H

HARDWARE.-- The physical parts of a computer-controlled system, such as circuit boards, chassis, peripheral devices, cables, etc.

HAZARD. -- The potential for harmful effects.

HEAT SINK. -- A part used to absorb heat.

Hertz (Hz) -- Frequency measured in cycles per second.

Hi-Pot Test --- A test that applies a high voltage to a conductor to assure the integrity of the surrounding insulation. See dielectric breakdown.

HOT JUNCTION. -- The junction of two dissimilar metals in a thermocouple circuit that is used to measure an unknown temperature. Also known as measurement junction.

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I

ICE POINT -- The temperature at which pure water changes from a liquid to a solid (freezes). 32¼F (0¼C).

ID -- Inside diameter.

IEEE. -- Abbreviation for Institute of Electrical and Electronics Engineers.

IEEE-488. -- See GPIB.

IMPEDANCE -- The total opposition of a circuit to the flow of alternating current. It includes resistance and reactance, and is measured in ohms.

INDUCTIVE LOAD -- Generated by a wire coil such as a relay, solenoid, motor or contactor. In an Inductive Load when the thermostat opens, a high voltage will be generated until all the current that is stored in the coil is discharged across the contacts. This can result in severe contact degradation.

INERT -- A material or gas that does not react chemically or biologically; for example, military thermostats are backfilled with an inert gas to prevent oxidation.

INFRARED -- A region of the electromagnetic spectrum with wavelengths ranging from one to 1,000 microns. These wavelengths are most suited for radiant heating and infrared (non-contact) temperature sensing.

INPUT BIAS CURRENT. -- The current that flows at the input due to internal circuitry and bias voltage. Also, the current that must be supplied to the high input measuring terminal (with zero input signal and offset voltage) to reduce the output indication to zero.

INPUT -- Process variable information that is supplied to the instrument.

INPUT SCALING -- The ability to scale input readings (readings in percent of full scale) to the engineering units of the process variable.

INPUT TYPE -- The signal type that is connected to an input, such as thermocouple, RTD, linear or process.

INRUSH CURRENT -- Measurement of the initial current flow in Capacitive or Lamp applications. Inrush currents can be as much as 15 times the rated current flow of the application.

Instrument Society of America (ISA) -- An engineering society that defines and maintains standards for scientific and technical measuring devices.

INPUT IMPEDANCE. -- The shunt resistance and capacitance (or inductance) as measured at the input terminals, not including effects of input bias or offset currents.

INPUT ISOLATION. -- On a switching card, the isolation from signal high to low (or guard) for a two-pole circuit. Specified as resistance and capacitance.

INPUT OFFSET CURRENT.-- The difference between the two currents that must be supplied to the input measuring terminals of a differential instrument to reduce the output indication to zero (with zero input voltage and zero offset voltage).

INPUT OFFSET VOLTAGE. -- The voltage that must be applied directly between the input measuring terminals, with bias current supplied by a resistance path, to reduce the output indication to zero.

INPUT/OUTPUT (I/O). -- The process of transferring data to and from a computer-controlled system using its communication channels, operator interface devices, data acquisition devices, or control interfaces.

INSERTION LOSS. -- The attenuation of signals due to routing them through a switching card. Specified as a decibel value over a frequency range.

INSULATOR. -- A material that does not significantly conduct electrical current

INTEGRATING CONVERSION.-- An analog to digital conversion process where the output results in a digital representation of the integral of the input signal over a specified time interval.

INTERNAL TRIGGER. -- A software-generated event that starts an operation. See also EXTERNAL TRIGGER.

INTERRUPT. -- A signal to the CPU indicating that the board detected the occurrence of a specified condition or event.

INTERRUPT LEVEL.-- A specific priority that ensures that high priority interrupts get serviced before low priority interrupts.

ION. -- This is what an atom becomes when an electron is separated from the atom, leaving it with a net positive charge or, if an electron is added, leaving it with a net negative charge.

ISA BUS.-- Industry Standard Architecture. The 16-bit wide bus architecture used in most MS-DOS and Windows computers. Sometimes called the AT bus.

ISOLATED OUTPUTS.-- Output signals where a common reference is not connected to either input terminal.

