ACQUISITION RATE. -- The rate at which the board acquires analog or digital data from an external signal input to the board. In the case of a scanning A/D, this is the aggregate conversion rate for all channels. (Number of channels times the sample rate per channel.)
ABSOLUTE ZERO -- The temperature at which substances possess minimal energy. Absolute zero is 0° Kelvin or 0° Rankine and is estimated to be -459.67° F (-273.15° C).
AC -- See alternating current.
AC LINE FREQUENCY -- The frequency of the alternating current power line measured in Hertz (Hz), usually 50 or 60Hz.
ACTUAL -- The present value of the controlled variable.
ACTUATION FORCE -- The amount of force required to complete the contact switching action. When this term is applied to thermostats, it refers to the amount of force generated by the bimetal disc, which opens or closes the contacts.
A/D. -- Abbreviation for analog-to-digital.
A/D (ANALOG-TO-DIGITAL) CONVERTER. -- An electronic device, often an integrated circuit, that converts an analog voltage to a digital value. All digital instruments use an A/D converter to convert the input signal into digital information.
ADDRESS -- A numerical identifier for a controller when used in computer communications.
ADMITTANCE.-- The reciprocal of the impedance. The admittance is the complex ratio of the current flowing through divided by the voltage across a device, circuit element, or network.
ACTUATION TIME. -- The time between application of the nominal relay coil voltage and the final closure of the relay contacts after the contact bounce interval. Also called operate time.
ALARM-- A signal that indicates that the process has exceeded or fallen below the set or limit point.
ALPHA (A)-- The temperature coefficient of the change in electrical resistance of a material measured in ohms/ohm/¼C. It indicates the basic change in electrical resistance in a material for each ¼C in temperature. Alpha is a defining parameter for resistance temperature detectors (RTDs). For example, common alphas for platinum RTDs are 0.00385 //¼C (DIN) or 0.003916 //¼C (JIS).
ALTERNATING CURRENT(~) -- An electric current that reverses at regular intervals, and alternates positive and negative values. (AC)
ALIASING. --Where the sampling rate is less than twice the input signal's highest frequency content.
AMERICAN WIRE GAUGE (AWG) --A standard of the dimensional. characteristics of wire used to conduct electrical current or signals. AWG is identical to the Brown and Sharpe (B & S) wire gauge.
AMMETER -- An instrument that measures the magnitude of an electric current in amperes.
AMPERE (Amp) -- A unit that defines the rate of flow of electricity (current) in a circuit. Units are one coulomb (6.25 x 1018 electrons) per second.
ANALOG -- A way to represent data by means of continuously variable quantities. Analog Output A continuously variable signal that is used to represent a value, such as the process value or set point value. Typical hardware configurations are 0-20mA, 4-20mA or 0-5VDC.
ANALOG RAMP. --BA voltage output of constant slope, dV/dt (Volts/second).
ANALOG TRIGGER. -- An event that occurs at a user-selected point on an analog input signal. The polarity, sensitivity, and hysteresis of the analog trigger can often be programmed.
ANGSTROM. -- A unit of length equal to 10-10 meters. Thus, there are ten angstroms to one nanometer (nm).
ANSI (American National Standards Institute) -- The United States government agency that defines and maintains technical standards. Approval Agencies Agencies such as Underwriters Lab (UL), CSA, VDE, that test and approve products for a particular use to a published standard.
APPLICATION PROGRAM.-- A computer program used to perform a particular kind of work, such as data acquisition.
ASME --American Society of Mechanical Engineers.
ASTM-- American Society for Testing and Materials.
ATMOSPHERE-- The ambient environment.
ATMOSPHERIC PRESSURE -- Standard Pressure exerted by the earth's atmosphere on bodies located within it. Standard atmospheric pressure is 14.7 psia (1.013 bar abs.) measured at sea level and 60¼F (15¼C).
AUTORANGING TIME. -- For instruments with autoranging capability, the time interval between application of a step input signal and its display, including the time for determining and changing to the correct range.
AUTORANGING. --The ability of an instrument to switch among ranges automatically. The ranges are usually in decade steps.
AUXILIARY OUTPUT -- An output that controls external activities that are not directly related to the primary, control output. For example, door latches, gas purges, lights and buzzers.
AWG -- See American Wire Gauge.
BACK END. -- Back-end refers to the package assembly and test stages of production in semiconductor manufacturing. It includes burn-in and environmental test functions.
BACKGROUND TASK. --An operation that can take place while another program or processing routine is running without apparent interruption to that program or routine. For example, an interrupt or DMA operation.
BANDWIDTH. --The highest frequency signal component that can pass through input amplifiers and/or filters without being attenuated.
BANDWIDTH. (DATA ACQUISITION) The range of frequencies that can be switched, conducted, or amplified within certain limits. Under given load conditions, bandwidth is defined by the -3dB (half-power) points.
BAUD RATE -- The rate of information transfer in serial communications, measured in bits per second.
BOILING POINT -- The equilibrium temperature between a liquid and a gaseous state. For example, the boiling point of water is 212¼F (100¼C) at standard atmospheric pressure.
BREAK-BEFORE-MAKE. -- Disconnecting the present circuit before connecting a new circuit.
BS -- British Standards The United Kingdom agency that defines and maintains technical standards.
BUFFER MEMORY. -- Temporary storage area for acquired or generated data.
