Glossary of Terms: F

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The temperature scale that sets the freezing point of water at 32°F and its boiling point at 212°F at standard atmospheric pressure. The formula for conversion to Celsius is: °C = 5/9 (°F - 32°F).
Failsafe Output
Output value where device goes to protect against effects of failure of the equipment, such as fuel shutoff in the event of loss of flame in a furnace or a sensor break.
Fall Time
Measure of the time required for the output voltage of a circuit to change from a high voltage level to a low voltage level, once a level change has started (90% to 10%).
False Pulse
Improper change of state of the output, usually associated with Turn-Off or Turn-On.
False Pulse Protection
Circuitry designed to clamp output Off until the power supply has time to reach proper voltage level. Typically 200-500 msec.
Unit of capacitance.
Fast Fourier Transfer (FFT)
An analysis algorithm - given a finite set of data points, FFT expresses the data in terms of its component frequencies.
Feedback Control
Error-driven control system where control signal to actuators is proportional to difference between a command signal and a feedback signal from the process variable being controlled.
Feedforward Control
Method of control that compensates for disturbance before its effect in output. Based on a model that relates output to input where disturbance occurs. In distillation, the disturbances are usually feed rate and feed compositions. Steady-state feedforward models usually combine with dynamic compensation functions to set manipulative variables and combined with feedback adjustment (trim) to correct for control model accuracy constraints.
Fiber Optics
Transparent fibers of glass or plastic used for conducting and guiding light energy. Fiber optics are used in photoelectrics as light pipes, consisting of a bundle of small optical fibers (glass) or single strand (plastic) housed inside a flexible sheathing.
FIFO buffer
First in, first out storage method. The first value placed in the buffer (queue) is the first value subsequently read.
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.
Attenuating components of a signal that are undesired. Reduce noise errors in a signal.
First-Order Lag
Term describing signal-delaying and signal-size-changing effects of a pad of control loop. Name comes from the form of the equation that represents the relation between output and input. Effects evaluated by a time constant.
Fixed Sensing Distance
Capacitive sensors with a fixed sensing distance do not have a potentiometer for sensitivity adjustment.
The motion of liquids or gases in response to a force.
Flush Mounting
See Shielded
FM Approved
An instrument that meets a set of specifications established by the Factory Mutual Research Corporation.
Force Differential
Difference between the operating force and the release force.
Force, Contact
The force holding closed contacts together.
Foreground Suppression
The ability of a special set of photo electric sensors to detect targets irrespective of color and surface properties provided the background is defined. Sensors with foreground capabilities exhibit no blind range.
Fork and Angle Sensors
Emitter and receiver are integrated in a fork or angle housing facing each other. These one-piece units require none of the difficult alignment and wiring tasks of their two-piece counterparts.
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.
Frequency, Natural
The frequency of free (not forced) oscillations of the sensing element of a fully assembled sensor.
Frequency Counter
Counts digital pulses over a defined gate time. A typical gate time is between 0.1 and 10 seconds.
Frequency Output
An output in the form of frequency, which varies as a function of the applied pressure.
Frequency Response
Response of a component, instrument, or control system to input signals at varying frequencies.
Front Panel
The front surface of a unit, generally containing switches and indicator lights.
Full Overtravel Force
Force required to depress the plunger of a switch to the full overtravel point.
Full Overtravel Point
The position of the plunger beyond which further overtravel would cause damage to the switch or actuator.
Full Scale Output (Span)
(1) Algebraic difference between output curve end points (outputs at specified upper and lower output limits). (2) The difference between the minimum output (normally zero) of a data acquisition device and the rated capacity.
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