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J

JIT -- Just-In-Time is a manufacturing concept using work cells to reduce leadtimes and lower costs. Some people refer to Just-In-Time delivery which is another way to say on-time delivery.

Joint Industrial Standards (JIS) -- A Japanese agency thatm establishes and maintains standards for equipment and components. Also known as JISC (Japanese Industrial Standards Committee), its function is similar to Germany's Deutsche Industrial Norm (DIN).

JOULE -- A basic unit of heat energy, equal to the work done when a current of 1 ampere is passed through a resistance of one ohm for one second.

JUNCTION -- The point where two dissimilar metal conductors join to form a thermocouple.

• Cold Junction -- Connection point between thermocouple metals and the electronic instrument. See reference junction.
• Exposed Junction -- A type of thermocouple probe in which the hot, or measuring, junction protrudes beyond the sheath material and is fully exposed to the substance being measured. It usually gives the fastest response time. No electrical isolation provided.
• Grounded Junction -- Type of thermocouple probe in which the hot, or measuring junction, is an integral part of the sheath material. No electrical isolation is provided.
• Isolated Junction -- A form of thermocouple probe construction in which the measuring junction is fully enclosed in a protective sheath and electrically isolated from it. Commonly called an ungrounded junction.
• Measuring Junction -- The thermocouple junction that is affixed to or inserted into thematerial being measured. Also called hot junction.
• Reference Junction -- The junction in a thermocouple circuit held at a stable, known temperature (cold junction). Standard reference temperature is 32¼F (0¼C).
• Thermocouple Junction -- The point where the two dissimilar metal conductors join. In a typical thermocouple circuit, there is a measuring junction and a reference junction. See measuring junction and reference junction.

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K

KELVIN CONTACTS. -- A means for testing or making measurements in electronic devices and circuits, particularly when low values are being measured. Two sets of leads are used at each test point, similar with respect to thickness, material and length; one set carries the test signal and the other connects with the measuring instrument. The effect of resistance in the leads is thus eliminated.

Kelvin (K) -- An absolute temperature scale. Zero Kelvin is absolute zero. No degree symbol (¼) is used with the Kelvin scale. (0¼C = 273.15K, 100¼C = 373.15K).

Kilo (k) -- A prefix meaning thousand.

Kilowatt (kW) -- Unit of electrical power equal to 1000 wattsor 3412 Btus per hour when the power factor equals 1.0.

Kilowatt Hour (kWh) -- Unit of electrical energy, or work, expended by one kilowatt in one hour. Also expressed as 1000watt hours.

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L

LCZ METER.-- Inductance (L), capacitance (C), impedance (Z) meter. A general purpose instrument for measuring component L, C. and Z. Sometimes called LCR meter This instrument may be applied to C-V testing, but typically lacks features optimized for C-V. See C-V METER.

LEAKAGE CURRENT. -- Error current that can degrade sensitive measurements. Leakage current is any unwanted current that flows when test voltage is applied. The ideal leakage current is zero. Leakage currents can originate in instruments, cables, or the device being tested. Even high resistance paths between low current conductors and nearby voltage sources can generate significant leakage currents.

LEAKAGE RESISTANCE. -- The resistance calculated from the dependence of leakage current on applied voltage. leakage resistance = V / leakage current

LEAST SIGNIFICANT BIT (LSB). -- The lowest order bit in a digital quantity.

LED -- See light emitting diode.

LEG-- One connection in an electric circuit.

LIFE CYCLES -- The endurance rating of a device expressed in number of operations with the stated electrical load applied.

LIGHT EMITTING DIODE(LED) -- A solid state electronic device that glows when electric current passes through it.

LINEARITY. -- The maximum deviation from a straight line between instrument readings at zero and full range. It is expressed in ppm at a specific temperature.

LINEAR INPUT -- A process input that represents a straight-line function.

LINEARITY -- The deviations in response from an expected or theoretical straight line value for instruments and transducers. Also called linearity error.

LINEARIZATION INPUT -- See linearization, square root.

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.

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.

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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M

MAGNETIC COILS -- Applications involving Magnetic Coils such as solenoids, relays and contactors are inductive loads which do not usually produce high Inrush Currents. However, when this type of load is turned off, the magnetic field collapses resulting in arcing across open contacts. This can cause serious contact deterioration.

