I - L

illuminance Luminous flux per unit area over a uniformly illuminated surface.

immersion length Of a thermometer, the distance along the thermometer body from the boundary of the medium whose temperature is being determined to the free end of the well, bulb or element, if unprotected.

impedance The complex ratio of a forcelike parameter to a related velocity like parameter - for instance, force to velocity, pressure volume velocity, electric voltage to current, temperature to heat flow, or electric field strength to magnetic field strength.

Impedance, input 1. The impedance (presented to the excitation source) measured across the excitation terminals of a transducer. 2. Impedance presented by a device to the source. 3. The impedance presented by a device or system output element to the input.

impedance, load Impedance presented to the output of a device by the load.

impedance, output 1. The impedance across the output terminals of a transducer presented by the transducer to the associated external circuitry. 2. Impedance presented by a device to the load . 3. The internal impedance of an output element which limits that element's ability to deliver power.

impedance, source Impedance presented to the input of a device by the source.

inches water gauge (wig.) Usual term for expressing a measurement of relatively low pressures or differentials by means of a U tube. One inch w.g. equals 5.2 lb per sq fit or 0.036 lb per sq in.

indined tube manometer A glass tube manometer having one leg inclined from the vertical to give more precise readings.

incomplete combustion The partial oxidation of the combustible constituents of a fuel.

incompressible Liquids are referred to as being incompressible since their change in volume due to pressure is negligible.

Inconel A series of International Nickel Co. high nickel, chromium and iron alloys characterised by inertness to certain corrosive fluids.

increased safety A type of protection by which measures are applied so as to reduce the probability of excessive temperatures and of the occurrence of arcs or sparks in the interior and on the external parts of electrical apparatus which does not produce them in normal service and which is intended for use in hazardous locations defined by the IEC as Zone 1. Referred to by IEC as type "Ex e" protection.

independent variable 1. A process or control system parameter that can change only due to external stimulus. 2. A parameter whose variations, intentional or unintentional, induce changes in other parameters according to predetermined relationships.

induced draft Airflow through a device such as a firebox or drying unit which is produced by placing a fan or suction jets in the exit duct.

inductance 1. In an electrical circuit, the property that tends to oppose changes in current magnitude or direction. 2. In electromagnetic devices, generating electromotive force in a conductor by means of relative motion between the conductor and a magnetic field such that the conductor cuts magnetic lines of force.

induction heating Raising the temperature of an electrically conductive material by electromagnetically inducing eddy currents in the material.

infrared Any electromagnetic wave whose wavelength is 0.78 to 300 Zm.

infrared absorption moisture detector An instrument for determining moisture content of a material such as sheet paper; moisture content can be read directly by determining the ratio of two beam intensities, one at a wavelength within the resonant absorption band for water and the other at a wavelength just outside the band.

infrared spectroscopy A technique for determining the molecular species present in a material, and measuring their concentrations, by detecting the characteristic wavelengths at which the material absorbs infrared energy and measuring the relative drop in intensity associated with each absorption band.

inherent flow characteristic The relationship between the flow rate through a valve and the travel of the closure member as the closure member is moved from the closed position to rated travel with constant pressure drop across the valve

integer programming 1. In operations research, a class of procedures for locating the maximum or minimum of a function subject to constraints, where some or all variables must have integer values.

integral 1. This control action will cause the output signal to change according to the summation of the input signal values sampled at regular intervals up to the present time. 2. Mathematically it is the reciprocal of reset.

integral absolute error (IAE) A measure of controller error defined by the integral of the absolute value of a time dependent error function; used in tuning automatic controllers to respond properly to process transients.

integral action A type of controller function where the output (control) signal or action is a time integral of the input (sensor) signal.

integral action rate (reset rate) 1. Of proportional-plus-integral or proportional-plus-integral plus-derivative control action devices; for a step input, the ratio of the initial rate of change of output due to integral control action to the change in steady state output due to proportional control action. NOTE: Integral action rate is often expressed as the number of repeats per minute because it is equal to the number of times per minute that the proportional response to a step input is repeated by the initial integral response. 2. Of integral control action devices; for a step input, the ratio of the initial rate of change of output to the input change.

integral control Form of control action that returns the value of the controlled variable to the set point when sustained offset occurs without this action. Also called "reset control.

integral control action (reset) Control action in which the output is proportional to the time integral of the error input, i.e., the rate of change of output is proportional to the error input.

integral orifice A differential pressure measuring technique for small flow rates in which the fluid flows through a miniature orifice plate integral with a special flow fitting.

integral time absolute error (ITAE) A measure of controller error defined by the integral of the product of time and the absolute value of a time dependent error function; whereas the absolute value prevents opposite excursions in the process variable from canceling each other, the multiplication by time places a more severe penalty on sustained transients.

