Process Control Common Terms

Quiescent Supply Current
-the supply current being drawn when the pressure sensor is at null.

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Ratiometric (Ratiometricity Error)
-at a given supply voltage, sensor output is a proportion of that supply voltage. Ratiometricity error is the change in this proportion resulting from any change to the supply voltage. Usually expressed as a percent of full scale output.

- 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.

-the measurand values over which the sensor is intended to measure, specified by the upper and lower limits.

-a device that converts alternating current into direct current.

Reed Technology
-technology where the reed contacts are designed to be actuated by a magnet. When a magnetic field is brought close to the reed contacts, the contacts are drawn together to make the circuit.

Reflective Scan
-a scanning technique in which the light source is aimed at a reflective surface to illuminate the photosensor. Retroreflective, specular, diffuse scan and convergent beam are all reflective scan techniques.

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.

Regulation %
-the ratio of voltage extremes due to loading or line fluctuations. The process of holding constant a quantity such as voltage by means of a system that automatically corrects errors. For example, as more current is drawn from a battery or power supply, the output voltage tends to decrease (load regulation). With a power supply derived from AC, the DC output voltage can vary with the variation in AC voltage (line regulation).

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.

Release Force
-the level to which force on the plunger must be reduced to allow the contacts to snap from the operated contact position to the normal contact position.

Release Point
-that position of the plunger at which the contacts snap from the operated contact position to the normal contact position.

Release Travel
-as an operating characteristic of a switch, release travel is the distance through which the plunger moves when traveled from the release point to the free position. As a characteristic of the actuation applied to the switch, release travel is the distance the plunger is released past the release point.

-the ability of a sensor to reproduce output readings when the same value is applied to it consecutively in the same direction, for a specified number of cycles, or specified time duration.

-the magnitude of output step changes as the pressure is continuously varied over the range. This term applies primarily to potentiometric sensors. Resolution is best specified as average and maximum resolution. Usually expressed in percent of full scale output.

Response Time
-the time it takes for a device to respond to an input signal. The sum of the sensor, amplifier, and output response is the total response time.

Retroreflective Scan
-the reflective scan technique that uses a special reflector (retroreflector) to return light along the same path it was sent.

Reverse Polarity Protection
-circuitry, usually a diode which prevents current from flowing into the control in case of accidental mis-wiring of the plus (+) or minus (-) terminals, preventing damage to the unit.

-the alternating component of voltage from a rectifier or generator. A slight fluctuation in the intensity of a steady current.

Rise Time
-a measure (10% to 90%) of the time required for an output voltage to rise from a state of low voltage to a high voltage level, once a level change has started.

Room Conditions
-ambient environmental conditions under which sensors must commonly operate, which have been established as follows: (a) Temperature: 25 + -10 %C (77 + - 18 degrees F). (b) Relative humidity: 90% or less. (c) Barometric pressure: 26 to 32 inches Hg. Note: Tolerances closer than shown are frequently specified for sensor calibration and test environments.

- 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.

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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.

Saturation Voltage
-the voltage drop appearing across a control device that is fully turned On.

Scan Technique
-a method of scanning objects. The two general categories are through and reflective scan.

Self-Contained Control
-a photoelectric control in which all three phases of control - sensing, signal conditioning, and output - occur in a single device.

Self-Contained Sensor
-a proximity sensor in which all three phases of control, sensing, signal conditioning, and output, occur in a single device.

Sensing Distance
-the maximum recommended distance between the sensor and standard target at which sensor will effectively and reliably detect the target.

Sensing Element
-that part of a sensor which responds directly to changes in input pressure.

-maximum recommended distance between the sensor and standard target at which sensor will effectively and reliably detect the target.

Sensitivity Shift
-a change in sensitivity resulting from an environmental change such as temperature.

-a sensing element. The basic element that usually changes some physical parameter to an electrical signal.

Series Circuit
-a circuit in which current has only one path to follow.

- The desired value of a controlled variable

Shielded Sensor
-a sensor which "senses" only to the front of its face and ignores metals to its side. The presence of such side metal, however, may cause a slight shift in operating characteristics.

Signal Conditioning
-to process the form or mode of a signal so as to make it intelligible to or compatible with, a given device, including such manipulation as pulse shaping, pulse clipping, digitizing, and linearizing.

Signal Ratio
-1) broadly, the comparison of light seen by a photosensor when the beam is blocked to the light seen when the beam is not blocked; 2) More specifically, the comparison of photocell resistance when sensor is dark to when it is illuminated. Proper control application involves establishing a large dark-to-light ratio.

Single-Pole Double-Throw (SPDT)
-switch which may either make or break a circuit, depending on how it is wired.

Single-Pole Single-Throw (SPST)
-switch with only one moving and one stationary contact. Available either normally open (N.O.) or normally closed (N.C.).

Shock, Mechanical
-a short pulse of acceleration, usually lasting only a few milliseconds. A typical shock test pulse is a half-sine acceleration wave having 100g peak and .007 second duration.

-the condition whereby the target approaches the sensing face of the proximity sensor in such a direction that its center will cross the axis of the sensing face at right angles.

Slight Contamination
-indoor locations, non-industrial areas, office buildings.

Snap Action
-in strict terms, snap action is a property of a switch such that the moving contact accelerates without added travel of the plunger beyond that travel which was required to separate the contacts. National Electrical Manufacturers Association defines snap action as "a rapid motion of the contacts from one position to another position, or their return. This action is relatively independent of the rate of travel of the actuator." The word "relatively" is important. In actual fact, the acceleration of the moving contact is partially dependent upon the velocity of the plunger. The important point is that, once the plunger reaches the operating or release point, the movable contact immediately transfers to its opposite position without further travel of the plunger. A non-snap acting switch lacks this feature.