MAINFRAME. -- A self-contained instrument in a cabinet, which provides a measurement or connection capability without requiring other instruments in the circuit. Some mainframes may be designated as a "master" or "slave."

MAKE-BEFORE-BREAK. -- Connecting a new circuit before disconnecting the present circuit.

MASTER. -- A mainframe that has control of other mainframes (slaves) through an external connection. A slave unit adds capacity or functions to the master. The master/slave combination has one IEEE-488 bus address. See also SLAVE and MAINFRAME.

MAXIMUM ALLOWABLE INPUT.-- The maximum DC plus peak AC value (voltage or current) that can be applied between the high and low input measuring terminals without damaging the instrument.

MAXIMUM LOAD IMPEDANCE -- The largest load that the output device can operate. Usually specified in ohms.

MAXIMUM OPERATING TEMPERATURE -- The highest temperature at which a device can operate safely or with expected normal service life.

MAXIMUM POWER RATING -- The maximum operating power at which a device can operate safely or with expected normal operating life.

Mega (M) -- A prefix that means one 106 (one million in the US).

MELTING POINT -- The temperature at which a substance changes from a solid to liquid state. This is the same as the freezing point of pure materials

MICRON -- A unit describing wavelength. One micron is equivalent to 1 x 10-6 meters.

MICROVOLT (µV) -- One 10-6 of a volt (one millionth in the US). Mil One thousandth of an inch, or 0.001 inches in decimal form.

Milliampere (mA) -- One thousandth of an ampere.

Millivolt (mV) -- One 10-3 (thousandth) of a volt.

MINIMUM LOAD CURRENT -- The smallest load current required insuring proper operation of an output-switching device.

MINIMUM OUTPUT IMPEDANCE -- See offstate impedance.

MULTIPLEX -- Connecting one instrument to multiple devices under test or multiple instruments to one device under test.

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N

N-TYPE. The conductivity type of a semiconductor material when the majority of carriers are electrons, hence negative. N-type silicon is doped with dopant atoms from column V of the periodic table, such asphosphorus.

NANOVOLTMETER. -- A sensitive DC voltmeter (typically one decade more sensitive than a digital multimeter) with a low thermal input connection.

National Bureau of Standards (NBS) -- Now called the National Institute of Standards Technology (NIST).

National Electrical Code (NEC) -- A set of specifications devised for the safe application and use of electric power and devices in the United States.

National Electrical Manufacturers Association (NEMA) -- A United States association that establishes specifications and ratings for electrical components and apparatuses. Conformance by manufacturers is voluntary.

National Institute of Standards and Technology (NIST) -- A United States government agency responsible for establishing scientific and technical standards. Formerly the National Bureau of Standards.

National Pipe Thread (NPT) -- The taper pipe thread standard used in North America.

NBS -- See National Bureau of Standards.

NEC -- See National Electrical Code.

Negative Temperature Coefficient -- A decrease in electrical resistance that occurs with a temperature increase. See thermistor.

NEMA -- See National Electrical Manufacturers Association.

NEMA 4X -- A NEMA specification for determining resistance to moisture infiltration and corrosion resistance. This rating certifies the controller as washable and corrosion resistant.

NIST -- See National Institute of Standards and Technology.

NOISE. -- An undesirable electrical signal from an external source such as an AC power line, motors, generators, transformers, fluorescent lights, CRT displays, computers, radio transmitters, and others.

NOISE SUPPRESSION -- The use of components to reduce electrical interference that is caused by making or breaking electrical contact, or by inductors.

NON-EQUILIBRIUM. -- The condition in a semiconductor device or material when there is still a tendency for its macroscopic properties to change with time. In other words, there still remains some settling to occur before equilibrium is achieved. Devices in which current always flows, like diodes and bipolar transistors, do not ever reach equilibrium.

NORMAL MODE REJECTION RATIO (NMRR). -- The ability of an instrument to reject interference(usually of line frequency) across its input terminals. Usually expressed in decibels at a frequency.

NORMAL MODE VOLTAGE. -- A voltage applied between the input high and input low terminals of an instrument.

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