Acceleration is the rate of change of velocity with time. Change in velocity or the acceleration of an object can be from a change in its speed, direction, or both. Acceleration, a, is expressed mathematically with the equation, a = (vf – vi)/t , where vf is the final velocity, vi , the initial velocity, and t is time. A decrease in velocity, where vf is smaller than vi , is a negative acceleration, also referred to as deceleration. The acceleration unit g is equal to the local gravitational field, which is equivalent to free-fall acceleration. Acceleration can be static or dynamic.
Accelerometer
A device that senses the inertial reaction of a proof-mass for the purpose of measuring linear or angular acceleration and provides a usable output in proportion to the applied acceleration. See Angular Accelerometer and Linear Accelerometer for additional information.
Alignment Error
The angular deviation between the orthogonal axes coordinate systems of the sensing element and the sensor input. The alignment reference for the sensor input is typically defined by the mounting base and one side of the sensor. It is then up to the user to use the defined reference surface of the sensor to align it within the application, however, this then yields another alignment error. The user can align the sensor manually or can compensate for alignment errors electrically or with software utilizing alignment data. See also input axis misalignment, output axis misalignment, and pendulous axis misalignment.
Angular Accelerometer
A device that senses angular (rotational) acceleration about its input axis and provides an output from a moment of inertia acting on a proof-mass, which is proportional to the angular acceleration input. The input is typically in units of radians per second squared (rad/sec 2 ).
Anisoelasticity
The inequality of compliance a structure has in different directions. As an example, the moving system of a Jewell taut-band mechanism is anisoelastic.
Bandwidth (BW)
Bandwidth refers to the dynamic response of the sensor. It is the range of frequency over which the sensor will respond to an input with an output that does not fall more than 3 dB below the flat (full) amplitude. Force balance sensors typically have a bandwidth from 1-200Hz, whereas MEMS typically ranges from 1-1000Hz. Electrolytic sensors tend to have a low bandwidth of < 10Hz – Electrolytics are intended for measurements of stationary objects. Bandwidth can be intentionally influenced by dampening employed via electrical filters or shock absorbing oil added to the torquer assemblies in the case of some force balance assemblies. These technologies can be used to remove unwanted elements of the vibrational profile in the operating environment, by manipulation of the sensors dampening ratio (see the dampening ratio below for more).
Bias
The output of a sensor with no acceleration or tilt applied, which is independent of misalignment. Bias of Jewell forced balance devices results primarily from the combination of residual mechanical torque in the torquer suspension and electrical biases in the electronics circuitry.
The basic Jewell equation is: Bias = (Output0° + Output180°) / 2 . Bias in g is obtained by dividing the result by scale factor. The output of an inertial sensor held stationary on a flat surface is from bias combined with misalignment and noise.
Bias Temperature Sensitivity (BTS)
The sensitivity defined by the ratio of a change in output bias for a corresponding change in temperature. Refer to Temperature Sensitivity in the appendix for the equation used to calculate BTS.
Bias Voltage Sensitivity
The bias change resulting from a change of the input voltage to the sensor.
Bias Uncertainty
See Repeatability, Turn-on.
Bonding Resistance
The resistance present between two specified conducting points, such as the sensor mounting base and the surface on which the sensor is mounted or the sensor mounting base and connector shell. It is usually measured in milliohms.
Capturing
The restraint of a proof mass to a specific reference position using a torquer in a servo loop as the control. Jewell inclinometers and accelerometers employ capturing technology.
Case
The enclosure of the transducer and the structure that provides the mounting surfaces to establish the reference axes.
Center of Gravity (CG)
The center of mass of a distribution of mass (of an object such as the proof mass) in space is the unique point at which the weighted relative position of the distributed mass sums to zero, meaning the distribution of mass is balanced around the center of mass.
Center of Seismic Mass (csm)
The Center of Seismic Mass defines the physical location of the proof mass of an inertial transducer within its package. It is the point within the sensor where acceleration forces are summed to produce the output. Typically this applies to accelerometer applications.
Characteristic Time
The time required for the output to reach 63% of its final value for a step input. It is also referred to as the time constant.
Cross Acceleration
The acceleration applied in a plane normal to an accelerometer input reference axis.
