Upscale Break Protection
A form of break detection for burned-out thermocouples. Signals the operator that the thermocouple has burned out.
Volt Amperes (VA)
A measurement of apparent power. The product of voltage and current in a reactive circuit. V (voltage) ·I (current) = VA. The term watt is used for real power.
Voltage (V)
The difference in electrical potential between two points in a circuit. It's the push or pressure behind current flow through a circuit. One volt (V) is the difference in potential required to move one coulomb of charge between two points in a circuit, consuming one joule of energy. In other words, one volt (V) is equal to one ampere of current (I) flowing through one ohm of resistance (R), or V = IR.
Watt (W)
A measurement of real power. The product of voltage and current in a resistive circuit. V (voltage) ·I (current) = P (power in watts).
Zero Cross
Action that provides output switching only at or near the zero-voltage crossing points of the AC sine wave. See Burst Fire.
Zero Switching
See Zero Cross.


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