Integrated Systems Digital Network (ISDN) A suite of protocols being defined by CCITT to provide voice and data services over wide area networks (WANs).

interacting control Control action produced by an algorithm whose various terms are interdependent.

interaction A phenomena, characteristic of a multivariable process, in which the effect of a manipulative variable change in one control loop not only affects its own controlled variable, but also the controlled variable in another loop.

intercooler A heat exchanger in the path of fluid flow between stages of a compressor to cool the fluid and allow it to be further compressed at lower power demand.

interference, common mode A form of interference which appears between measuring circuit terminals and ground.

interference, electromagnetic Any spurious effect produced in the circuits or elements of a device by external electromagnetic fields. NOTE: A special case of interference from radio transmitters is known as "radio frequency interference (RFI)"

interference, normal-mode A form of interference which appears between measuring circuit terminals.

interference pattern The pattern of some characteristic of a stationary wave produced by superimposing one wave train on another.

interlock 1. To arrange the control of machines or devices so that their operation is interdependent in order to assure their proper coordination. 2. Instrument which will not allow one part of a process to function unless another part is functioning. 3. A device such as a switch that prevents a piece of equipment from operating when a hazard exists.

intermodulation The modulation of the components of a complex wave by each other, producing new waves whose frequencies are equal to the sums and differences of integral multiples of the component frequencies of the original complex wave.

intermodulation distortion (IMD) 1. Distortion caused by interaction of two or more signals. 2. Defined as 20 log (rms sum of the sum and difference distortion products)/(rms amplitude of the fundamental).

intrinsically safe circuit A circuit in which no spark nor any thermal effect produced under prescribed test conditions (which include normal operation and specified fault conditions) is capable of causing ignition of a given explosive atmosphere.

intrinsic safety 1. A type of protection in which a portion of the electrical system contains only intrinsically safe equipment (apparatus, circuits, and wiring) that is incapable of causing ignition in the surrounding atmosphere. No single device or wiring is intrinsically safe by itself (except for battery-operated self-contained apparatus such as portable pagers, transceivers, gas detectors, etc., which are specifically designed as intrinsically safe self-contained devices) but is intrinsically safe only when employed in a properly designed intrinsically safe system. This type of protection is referred to by IEC as "Ex I." see also "associated equipment (apparatus). 2. Design methodology for a circuit or an assembly of circuits in which any spark or thermal effect produced under normal operating and specified fault conditions is not capable under prescribed test conditions of causing ignition of a given explosive atmosphere. 3. A method to provide safe operation of electric process control instrumentation where hazardous atmospheres exist. The method keeps the available electrical energy so low that ignition of the hazardous atmosphere cannot occur. 4. A protection technique based upon the restriction of electrical energy within apparatus and of interconnecting wiring, exposed to a potentially explosive atmosphere, to a level below that which can cause ignition by either sparking or heating effects. Because of the method by which intrinsic safety is achieved, it is necessary to ensure that not only the electrical apparatus exposed to the potentially explosive atmosphere but also other electrical apparatus with which it is interconnected is suitably constructed.

intrinsic safety barrier 1. A component containing a network designed to limit the energy (voltage and current) available to the protected circuit in the hazardous (classified) location, under specified fault conditions. 2. A device inserted in wire between process control instrumentation and the point where the wire passes into the hazardous area. It limits the voltage and current on the wire to safe levels.

intrinsic safety ground bus A grounding system which has a dedicated conductor separate from the power system so that ground currents will not normally flow and which is reliably connected to a ground electrode in accordance with the applicable standard.

inversion temperature In a thermocouple, the temperature of the "hot" junction when the thermoelectric emf of the circuit is equal to zero.

ion exchange A chemical process for removing unwanted dissolved ions from water by inducing an ion exchange reaction (either cation or anion) as the water passes through a bed of special resin containing the substitute ion.

ion exchange resin A synthetic organic compound (resin) that can remove unwanted ions from a dilute solution by combining with them or by exchanging them for ions that produce desirable or neutral effects.

ionisation The process of splitting a neutral molecule into positive and negative ions, or of detaching one or more electrons from a neutral atom.

isentropic Proceeding at constant entropy.

isentropic exponent A ratio defined by the specific heat at constant pressure divided by the specific heat at constant volume.

isobaric Proceeding at constant pressure.

isotope Any of two or more nuclides that have the same number of protons in their nuclei but different numbers of neutrons; such atoms are of the same element, and thus cannot be separated from each other by chemical means, but because they have different masses can be separated by physical means.

iterate To repeatedly execute a loop or series of steps. For example, a loop in a routine. iterative

jacketed valve A valve body cast with a double wall or provided with a double wall by welding material around the body so as to form a passage for a heating or cooling medium. Also refers to valves which are enclosed in split metal jackets having internal heat passageways or electric heaters. Also referred to as "steam jacketed" or "vacuum jacketed. " In a vacuum jacketed valve, a vacuum is created in the space between the body and secondary outer wall to reduce the transfer of heat by convection from the atmosphere to the internal process fluid, usually cryogenic.

Kalman filter A technique for calculating the optimum estimates of process variables in the presence of noise; the technique, which generates recursion formulas suitable for computer solutions, also can be used to design an optimal controller.

Kernaugh map A tubular arrangement that facilitates the combination and elimination of logical functions by listing similar logical expressions, thereby taking advantage of the human brain's ability to recognise visual patterns to perform the minimisation.