Cross Axis Sensitivity
The proportionality constant (g/g) that relates a variation of accelerometer output to cross acceleration. This sensitivity can vary, depending on the direction of cross acceleration.
Damping & Damping Ratio (z)
Damping is defined as the energy dissipating characteristic which, together with the natural frequency, determines the limit of frequency response and the response time characteristics of an accelerometer/inclinometer. In response to a step change of input, an under-damped (periodic) system oscillates about its final steady value before coming to rest at that steady value; an over-damped (aperiodic) system comes to rest without overshoot, and a critically damped system is at the point of change between the under- and over-damped conditions. Viscous damping uses the viscosity of fluids (liquids or gases) to produce damping. Magnetic damping uses current induced in electrical conductors by changes in magnetic flux to produce damping.
The damping ratio (ζ) characterizes the shape of the sensor dynamic response. It is defined as the ratio of actual damping compared to critical damping for a second-order system. Typically, for Jewell sensors the ratio equals 0.707 of the Full-Scale Output (FSO) but it can range from 0.5 to 1 depending on the specific product or customization.
Damping Fluid
A fluid, which can be either a liquid or gas, used to provide a viscous damping force or torque on an inertial sensing element.
Dielectric Strength
The maximum electric field an insulating material can withstand without breaking down (i.e. without experiencing failure of its insulating properties). A sensor is normally tested to a maximum leakage current requirement, with the measured made between the sensor’s isolated electronic circuitry and case. The voltage potential for dielectric strength may be either AC or DC.
Error Band
A band defined about the specified output function containing the output data. The error band contains the composite effects of non-linearity, resolution, non-repeatability, hysteresis, and other uncertainties in the output data.
Excitation
The external electrical voltage and/or current applied to a sensor for proper operation. See also input voltage.
Dynamic Range
The ratio of sensor input range to the threshold.
Frequency Response
The response characteristic an output has with the input changing in frequency. See bandwidth, natural frequency, and damping ratio.
Full Range Output (FRO)
The total change in output the sensor has for its full positive to negative range of input. For example, an inclinometer with a ±90° input has a full range output of ±5 Volts, or 10 Volts.
Full-Scale Output (FSO)
The change in output the sensor has for an input going from zero to positive or negative full scale. For example, a standard ±90° inclinometer has a full-scale output of 5 Volts for an input change of +90° or -90°. (This term is sometimes mistakenly used to specify Full Range Output.)
Galvanometer
A galvanometer is an electromechanical device that consists basically of a coil of wire, which is pivoted or suspended within the magnetic field of a permanent magnet to allow limited rotation of the coil when a direct current flows through it. The most common use of the galvanometer is for a measuring device having a pointer attached to the coil, referred to as a meter mechanism.
When applying the galvanometer to an inertial device it functions as a torque motor or torquer when a proof mass on a short arm is substituted for the pointer. The d’Arsonval galvanometer is named after its developer, French physicist, and physician Jaques-Arsène d’Arsonval.
Gravitational Acceleration Unit (g)
The unit of acceleration equal in magnitude to the local value of gravity, unless otherwise specified.
Hysteresis
The maximum separation occurring between up-scale-going and down-scale-going sensor outputs over a full range traverse, unless otherwise specified. It is generally expressed as an equivalent input.
Inclinometer
A sensor for measuring an inclination relative to the horizontal of an axis referenced to Earth’s gravity. Tilt values are usually defined in “degrees”. Inclinometers are sometimes referred to as clinometers or tilt sensors.
Inertial Sensor
A device for measuring position, attitude or motion and whose references used to make the measurement are completely internal, except possibly for initialization.
Input Axis
The axis along or about which an input causes a maximum output from the sensor. It is also referred to as the Sensitive Axis.
Input Axis Misalignment (MIA)
The angle the sensor has between its input axis and associated input reference axis when at its minimal output condition. (IEEE standards use both direction cosines and right-handed Euler angles, depending on the principal field of application. Other conventions, differing both in signs and designation of axes, are sometimes used. When components are reported, the convention should always be identified.) The input axis misalignment is the geometric sum of the output axis and the pendulous axis misalignments relative to the sensor’s base and a reference side respectively:
Input Range
The region between the input limits of a sensor within which a quantity is measured, expressed by stating the lower- and upper-range values. Examples would be tilt input limits of -30° to +30° or input acceleration limits of -0.5 g to +0.5 g, with ranges expressed as ±30° or ±0.5 g respectively.
