Glossary of Terms

ACCURACY:The quality of closeness to a specified value under stated conditions. Usually expressed as uncertainty - the maximum deviation to be expected between a reading or output and the actual value being measured.

AMBIENT TEMPERATURE: The average temperature of air surrounding the device or equipment.

ANALOG: A parameter which varies in a continuous, rather than incremental or discrete-step manner.

CALIBRATE: To determine the indication or output of a device with respect to a standard.

CELSIUS: Temperature scale where 0° = freezing and 100° = boiling point of water at sea level. Formerly known as Centigrade.

DIGIT: A measure of the display span of a meter. By convention, a full digit can assume any value from 0 through 9, a 1/2 digit will display a 0 or 1 and overrange at 2, a 3/4 digit will display up to 3 and overrange at 4. A meter with a display span of ±3999 counts is a 33/4 digit meter.

EMI: Electromagnetic interference.

EXPANDED SCALE: An arrangement that expands a specific portion of an overall range to occupy a larger portion of the full-scale length than it normally would.

EXPLOSION PROOF: An enclosure capable of withstanding an explosion of a specified gas or vapor which may occur within it and of preventing the ignition of a specified gas or vapor surrounding the enclosures by sparks, flashes, heat or explosion of the gas vapor within.

FORM: A method for specifying relay contact configuration: Form A SPST-NO single pole, single throw, normally open Form B SPST-NC single pole, single throw, normally closed Form C SPDT single pole, double throw Change-Over CO same as SPDT

FULL SCALE VALUE: The arithmetic sum of the two end-scale values (may not apply to some specialized meters, such as power factor). When zero is not on the scale, the full-scale value is the higher end-scale value. Examples: Scale Full Scale Value 0-10 ppm, 0 - 100ppm, 0 - 500ppm, 0 - 1000ppm, 0 - 2500ppm, 0 - 10000ppm, 0 - 1%, 0 - 5%, 0 - 10%, 0 - 25%

GROUND: Reference point for an electrical system. Often used to indicate an earth connection or negative side of a DC supply.

HYSTERESIS: The difference in an output or activation point due to rising vs. falling input signals.

IMPEDANCE: The total opposition to electrical flow, including both resistive and reactive elements.

INPUT RESISTANCE (INPUT IMPEDANCE): DC (or AC) resistance measured across the input terminals with signal leads disconnected.

KELVIN (K): The basic temperature unit of the thermodynamic scale. 0°C= 273K

LINEARITY ERROR: A measure of the departure from a straight-line response in the relationship between two quantities, where the change in one quantity is directly proportional to a change in the other quantity. Usually expressed as a maximum percent.

NEMA ENCLOSURE: A rating standard from the National Electrical Manufacturers Association which defines intended use and degree of environmental protection.

OFFSET: The non-zero output of a device for zero input.

OVERRANGE: A reading that exceeds full scale. OVERSHOOT The amount by which a meter or process exceeds the final value during

PPM (PARTS PER MILLION): A convenient format to express very small numbers, such as temperature coefficients. 100ppm is 0.01%. PPM is also written as "trace" as in trace oxygen.

PSIA (PSI Absolute): A pressure reading using vacuum as the reference.

PSIG (PSI Gauge): A pressure reading using ambient air pressure as the reference.

RANGE: The span of values over which a meter will function without entering overload condition, e.g. 0-100ppm

REPEATABILITY: The ability of an instrument to register the same reading in successive measurements of the same input.

RESOLUTION: The degree to which nearly equal values of a quantity can be discriminated. In digital meters, the value represented by a one-digit change in the least-significant digit.

RTD (RESISTANCE TEMPERATURE DETECTOR): A metallic sensor where resistance increases in a predictable manner with increasing temperature.

SENSITIVITY: The minimum change in input to which a device can respond.

SHIELD: A protective conductive covering that provides a least resistance path to ground for external interference.

SPAN ADJUSTMENT: The ability to adjust the gain of a process or meter so the display span corresponds to a specified signal span.

TEMPERATURE COEFFICIENT: The change in a parameter produced by a change in temperature. Normally expressed in %/°C or ppm/°C.

THERMISTOR: A semiconductor material which exhibits a known electrical resistance vs. temperature.

T90: The time required for a sensor to respond to 90% of its total change resulting from a step input.

TWO-WIRE TRANSMITTER: A signal conditioner in which the signal output and power input share two wires, thus minimizing wiring.

VOLT: The unit of electromotive force. One volt applied to a resistance of one ohm produces a current of one ampere.

WATT (W): Unit of real (effective) electrical power. W = VA x PF in a sinusoidal circuit.

ZERO ADJUSTMENT: The ability to adjust a signal conditioner or meter so that zero output or zero display corresponds to a specific input signal, such as 0V or 4mA.