This glossary contains ANSI terms and definitions related to meters and instrumentation.
Accuracy: The quality of closeness to a specified value under stated reference conditions. Accuracy is quantitatively expressed by uncertainty.
Accuracy, intrinsic: The limit of the accuracy of an instrument when used under reference conditions. Accuracy is expressed in percentage of the fiducial value. This concept of accuracy is concerned with the intrinsic qualities of the instrument as opposed to the variation in indication that may arise when the instrument is used under conditions other than the reference conditions. Intrinsic accuracy is the uncertainty of the instrument in the "as received" condition, without the applications of corrections from a chart, curve or tabulation.Accuracy, rated (class): The assigned classification, which represents the value of uncertainty that the intrinsic accuracy of the instrument will not exceed.
Balance: The change in the position of the pointer from zero when the axis of the moving element moves from the vertical position to the horizontal position. The balance is expressed as percentage of the scale length.
Current, maximum: The specified current in an instrument, such as a wattmeter or power-factor meter that will not cause breakdown or sustain any observable physical degradation when applied continuously at the maximum rated operating temperature in the instrument energized at rated values.
Current, rated: The specified current that an instrument is designed to carry continuously under the usual service conditions.
Damping: The manner in which the pointer settles at its steady indication after a change in the value of the measured quantity. There are two general classes of damped motion, as follows:
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Periodic, in which the pointer oscillates about the final position before coming to rest. |
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Aperiodic, in which the pointer comes to rest without overshooting the rest position. |
The pointer of change between periodic and aperiodic damping is called "critical damping". A critically damped instrument is when overshoot is present but does not exceed an amount equal to half the rated accuracy of the instrument.
Damping factor: The ratio of the deviation of the pointer in two consecutive swings from the position of equilibrium, the greater deviation being divided by the lesser. The deviations are expressed in angular degrees.
End-scale value: The value of the actuating electrical quantity that corresponds to end-scale indication. When zero is not at the end or at the electrical center of the scale, the higher value is used.
Excitation: The electrical quantity applied to the instrument to cause the instrument to indicate as intended.
Fiducial value: The value to which reference is made in order to specify the accuracy of an instrument.
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Instrument Fiducial Value |
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0 -150 |
150 |
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50-0-150 |
200 |
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150-0-150 |
300 |
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90-140 |
140 |
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Full-scale value: The arithmetic sum of the two end-scale values. When zero is not on the scale, the full -- scale value is the higher end-scale value.
| Examples: |
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Instrument |
Full-scale Value |
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0-150 |
150* |
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50-0-150 |
200* |
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150-0-150 |
300* |
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90-140** |
400* |
* When zero is on the scale.
** When zero is not on the scale.
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Note: Certain instruments, such as power-factor meters, are necessarily exempted from this definition.
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Instrument, expanded scale: An instrument in which the normal deflection has been modified to expand a portion of the scale for improved readability.
Instrument, moving-iron: An instrument comprising a movable piece of ferromagnetic material that is actuated by a fixed coil carrying a current or by a fixed piece of ferromagnetic material magnetized by the current.
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Instrument, offset-zero: An instrument in which the scale markings are projected onto the dial by optical means.
Instrument, panel: A panel-mounted light-duty instrument that is used in portable, semi-portable or pertinent installations.
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Instrument, permanent-magnet moving-coil: An instrument designed that operationally depends on the reaction between the current in a movable coil(s) and the field of a fixed permanent magnet.
Instrument, self-contained: An instrument that has all necessary equipment and components built into the case or integrated within.
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Instrument, suppressed-zero: An instrument in which the lower portion of a given scale does not appear and cannot be indicated and where the zero is displaced outside the scale by mechanical or electrical means.
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Instrument, switchboard: A panel-mounted heavy-duty instrument that can be easily read from a distance and is designed for extended operation and permanent installation in severe environments such as may be encountered in industrial applications.
Instrument, thermocouple-type: An instrument in which the current heats a thermocouple. A permanent magnet moving-coil instrument measures the electromotive force from the thermocouple.
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Instrument, shunt: A resistor intended to be connected in parallel with an associated instrument in order to produce a higher current range that can be obtained by the instrument alone. The resistance of the shunt may be so chosen that the ratio of current measured by the combination to the current measured by the instrument alone is known.
