static characteristics of instruments:The attributes collectively known as the static characteristics of instruments and are given in the datasheet for a particular instrument.
The 11 main Static characteristics of instruments are given below;
- Accuracy and inaccuracy (measurement uncertainty)
- Precision/repeatability/reproducibility
- Tolerance
- Range or span
- Linearity
- Sensitivity of measurement
- Threshold
- Resolution
- Sensitivity to disturbance
- Hysteresis effects
- Dead space
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Static characteristics of instruments
What do you mean by theย Static characteristics of instruments?
static characteristics of instruments:The attributesย collectively known as the static characteristics of instruments and are given in theย datasheet for a particular instrument.ย
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It is important to note that the values quotedย for instrument characteristics in such a data sheet only apply when the instrument is usedย under specified standard calibration conditions.ย
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Due allowance must be made forย variations in the characteristics when the instrument is used in other conditions.
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What are the Static characteristics of instruments?
Accuracy and inaccuracy (measurement of uncertainty)
The accuracy of an instrument is a measure of how close the output reading of theย instrument is to the correct value.ย
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In practice, it is more usual to quote the inaccuracyย figure rather than the accuracy figure for an instrument.ย
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Inaccuracy is the extent toย which reading might be wrong and is often quoted as a percentage of the full-scaleย (f.s.) reading of an instrument.ย
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If, for example, a pressure gauge of range 0โ10 barย has a quoted inaccuracy ofย +/-ย 1.0% f.s. (+/- 1% of full-scale reading), then the maximumย error to be expected in any reading is 0.1 bar.ย
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This means that when the instrument isย reading 1.0 bar, the possible error is 10% of this value.
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ย For this reason, it is an importantย system design rule that instruments are chosen such that their range is appropriate to theย spread of values being measured, in order that the best possible accuracy is maintainedย in instrument readings.ย
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Thus, if we were measuring pressures with expected valuesย between 0 and 1 bar, we would not use an instrument with a range of 0โ10 bar.
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ย Theย term measurement uncertainty is frequently used in place of inaccuracy.
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Precision/repeatability/reproducibility
Precision is a term that describes an instrumentโs degree of freedom from randomย errors.ย
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If a large number of readings are taken of the same quantity by a high precisionย instrument, then the spread of readings will be very small.ย
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Precision is often, thoughย incorrectly, confused with accuracy.
ย High precision does not imply anything aboutย measurement accuracy.ย
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A high precision instrument may have low accuracy.
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ย Lowย accuracy measurements from a high precision instrument are normally caused by aย bias in the measurements, which is removable by recalibration.
repeatability and reproducibility
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What areย repeatability and reproducibility?
The terms repeatability and reproducibility mean approximately the same but areย applied in different contexts.ย
Repeatability
Repeatability describes the closenessย of output readings when the same input is applied repetitively over a short periodย of time, with the same measurement conditions, same instrument and observer, sameย location and same conditions of use maintained throughout.
ย Reproducibility
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ย Reproducibility describesย the closeness of output readings for the same input when there are changes in theย method of measurement, observer, measuring instrument, location, conditions of useย and time of measurement.
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ย Both terms thus describe the spread of output readings forย the same input.ย
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This spread is referred to as repeatability if the measurement conditionsย are constant and as reproducibility, if the measurement conditions vary.
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The degree of repeatability or reproducibility in measurements from an instrument isย an alternative way of expressing its precision.ย
Tolerance
Tolerance is a term that is closely related to accuracy and defines the maximumย error that is to be expected in some value.ย
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Whilst it is not, strictly speaking, a staticย characteristic of measuring instruments, it is mentioned here because the accuracy ofย some instruments is sometimes quoted as a tolerance figure.ย
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When used correctly,ย tolerance describes the maximum deviation of a manufactured component from someย specified value.ย
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For instance,ย ย electric circuit components such as resistors have
tolerances of perhaps 5%.
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ย One resistor is chosen at random from a batch having a nominal
value 1000W and tolerance 5% might have an actual value anywhere between 950Wย and 1050 W.
Range or span
The range or span of an instrument defines the minimum and maximum values of theย quantity that the instrument is designed to measure.
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Linearity
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It is normally desirable that the output reading of an instrument is linearly proportionalย to the quantity being measured.ย
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In a plot of the typicalย output readings of an instrument when a sequence of input quantities are applied toย it, the normal procedure is to draw a good fit straight line through the points.
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The non-linearity is then defined as the maximum deviation of any of theย output readings marked points from this straight line.ย
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Non-linearity is usually expressed asย a percentage of full-scale reading.
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Sensitivity of measurement
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The sensitivity of measurement is a measure of the change in instrument output thatย occurs when the quantity being measured changes by a given amount.ย
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Thus, sensitivityย is the ratio ofย scale deflection to theย value of measurand producing deflection.
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The sensitivity of measurement is, therefore, the slope of the straight line drawn based on the values.
