Glossary of Terms: L

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Lag
Relative measure of time delay between two events, states, or mechanisms. The delay between the output of a signal and the response of the instrument to which the signal is sent.
Lag Time
The amount of time after the dead time that the process variable takes to move 63.3% of its final value after a step change in valve position.
LAN
Local area network. A data communication system connecting devices in the same vicinity. Data is transferred without the use of public communications. Examples of LANs are Ethernet, token ring and Modbus.
Latching Logic
Signal modification that causes the output to be energized and remain energized (maintain output). Latched output may be immediate or delayed. Usually, the latch is released by closing a circuit between the reset (RS/D) terminal and ground.
Latching Relay
Device or program element that retains a changed state without power.
lbf
One pound of force.
lbm
One pound of mass. In standard earth gravity a one pound mass weighs one pound of force. On the moon's surface the same one pound mass would only weigh 1/6th of a pound of force.
LCP
Liquid Crystal Polymer
Leakage
A low resistance path in materials that are normally insulators.
Leakage Current
The current which flows through an output in the "Off" state due to semiconductor characteristics. Normally only considered for two-wire devices.
Leakage Rate
Maximum rate at which a fluid is permitted or determined to leak through a seal. The type of fluid, the deferential pressure across the seal, the direction of leakage, and the location of the seal must be specified.
Least Squares Line
Straight line for which the sum of the squares of the residuals (deviations) is minimized. This method is used to calculate best fit straight line (BFSL) linearity.
LIFO
Last in, first out. Describes a stack method of data storage where the last data captured is first read.
Light Emitting Diode (LED)
A a solid state electronic device that glows when electric current passes through it.
Limit Control
Sensing device that shuts down an operation or terminates a process step when a prescribed limiting condition is reached.
Limit Controller
A highly reliable, discrete safety device (redundant to the primary controller) that monitors and limits a point in the process. When that point exceeds or falls below the limit setpoint, the controller interrupts power through the load circuit.
Linear (Output)
Output that is a continuous amplified version of its input. That is, the output is a predetermined variation of its input.
Linear Output
An output which provides a straight-line relation between output signal (voltage or current) and target movement.
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.
Linearity
Closeness of a calibration curve to a specified straight line. Expressed as maximum deviation of any calibration point on a specified straight line, during any one calibration cycle.
Linearity Error
Deviation of the sensor output curve from a specified straight line. Linearity error is usually expressed as a percent of full scale output.
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.
Liquid Level Sensors
Liquid levels can be measured simply and accurately using infrared light, without the need for any electrical or thermal connection between the target medium and sensor.
Load
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.
Load Cell
A transducer that converts a force into an electrical signal. It normally comprises four strain gauges in a Wheatstone bridge arrangement.
Load Current
The maximum amount of current that a proximity sensor will switch through its load. Load current for a particular device can be calculated by dividing the load voltage by the load resistance. Attempting to switch a higher load current than the sensor is rated for will result in sensor failure. Units = Amps/milliamps (DC) or Amps RMS/milliamps RMS (AC).
Load Impedance
Measure of opposition to a sinusoidal alternating electric current presented to the output terminals of a sensor by the associated external circuitry.
Load Upset
An upset to the process created by something other than changing the setpoint.
Local Automatic Mode
Mode in which the controller operates from local setpoint and automatically adjusts output to maintain setpoint at desired value.
Local Setpoint
Setpoint determined by controller or recorder.
Lockout
Condition that prevents unauthorized configuration or calibration changes.
Logic
Modification of an input signal that produces delayed, pulsed, latched, or other output response. Logic circuitry is sometimes an integral part of the control, but more often, a separate plug-in card or module.
Logic Gate
Used in solving math problems through repeated use of simple functions that define basic concepts (i.e., AND, OR, NOR, etc.). A logic gate performs a logical operation on one or more logic inputs and produces a single logic output. The logic normally performed is Boolean logic and is most commonly found in digital circuits. Logic gates are primarily implemented electronically using diodes or transistors, but can also be constructed using electromagnetic relays, fluidics, optics, or even mechanical elements.
Low Energy Circuit
Qualitative term having no exact numeric definition. It usually refers to a circuit having such low voltage and current that there are no significant thermal effects at the contact interface.
Low Limit
Control shuts down system if a condition drops below minimum value for safe operation.
Low Output Limit
Lowest value of output below which a controller's automatic output cannot exceed.
Low Pass Filter
A filter that lets through the lower frequencies and attenuates (reduces) frequencies higher than the cutoff value. Choose the cutoff frequency to be compatible with the unwanted frequencies, the frequencies present in the signal you are measuring, and the sampling rate of the analog-to-digital converter.
Low Reset Limit
Lowest value of output beyond which no reset should occur.
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