- To Maintain the process variable at a desired value

-the algebraic difference between limits of the pressure range.

Specular Scan
-a reflective scan technique in which reflection from a shiny surface illuminates the photosensor, which must be precisely positioned to receive the reflected light. The angle of incidence equals the angle of reflection.

Stability of an Operating Characteristic
-the extent to which an operating characteristic such as operating point remains constant during a specified number of cycles of switch operation, under specified conditions of actuation, electrical loading and environment. Most clearly expressed as a graph of the characteristic versus cycles of switch operation.

Standard Target
-an object used for making comparative measurements of operating distance. A square of mild steel, 1mm thick. The length of the side of the square is equal to either: A: the diameter of the circle inscribed on the active surface of the sensitive face of the sensor, or B: three times the rated operating distance, whichever is the greater.

Storage Temperature Range
-the minimum and maximum specified temperature which may be applied to the pressure sensor without causing a permanent change in the output characteristics.

Strain Gage
-a sensing device providing a change in electrical resistance proportional to the level of applied stress.

-the change of state of a materiel from solid to vapor and back to solid without going through a liquid state.

Supply Current
-units = Amps or milliamps. The amount of current necessary to maintain operation of a photoelectric control, proximity sensor or control base. Sometimes referred to as Current Consumption.

Supply Voltage
-units + Volts. The range of power required to maintain proper operation of a photoelectric control, proximity sensor or control base. The difference in potential (or range of difference in potential) necessary to operate the unit.

Switching Frequency
-the actual number of targets to which the sensor can respond in a given time period, usually expressed as Hertz (cycles per second).

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-the part or piece being detected.

Terminal Base Linearity
-T.B.L. (End Point Linearity) - a method of defining linearity. The maximum deviation of any data point on a sensor output curve from a straight line drawn between the end data points on that output curve. (T.B.L. is approximately twice the magnitude of B.F.S.L.).

Terminal Line
-a theoretical slope for which the theoretical end points are normalized at 0 and 100% of both measurand and output. Interchangeability error is referenced to this line.

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

Temperature Error
-the maximum change in output, at any input pressure within the specified range, resulting from a change in temperature.

Thermal Drift Chart
-a chart illustrating sensor operating variance due to changes in temperature.

-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

-technology using silk screened pastes to form conductor, resistor, themistors, and insulator patterns; screened onto the substrate (usually ceramic) and cured by firing at elevated temperatures.

Thin Film
-a technology using vacuum deposition of conductors and dielectric materials onto a substrate (frequently silicon) to form an electrical circuit.

Threshold Response
-a control type that responds to the change in input signal level. Plug-in amplifiers are either threshold or transition responsive.

-the number of circuits that each individual pole of a switch can control. The number of throws is completely independent of the number of poles and number of breaks. A single-pole double-throw single-break switch connects the common terminal of the switch to the normally closed terminal when the plunger is free, but connects the common terminal to the normally open terminal when the plunger is depressed. A single-pole single-throw single-break switch has a common terminal and either a normally open terminal or a normally closed terminal but not both.

Thru Scan
-a scanning technique in which the emitter (light source) is aimed directly at the receiver. Also called direct scan and transmitted scan, since light is transmitted directly, not reflected to the sensor. Presently, it is the only scanning technique commonly used to scan distances greater than 40 feet.

Time Delay Before Availability
-also know as False Pulse Protection. Outputs are turned Off when power is first applied during this time period.

Total Travel
-the distance from the plunger free position to the full overtravel point.

Transient Protection
-circuitry to guard against spikes induced on the supply lines by inductive sources such as heavy motors or solenoids turning On and Off.

-in electronic usage, usually refers to an unwanted, temporary, large increase or decrease in a current or supply voltage that only occurs occasionally. Almost always due to reactive components during rapid changes in voltage or current.

Transition Responsive
-a control type that responds to the rate of change in light intensity rather than the level change. Used to detect fast moving objects that cause little change in light intensity level.

-allows light to pass through. Detecting translucent objects is often best done with retroreflective scan, during which the light must pass through the object twice, thereby causing more of a signal change (larger signal ratio).

-a transducer with a current loop output, typically 4 to 20 mA, enabling transmission of a signal over a longer distance.

-a generic term for Transistor Logic which is used extensively in digital electronics systems.

TTL Compatibility
-TTL (transistor-transistor-logic) requires NPN (current sinking) input signals. Reliable operation demands maximum input sensor voltage drop of 0.8 V. Most TTL compatible interface devices have voltage drops of less than 0.7V.

-(as used herein): refers to the target value or where a range is given, represents an estimate of where 2/3 of the total population of several production runs would be.

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-Underwriter's Laboratories, Inc., a non-profit organization that establishes, maintains and operates laboratories for the examination and testing of devices, systems and materials primarily for safety.

- The amount by which a process variable falls below the setpoint before stabilizing.

Unidirectional Differential Pressure Sensor
-a differential pressure sensor requiring the greater input pressure to be applied to a specified pressure port.

Unshielded Sensor
-a sensor with limited side and front sensing capabilities.

Usable Sensing Distance
-sensing distance after temperature range tolerance and manufacturers tolerance are taken into account.

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-the rate at which the position of a moving object is changing. Velocity has two characteristics: magnitude (speed) and direction.

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.

Voltage Drop
-units = Volts (DC) or Volts RMS (AC). Sometimes referred to as Saturation Voltage. In any solid state control that switches a load, there will be some voltage dropped across the output. This voltage drop or saturation voltage will often vary with the amount of current going through the output section and the load. It should be specified with current conditions.

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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).

Welding, Contact -one of several conditions that may cause switch contacts to fail to separate at the intended point of plunger travel. As the name implies, the contacts literally are welded together as a result of the electrical and thermal effects at the contact interface.

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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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