Input Reference Axis
The axis direction that is nominally parallel to the input axis as defined by the case mounting surfaces or external case markings, or both.
Input Span
The algebraic difference between the upper and lower values that define the input range.
Input Voltage
The excitation or power supplied to a sensor for its operation within specification, providing the input voltage is within the required range.
Insulation Resistance
The resistance of an insulator or insulating material, which is typically determined by applying a DC voltage across the insulator while measuring the current through it and then dividing the applied voltage value by the measured current to calculate the insulation resistance in Ohms.
Linear accelerometer
A device used to measure translational acceleration along an input axis and produce an output proportional to the acceleration. A DC-responding accelerometer will respond to Earth’s gravitational field with an output equal to the vector component of acceleration applied to the input axis. A device meant to measure acceleration due to Earth’s gravity, with an output when the device is tilted, is commonly called an inclinometer or tilt sensor. Some types of accelerometers, such as piezoelectric accelerometers, respond only to dynamic inputs while others, including force-balanced accelerometers, respond to both static and dynamic inputs.
Mechanical Freedom
The maximum linear or angular displacement the proof mass of an accelerometer or inclinometer travels, relative to its case.
Natural Frequency (Fn)
Natural Frequency is defined as sensors Output frequency where the phase of the output lags the input by 90°. For Jewell Force-Balance sensors, the Natural Frequency typically is identical to the bandwidth of the device if the dampening ratio equals 0.707 of the Full-Scale Output (FSO).
Noise
See Output Noise
Non-linearity
The systematic deviation from the least-squares straight line for input-output data relationships which nominally can be represented by a linear equation. Refer to the appendix for more on non-linearity and the mathematical equations involved.
Operating Life
The accumulated time of operation throughout which an inertial sensor exhibits specified performance when maintained and calibrated in accordance with a specified schedule.
Operating Temperature Range
The range in temperature is defined by the extremes, at which the sensor is intended to operate within its specified thermal performance limits. Within this range of temperature all tolerances for temperature performance including error band, gradient error, zero shift, and sensitivity shift apply.
Output Axis
An axis of freedom provided with a pickoff which generates an output signal as a function of the output axis angle.
Output Axis Misalignment (MOA)
The angular deviation or alignment error the sensor has between its output axis (true sensing axis) and the output reference axis as defined by the case. The basic Jewell equation is: Output Axis Misalignment (MOA) = (Output0° – Output180°) / 2 . Dividing the result with Scale Factor gives the misalignment in g. The g-result is converted to misalignment in degrees using the arcsin function.
Output Noise
The AC component (rms, peak, or peak-to-peak) of a sensor’s DC output that is present when all sensor input variations, electrical and mechanical, are absent. When evaluating the level of output noise, it is important to minimize input variations by measuring noise at a location that is free of seismic disturbances. Accelerometers and inclinometers respond to even the slightest seismic disturbances within the pass-band and that can easily add to the noise measured at the output, increasing the overall broadband noise and resulting in an inaccurate output noise measurement. Lower input range units are particularly plagued with this measurement problem due to their increased sensitivity to seismic noise.
Output Impedance
The internal impedance of a device that is seen at its output terminals.
Output Range
The product of input range and scale factor.
Output Reference Axis
The axis direction that is nominally parallel to the output axis as defined by the case mounting surfaces or external case markings, or both.
Output Span
The algebraic difference between upper and lower values that define the output range.
Overload Capacity
The maximum acceleration a sensor is able to tolerate without a permanent change in the specified performance characteristics when exposed to accelerations beyond the normal operating range.
Pendulosity
The product of the mass and the distance from the center of the mass to the center of the support or pivot measured along the pendulous axis.
Pendulous Accelerometer
An accelerometer employing a proof mass which is suspended in a manner permitting it to rotate about an axis that is perpendicular to an input axis.
