- 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.
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- 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.
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- 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.
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- 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.
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- Latching Relay
- Device or program element that retains a changed state without power.
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- lbf
- One pound of force.
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- 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.
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- LCP
- Liquid Crystal Polymer
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- Leakage
- A low resistance path in materials that are normally
insulators.
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- Leakage Current
- The current which flows through an output in the "Off"
state due to semiconductor characteristics. Normally only
considered for two-wire devices.
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- 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.
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- 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.
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- LIFO
- Last in, first out. Describes a stack method of data storage where the last
data captured is first read.
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- Light Emitting
Diode (LED)
- A a solid state electronic device that glows when
electric current passes through it.
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- Limit Control
- Sensing device that shuts down an operation or terminates a process step
when a prescribed limiting condition is reached.
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- 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.
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- Linear (Output)
- Output that is a continuous amplified version of its input. That is, the
output is a predetermined variation of its input.
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- Linear Output
- An output which provides a straight-line relation
between output signal (voltage or current) and target
movement.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- Load Cell
- A transducer that converts a force into an electrical signal. It normally
comprises four strain gauges in a Wheatstone bridge arrangement.
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- 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).
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- Load Impedance
- Measure of opposition to a sinusoidal alternating electric current presented
to the output terminals of a sensor by the associated external circuitry.
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- Load Upset
- An upset to the process created by something other than changing the
setpoint.
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- Local Automatic Mode
- Mode in which the controller operates from local setpoint and automatically
adjusts output to maintain setpoint at desired value.
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- Local Setpoint
- Setpoint determined by controller or recorder.
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- Lockout
- Condition that prevents unauthorized configuration or calibration changes.
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- 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.
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- 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.
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- 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.
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- Low Limit
- Control shuts down system if a condition drops below minimum value for safe
operation.
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- Low Output Limit
- Lowest value of output below which a controller's automatic output cannot
exceed.
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- 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.
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- Low Reset Limit
- Lowest value of output beyond which no reset should occur.
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