BURST MODE. -- A data acquisition mode in which a group of analog input channels are scanned at an interval determined by the pacer clock and the signal from each channel within the scan is converted at a higher rate determined by the burst mode conversion clock. This mode minimizes the skew between channels.
BUS. -- An interconnection system that allows each part of a computer to communicate with the other parts.
BYTE. -- A group of eight bits.
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CALIBRATION -- The comparison of a measuring device (an unknown) against an equal or better standard.
CALIBRATION OFFSET -- An adjustment to eliminate the difference between the indicated value and the actual process value.
CALORIE -- A unit of energy defined as the amount of heat energy required to raise the temperature of one gram of water 1¼C at 15¼C.
CAPACITANCE.-- In a capacitor or system of conductors and dielectrics, the property that permits the storage of electrically separated charges when potential differences exist between the conductors. Capacitance is related to charge and voltage as follows: C = Q/V, where C is the capacitance in farads, Q is the charge in coulombs, and V is the voltage in volts.
CE -- A manufacturer's mark that demonstrates compliance with European Union (EU) laws governing products sold in Europe. CE-Compliant Compliant with the essential requirements of European directives pertaining to safety and/or electromagnetic compatibility.
CELSIUS - (Formerly known as Centigrade) A temperature scale in which water freezes at 0¼C and boils at 100¼C at standard atmospheric pressure. The formula for conversion to the Fahrenheit scale is: ¼F= (1.8 á¼C) + 32.
CENTRAL PROCESSING UNIT (CPU) -- The unit of a computing system that includes circuits controlling the interpretation of instructions and their execution.
CHARGE COUPLED DEVICE (CCD). A semiconductor device, often used for sensing light, which operates by storing charge on capacitors and selectively moving that charge through the device by manipulating voltages on its electrodes.
CIRCUIT -- Any closed path for electrical current. A configuration of electrically or electromagnetically-connected components or devices.
COAXIAL CABLE -- A cylindrical transmission cable made of an insulated conductor or conductors centered inside a metallic tube or shield. It isolates the signal-carrying conductor from electrical interference or noise.
COLD JUNCTION. -- The junction in a thermocouple circuit that is held at a stable known temperature. Also known as reference junction.
COLD-JUNCTION COMPENSATION. --A method of compensating for ambient temperature variations in thermocouple circuits.
COMMON MODE LINE FILTER -- A device to filter noise signals on both power lines with respect to ground.
COMMON MODE INPUT ISOLATION. -- On a switching card, the isolation from signal high and low to guard (or shield) for a 3-pole circuit, or from signal high and low to chassis ground for a 2-pole circuit. Specified as resistance and capacitance.
COMMON MODE REJECTION RATIO (CMRR). -- The ability of an instrument to reject interference from a common voltage at its input terminals with respect to ground. Usually expressed in decibels at a frequency.
COMMON MODE VOLTAGE.-- A voltage between input low and chassis ground of an instrument.
COMPLIANCE CURRENT. -- The maximum output current of a constant voltage source. Also known as current limit.
COMPLIANCE VOLTAGE. -- The maximum output voltage of a constant current source. Also known as voltage limit.
COMPUTER GROUND -- A line for the ground connections to computers or microprocessor-based systems. It is isolated from safety ground.
CONDUCTANCE (G). -- The ability to conduct electricity. Defined by G =Re (I/V) where G is the conductance in Siemens, I is the current in Amps, and V is the voltage in Volts.
CONSTANTAN -- A generic designation for a thermocouple alloy made of 55 percent copper and 45 percent nickel that is used as the negative conductor in ANSI Type E, J, and T thermocouples.
CONTACT -- The surfaces of current carrying members at which electrical circuits are opened and closed.
CONTACT RESISTANCE -- The amount of resistance generated between two electrical contacts.
CONTACT BOUNCE. -- The intermittent and undesired opening of relay contacts during closure, or closing of relay contacts during opening.
CONVERSION. -- A process where a signal is changed from an analog to digital (A-D) representation, or digital to analog (D-A).
CONVERSION RATE. -- The rate at which sampled analog data is converted to digital data or digital data is converted to analog data.
COUNTER/TIMER. -- A circuit that counts pulses or measures pulse duration.
CPS (Cycles per second) -- Frequency. Also referred to as Hertz.
CREST FACTOR. -- The ratio of the peak value to the root-mean-square (rms) value of a waveform.
CROSSTALK. -- The coupling of a signal from one input to another (or from one channel to another or to the output) by conduction or radiation. Crosstalk is expressed in decibels at a specified load and up to a specific frequency.
CSA (Canadian Standards Association)-- An independent testing laboratory that establishes commercial and industrial standards tests products and certifies them.
C-UL Canadian recognition of Underwriters Laboratories, Inc.-- (UL) approval of a particular product class, such as UL 508. In some instances, C-UL approval may stand in lieu of Canadian Standards Association (CSA) approval. All references to C-UL stem from the original UL file only, resident at the location of UL approval. See CSA, UL.
CURRENT -- The rate of flow of electricity. The unit of measure is the ampere (A). 1 ampere = 1 coulomb per second. Current Transformer -- A transformer designed for measuring electrical current.
CYCLES -- The number of times an Alternating Current changes direction in one second is referred to as a cycle (also called Hertz-Hz).