Pendulous Axis
The axis defined by a line through the mass center of the proof mass, perpendicular to, and intersecting the output axis in pendulous devices. The positive direction is defined from the output axis to the proof mass.
Pendulous Axis Misalignment (MPA)
The angular deviation or alignment error the sensor has between the pendulous axis (true sensing axis) and the pendulous reference axis as defined by the mounting case. The basic Jewell equation is: Pendulous Axis Misalignment (MPA) = (Output-90° – Output+90°) / 2 . The misalignment may at times be measured at angles other than ±90°.
Pendulous Reference Axis
The axis direction that is nominally parallel to the pendulous axis as defined by the case mounting surfaces or external case markings or both.
Pendulosity
A pendulous accelerometer’s pendulosity, p, is the proof mass multiplied by the length from the mass’s Center of Gravity (CG) to the hinge (pivots or taut band).
Pickoff
A device that produces a signal output as a function of the relative linear or angular displacement between two elements. It is also referred to as a position detector.
Piezoelectric Accelerometer
A device that employs a piezoelectric material as the sensing element. It is generally used as a vibration sensor and will not respond to a static acceleration input.
Pitch
Angular displacement about an axis that is parallel to the lateral axis of a body.
Proof Mass
The effective mass whose inertia transforms an acceleration or tilt along, or about, an input axis into a force or torque. The effective mass takes into consideration flotation and contributing parts of the suspension.
Proportional-Integral-Derivative (PID) Controller
A PID Controller is used to measure a process variable in order to regulate the process with respect to the desired setpoint. In principle, the controller reads a process output, compares it to the setpoint to calculate an error, and then adjusts the process to minimize the error. The process is controlled by the weighted sum of three separate constant parameters. There is the proportional (P) term, which is a multiple of the present error and is referred to as the proportional gain constant. The integral (I) term, which is proportional to both the magnitude and duration of the error and is multiplied by the integral gain to accelerate process correction towards the setpoint. Finally, the derivative (D) term, determined by the slope of the change in error over time and multiplied by the derivative gain, slows the rate of change to reduce overshoot and improve the stability of the control loop. The PID controller is synonymous with the servo loop that is used for control of the torquer in Jewell inertial sensors.
Range
The physical values over which a sensor is intended to measure an input, specified by upper and lower limits. For example, an inclinometer with a range of 30° will measure tilt from -30° through +30°.
Rectification Error
A steady-state output error that occurs while vibratory disturbances are acting on a sensor. Anisoelasticity is one reason for rectification error.
Repeatability
The closeness of agreement among repeated measurements of the same variable and at the same sensor conditions following changes in conditions or when non-operating periods occur between measurements. It is often referred to as non-repeatability.
Repeatability, Turn-on
Uncertainty of an output from a sensor when it may yield different outputs under the same input conditions after the sensor has been powered off and then back on. This is referred to as Bias Uncertainty when the sensor is at zero-g input and power cycled off and on.
Resolution
The largest absolute value of the minimum change in input needed, for inputs greater than the threshold, to produce a change in output equal to at least 50% of the output expected using the nominal scale factor. Typically Jewell measures it at the full-scale input or at one-half the full-scale input.
Roll
Angular displacement about an axis that is parallel to the longitudinal axis of a body.
Root-Sum-Square (RSS) Error
The resultant error for several error sources (bias, misalignment, thermal, etc.) evaluated with the root-sum-square mathematical expression, which involves squaring each error value, summing the squares, and then taking the square root of the sum.
Scale Factor (SF)
The ratio of a change in output to a change in the input intended to be measured. Scale Factor is generally evaluated as the slope of the straight line that can be fitted by the method of least squares to input-output data obtained by varying the input cyclically over the input range. Jewell typically specifies scale factor as the sensor units of output per g, such as Volts/g for example.
Scale Factor Temperature Sensitivity (SFTS)
The sensitivity defined by the ratio of change in scale factor to a corresponding change in temperature. Refer to Temperature Sensitivity in the appendix for the equation used to calculate SFTS.
Scale Factor Voltage Sensitivity
The scale factor change resulting from a change of the input voltage to the sensor.