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ย If, for example, a pressure of 2 bar produces a deflection of 10 degrees inย a pressure transducer, the sensitivity of the instrument is 5 degrees/barย
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(assuming thatย the deflection is zero with zero pressure applied).
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Threshold
If the input to an instrument is gradually increased from zero, the input will have toย reach a certain minimum level before the change in the instrument output reading isย of a large enough magnitude to be detectable.ย
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This minimum level of input is knownย as the threshold of the instrument.
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ย Manufacturers vary in the way that they specify aย threshold for instruments.ย
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Some quote absolute values, whereas others quote thresholdย as a percentage of full-scale readings.ย
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As an illustration, a car speedometer typically hasย a threshold of about 15 km/h. This means that, if the vehicle starts from rest and accelerates,ย no output reading is observed on the speedometer until the speed reaches 15 km/h.
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Resolution
When an instrument is showing a particular output reading, there is a lower limit on theย magnitude of the change in the input measured quantity that produces an observableย change in the instrument output.ย
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Like threshold, the resolution is sometimes specified as anย absolute value and sometimes as a percentage of f.s. deflection.ย
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One of the major factorsย influencing the resolution of an instrument is how finely its output scale is divided intoย subdivisions.ย
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Using a car speedometer as an example again, this has subdivisions of typically 20 km/h.
This means that when the needle is between the scale markings, we cannot estimate speed more accurately than to the nearest 5 km/h. This figure of 5 km/h thus represents the resolution of the instrument.
Sensitivity to disturbance
All calibrations and specifications of an instrument are only valid under controlledย conditions of temperature, pressure etc.ย
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These standard ambient conditions are usuallyย defined in the instrument specification.ย
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As variations occur in the ambient temperatureย etc., certain static instrument characteristics change, and the sensitivity to disturbance
is a measure of the magnitude of this change.ย
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Such environmental changes affectย instruments in two main ways, known as zero drift and sensitivity drift.ย
what is Zero drift?
Zero drift isย sometimes known by the alternative term, bias.
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Zero drift or bias describes the effect where the zero reading of an instrument isย modified by a change in ambient conditions.ย
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This causes a constant error that existsย over the full range of measurement of the instrument.ย
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The mechanical form of a bathroomย scale is a common example of an instrument that is prone to bias.
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ย It is quite usual toย find that there is a reading of perhaps 1 kg with no one stood on the scale.ย
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If someoneย of known weight 70 kg were to get on the scale, the reading would be 71 kg, andย if someone of known weight 100 kg were to get on the scale, the reading would beย 101 kg.ย
How can we remove the Zero drift of Instruments?
Zero drift is normally removable by calibration.ย
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In the case of the bathroomย scale just described, a thumbwheel is usually provided that can be turned until theย reading is zero with the scales unloaded, thus removing the bias.
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Zero drift is also commonly found in instruments like voltmeters that are affected byย ambient temperature changes.ย
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Typical units by which such zero drift is measured areย volts/ยฐC.
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ย This is often called the zero drift coefficient related to temperature changes.ย
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If the characteristic of an instrument is sensitive to several environmental parameters,ย then it will have several zero drift coefficients, one for each environmental parameter.
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What isย Sensitivity drift?
Sensitivity drift (also known as scale factor drift) defines the amount by which anย instrumentโs sensitivity of measurement varies as ambient conditions change.ย
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It isย quantified by sensitivity drift coefficients that define how much drift there is for a unitย change in each environmental parameter that the instrument characteristics are sensitiveย to.ย
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Many components within an instrument are affected by environmental fluctuations,ย such as temperature changes: for instance, the modulus of elasticity of spring isย temperature-dependent.ย
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Sensitivity drift is measured in units of theย form (angular degree/bar)/ยฐC.ย
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If an instrument suffers both zero drift and sensitivityย drift at the same time, then the typical modification of the output characteristic isย shown in Figure (c)
Hysteresis effects
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ย The above fig. illustrates the output characteristic of an instrument that exhibits hysteresis.
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If the input measured quantity to the instrument is steadily increased from a negativeย value, the output reading varies in the manner shown in curve (a).ย
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If the input variableย is then steadily decreased, the output varies in the manner shown in curve (b).ย
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Theย non-coincidence between these loading and unloading curves is known as hysteresis.
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Two quantities are defined, maximum input hysteresis and maximum output hysteresis,ย as shown in Figure above.ย
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These are normally expressed as a percentage of the full-scale
input or output reading respectively.
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Dead space
Dead space is defined as the range of different input values over which there is noย change in output value.ย
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Any instrument that exhibits hysteresis also displays deadย space, as marked on Figure explaining hysteresis characteristics.ย
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Some instruments that do not suffer from any significantย hysteresis can still exhibit a dead space in their output characteristics, however.
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Backlash in gears is a typical cause of dead space and results in the sort of instrumentย output characteristic shown in Figure below.
Backlash
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ย Backlash is commonly experienced in gearsetsย used to convert between translational and rotational motion (which is a commonย technique used to measure translational velocity).
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You can check the previous posts also.
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