Mechanical zero (as applicable to instruments that have mechanical restoring force): The position of the pointer at equilibrium when the instrument is not energized.
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Negligible influence: Any influence, such as that of ambient temperature, external field, etc. that is less than one-quarter of the rated accuracy.
Overshoot: The ratio of the overtravel of the pointer beyond a new steady deflection to the change in steady deflection when a new constant value of the measured quantity is suddenly applied. The overtravel and deflection are determined in angular measure, and the overshoot is usually expressed as a percentage.
Note 1: Since in some instruments, the ration depends on the magnitude of the deflection, a value corresponding to an internal deflection from zero to end-scale is used in determining the overshoot for rating purposes.
Note 2: Overshoot and damping factor have a reciprocal relationship. The percentage overshoot may be obtained by dividing 100 by the damping factor.
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Pointer shift due to tapping: The displacement in the position of a moving element that occurs when the instrument is tapped lightly. The displacement is observed by a change in the indication of the instrument.
Range: The region between the limits of display within which the input quantity is measured. Range is expressed by stating the lower and upper values of the display. (Examples: 0-100, 70-140, 50-0-50).
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Repeatability (hystersis): The ability of an instrument of repeat its indications when the pointer is deflected upscale compared to the indications taken when the pointer is deflected downscale. Repeatability is expressed as a percentage of the fiducial value.
Response time: The time required after an abrupt change of the measured quantity to a new constant value until the pointer or indication means first comes to apparent rest in its new position.
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Scale: The array of calibrated marks from which the input quantity may be read and interpreted.
Scale division: The interval between the centers of two consecutive scale marks.
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Scale length: The length of the path of motion from one end of the scale as indicated by the tip of the pointer. In the case of knife-edge pointers and others, extending beyond the scale division mark, the pointer shall be considered as ending at the outer end of the shortest scale division marks. In multi-scale instruments the longest scale shall be used to determine the scale length. The case of "antiparallax" instruments of the step-scale type with graduations on a raised step into the plane of and adjacent to the pointer tip, the scale length shall be determined by the end of the scale divisions adjacent to the pointer tip.
Series resistor (impedance): A resistor (impedance) intended to be connected in series with an associated instrument in order to produce a higher voltage range that can be obtained by the instrument alone.
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Shunt leads: Leads that connect a circuit of an instrument to an external instrument shunt. The resistance of these leads is taken into account in the adjustment of the instrument.
Sticking: The condition caused by physical interference with the normal motion of the moving element.
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Suspension, taut-band: A mechanical arrangement of two ribbons under tension, one at each end of the moving element. The ribbons support the moving element, allow it to rotate freely, provide the restoring torque, and conduct current to the moving element of moving coil instrument.
Symmetry (in an offset-zero instrument): The difference in indications when the polarity of a measured quantity is reversed expressed as a percentage of fiducial value.
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Symmetry error (of an offset zero instrument): The difference in indications when the polarity of a measured quantity is reversed, expressed as a percentage of fiducial value.
Tracking: The ability of an instrument to indicate at the scale mark being checked when energized by the proportional value of actual end-scale excitation.
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Tracking error: The error indication at a scale mark, expressed in percentage of fiducial value, when the instrument is energized by the proportional value of the actuated end-scale excitation. On offset-zero indicators, the higher end-scale value should be used as the reference value.
Transducer, self-contained: A device internal to an instrument that changes an electrical input into an electrical output, of either the same or a different kind, in such a manner that the desired characteristics of the input are measured by a permanent —magnet moving-coil mechanism.
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Voltage circuit (shunt circuit): a measuring circuit energized by a voltage that is a prime factor in determining the indication of the measured quantity. It may be the voltage directly involved supplied by a voltage transformer or a voltage divider.
Voltage drop (applied to current circuits): In a current measuring instrument, the value of the voltage between the terminals when the applied current corresponds to nominal end-scale deflection. In other instruments the voltage drop is the value of the voltage between the terminals at rated current.
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Voltage, maximum: The specified voltage in an instrument that will not cause electrical breakdown or any observable physical degradation when applied continuously at the maximum operating temperature of the instrument and with any other circuit in the instrument energized at rated values.
Voltage, rated: The specified voltage that an instrument (such as a wattmeter, power-factor meter, or frequency meter) is designed to carry continuously under usual service conditions. This is also the value of applied voltage used for test purposes. |