Sensitive Axis
See Input Axis
Sensitivity
The ratio of the sensor output to input range. It is usually in units of Volts per g (V/g) or other similar units of measure and is the basic sensor transfer function used in calculations such as a sensor output to its corresponding input acceleration or tilt.
Sensor
A device used to measure a physical input, such as temperature, light, pressure, displacement, acceleration, tilt, etc., and generate a usable output that can be read or interpreted.
Servo
The name given to a type of feedback system or mechanism whose operation is self-regulating.
Shock
A mechanical shock is a sudden acceleration of a short duration typically caused by an impact or explosion. Shock is measured in the same units as acceleration. As a performance characteristic, the shock is specified as the highest shock level that the device can be exposed to without causing a permanent change or damage to the unit. The specified shock can be applied an infinite number of times.
Standard Deviation (S)
The standard deviation of a data set indicates the amount of spread data has from a mean value. Jewell commonly uses the standard deviation to evaluate non-linearity data.
Stiction
Two solid objects pressing against each other (but not sliding) will require some minimum level of force parallel to the surface of contact to overcome static cohesion. Stiction is the force threshold that overcomes the cohesion but is not continuous. In the case where two surfaces having areas less than a micrometer come into close proximity (as in an accelerometer), they may adhere to one another, because, at this scale, electrostatic and/or Van der Waals and hydrogen bonding forces become significant. The phenomenon of two such surfaces being adhered together in this manner is called stiction. Stiction may also result from the presence of surface contamination.
Storage Temperature Range
The range in temperature defined by the extremes, within which the sensor can be exposed while being stored or in an unpowered state that will not cause damage or change in performance.
Survival Temperature Range
The range in temperature as defined by the extremes, within which the sensor will operate without damage or degradation of performance that is permanent. Although the sensor will survive exposure to this temperature range, specified performance is not guaranteed when outside the operating temperature range.
Thermal Sensitivity
Thermal sensitivity is the change in the output of the sensor due to a change in temperature within the operating temperature range.
Threshold
The largest absolute value of the minimum input needed to produce a change in output equal to at least 50% of the output expected using the nominal scale factor. It is measured at a nominally zero-g or zero degrees input.
Transducer
A device that receives energy in one form and converts it to another. Transducers are often used as a device to convert a physical or mechanical input to an electrical output that can then be measured.
Transverse Axis
A transverse axis is an axis perpendicular to the input or sensitive axis of the sensor. The output axis and the pendulous axis are both transverse axes.
Vibration
Vibration is a mechanical back-and-forth motion consisting of a single frequency or a range of frequencies. A sensor will respond to vibration with what is considered a useable output over its specified bandwidth. As an environmental characteristic, vibration is typically specified as the largest level, sine or random, that the sensor can be exposed to without physical damage or a permanent change in performance.
Vibration Rectification
See Rectification Error
Yaw
Angular displacement about an axis that is parallel to the vertical axis of a body. Gravity-referenced transducers or inertial sensors such as Jewell produces, will not respond to yaw.
Zero-Degree (0°) Output Temperature Sensitivity (ZTS)
The sensitivity defined by the ratio of change in output with the sensor at zero degrees to a corresponding change in temperature. Refer to temperature Sensitivity in the appendix for the equation used to calculate ZTS.
Degree of conformity of an indicated value to a recognized standard value, or ideal value.
Analog Signal
A signal representing a variable which may be continuously observed and continuously represented.
Attenuation
A decrease in signal magnitude between two points, or between two frequencies. The reciprocal of gain.
Bias
A voltage that is maintained at a point in a circuit.
Damping
The progressive reduction or suppression of oscillation in a device or system. Viscous damping uses the viscosity of liquids or gases to effect damping.
Differential Signal
A voltage measured with reference to another voltage that is isolated from power or signal ground. Differential measurement helps reject noise that originated from the ground. The scale factor for differential output is one half the scale factor for single-ended output. Differential output is measured between the +X and -X channels and between the +Y and -Y channels, respectively.
Digital Signal
Representation of information by a set of discrete values in accordance with a prescribed law. These values are represented by numbers.
Dynamic Response
The behavior of the output of a device as a function of the input, both with respect to time.