KByte 1024(210) bytes.

kelvin Metric unit for thermodynamic temperature. The Kelvin scale is an absolute temperature scale where the zero point is defined as absolute zero and the scale divisions (called kelvins) are equal to the scale divisions in the Celsius system. 0°C = approximately 273.16 kelvins or K.

Kennison nozzle A specially shaped nozzle designed for measuring flow through partially filled pipes; because of its self-scouring, nonclogging design, it is especially useful for measuring flow of liquids containing suspended solids or debris and at low flow rates or widely varying flow rates.

Keyphasor A sensing device to detect the passage of a point on the rotor. May be a magnetic, capacitive, eddy current, or photoelectric probe. The signal is used as a trigger for the external trigger input of other instruments, such as a vibration monitor.

kilogram Metric unit of mass.

kinematic viscosity Absolute viscosity of a fluid divided by its density.

kinetic energy 1. The energy of a working fluid caused by its motion. 2. Energy related to the fluid of dynamic pressure, 12 pV2.

Knudsen flow Gas flow in a long tube at pressures such that the mean free path of a gas molecule is significantly greater than the tube radius.

ladder diagram Symbolic representation of a control scheme. The power lines form the two sides of a ladder like structure, with the program elements arranged to form the rungs. The basic program elements are contacts and coils as in electromechanical logic systems.

lag 1. A relative measure of the time delay between two events, states, or mechanisms. 2. In control theory, a transfer function term in the form, lI(Ts + 1).

lambert A unit of luminance; it equals the uniform luminance of a perfectly diffusing surface emitting or reflecting light at one lumen per square centimeter.

laminar flow 1. A type of streamline flow most often observed in viscous fluids near solid boundaries, which is characterised by the tendency for fluid to remain in thin, parallel layers to maintain uniform velocity. 2. A nonturbulent flow regime in which the stream filaments glide along the pipe axially with essentially no transverse mixing. Also known as "viscous" or "streamline flow." 3. Flow under conditions in which forces due to viscosity are more significant than forces due to inertia.

Laplace transform For a functions its Laplace transform is the function F(y) defined as the integral over x from 0 to fi of the function e-yxf(x).

LASER Light amplification by stimulated emission of radiation. It is a source of EM radiation generally in the IR, visible, or W bands and is characterised by small divergence, coherence, and monochromaticity.

laser Doppler flowmeter An apparatus for determining flow velocity and velocity profile by measuring the Doppler shift in laser radiation scattered from particles in the moving fluid stream

latency In data processing, the time between the completion of the interpretation of an address and the start of the actual transfer from the addressed location. Latency includes the delay associated with access to storage devices such as disc drives.

latent heat Heat that does not cause a temperature change.

leakage classification Refer to ANSI B16. 104 to categorise seat leakage of control valve trim.

light-emitting diode (LED) A semiconductor diode which emits visible or infrared light. Light from an LED is incoherent spontaneous emission, as distinct from the coherent stimulated emission produced by diode lasers and other types of lasers.

limit cycle A sustained oscillation of finite amplitude.

linearity The closeness of a calibration curve to a specified straight line. Linearity is expressed as the maximum deviation of any calibration point on a specified straight line, during any one calibration cycle. It is expressed as "within + percent of full scale output".

linear programming (LP) A method of solution for problems in which a linear function of a number of variables is subject to a number of constraints in the form of linear inequalities.

linear variable differential transformer (LVDT) A position sensor consisting of a central primary coil and two secondary coils wound on the same core; a moving iron element linked to a mechanical member induces changes in self induction that are directly proportional to movement of the member.

linear variable reluctance transducer (LVRT) A position sensor consisting of a centre tapped coil and an opposing moving coil attached to a linear probe; the winding is continuous over the length of the core, instead of being segmented as in an LVDT.

liquid pressure recovery factor The ratio (F1) of the valve flow coefficient (Cv) based on the pressure drop at the vena contracta, to the usual valve flow coefficient (Cv) which is based on the overall pressure drop across the valve in non vapourising liquid service. These coefficients compare with the orifice metering coefficients of discharge for vena contracta taps and pipe taps, respectively.

Litre Abbreviated 1. The SI unit of volume; it equals 0.001 m3 or 1.057

load cell A transducer for the measurement of force or weight. Action is based on strain gauges mounted within the cell on a force beam.

load impedance The impedance presented to the output terminals of a transducer by the associated external circuitry.

longitudinal redundancy check (LRC) Error detection scheme that consists of a byte where each bit is calculated on the basis of the parity of all the bits in the block that have the same power of two.

longitudinal wave A wave in which the medium is displaced in a direction perpendicular to the wave front at all points along the wave.

loop, closed (feedback loop) A signal path which includes a forward path, a feedback path and a summing point, and forms a closed circuit.

loop gain The product of the gains of all the elements in a loop.

luminance The luminous intensity of any surface in a given direction per unit of projected area in a plane perpendicular to that direction.

luminosity Emissive power with respect to visible radiation.

luminous flux The amount of light passing a given point per unit time.

luminous intensity Luminous flux per unit solid angle.

lux Metric unit of illuminance.