Error
The algebraic difference between the indication and the ideal value of the measured signal. It is the quantity which algebraically subtracted from the indicated value gives the ideal value.
Gain
An increase (or change) in signal power (or amplitude) from one point in a circuit or system to another.
Hysteresis
Dependence of the output signal, after transients have decayed, upon the history of prior inputs and the direction of the current traverse.
Linearity
The maximum deviation of the calibration curve from a straight line so positioned as to minimize the maximum deviation.
Natural Frequency
The frequency of free oscillation in cycles per unit of time.
Range
The region over which useful signals can be detected.
Reliability
The probability specified that a device will perform its objective adequately, for the period of time, under the operating conditions specified.
Repeatability
The closeness of agreement among a number of consecutive measurements of the output for the same value of the input under the same operating conditions, approaching from the same direction, for full-range traverses.
Reproducibility
The closeness of agreement among repeated measurements of the output for the same value of input made under the same operating conditions over a period of time, approaching from both directions.
Resolution
The least interval between two adjacent discrete details which can be distinguished one from another.
Scale Factor
The factor by which the output signal of an instrument (volts, milliamps, frequency, etc.) should be multiplied to compute the value of the measured variable.
Settling Time
The time required, following the initiation of a specified stimulus to the system, for the output to enter and remain within a specified narrow band centered on its steady-state value. This is mainly applicable to our electrolytic tiltmeters and settling time can vary from one series to another. The 900 & 800 series have a small settling time while the 700, 600, Lily and miniature tilt sensor requires a longer period after any movement (including vibration or shock) occurs before accurate measurements can be taken.
Single-ended signal
A voltage measured with reference to power or signal ground. After receiving the tiltmeter, the customer has the option to choose between single-ended or different using the same tiltmeter depending on how the wires are connected. Some electrolytic datasheets will specify the tilt output signal as follows: ±8Vdc (single-ended) and ±16Vdc (differential) for example.
Span
The algebraic difference between the upper and lower range values.
Temperature Coefficient of Scale Factor
A coefficient describing the relationship of instrument scale factor to instrument temperature. Also called temperature coefficient of span.
Temperature Coefficient of Zero Shift
A coefficient describing the change in output bias as a function of instrument temperature
Time Constant
At any instant of a response to a step or impulse, the time constant (T) is the quotient of the instantaneous rate of change divided into the change still to be completed. For the output of a first-order system, T is the time required to complete 62.3% of the total rise or decay occurring as a result of the step or impulse.
Transient
The behavior of a variable during transition between two steady states.
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, 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:
Periodic, in which the pointer oscillates about the final position before coming to rest.
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.
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:
Instrument
Full-scale Value
0-150
150*
50-0-150
200*
150-0-150
300*
* When zero is on the scale.
90-140**
400*
** When zero is not on the scale.
Note : Certain instruments, such as power-factor meters, are necessarily exempted from this definition.
Instrument, offset-zero
An instrument in which the scale markings are projected onto the dial by optical means.
Instrument, self-contained
Dependence of the output signal, after transients have decayed, upon the history of prior inputs and the direction of the current traverse.
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.
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.
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.
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.
Repeatability (hysteresis)
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.
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.
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.
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.
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.
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.
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.
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:
Periodic, in which the pointer oscillates about the final position before coming to rest.
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.
Error, pivot friction
Error caused by friction between the pivots and the jewels.
Error, Zero shift (residual deflection)
The part of the deflection of a mechanically controlled moving element that remains after the excitation producing the deflection has been removed and all the measuring circuits are de-energized.
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:
Instrument
Full-scale Value
0-150
150*
50-0-150
200*
150-0-150
300*
* When zero is on the scale.
90-140**
400*
** When zero is not on the scale.
Note : Certain instruments, such as power-factor meters, are necessarily exempted from this definition.
Instrument, analog indicating
An instrument that continuously measures an electrical quantity and displays the result by a relationship between a pointer and gradual scale.
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.
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.
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.
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).
Repeatability (hysteresis)
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.
Scale
The array of calibrated marks from which the input quantity may be read and interpreted.
Sticking
The condition caused by physical interference with the normal motion of the moving element.
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.
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.
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.
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