ion performance of measurement
TRANSCRIPT
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Instrumentation and Measurement Techniques
Performance of measurement
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Objective
•Defining terms of performance of measurement systems
•Reliability of measurement system
•Discuss the requirement of measurement system
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Function of Instruments & Measurement Systems
Indicating instruments :Meter display, digital display
Example: speedometer in car, pressure gauge
Recording function : Data Keeping
Example: Printer, magnetic disc
Controlling function :Temperature, position, speed, liquid level, flow control.
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Performance assessment • An ideal measuring system is one where the output
signal has a linear relationship with the measurand.• Error is the difference between the indicated value
and the true value. • Measuring and control system performance can be
examined in two ways ,• Static performance –when steady or constant input
signals are applied• Dynamic performance – when changing input signals
are applied
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Static Performance Indication • Static sensitivity is defined as the ratio of the
change in output to the corresponding change in input under static or steady state conditions,
• For a system having static sensitivities of K1 ,K2,K3 ……, the overall system sensitivity is given by
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Static Performance Indication• Dynamic performance of both measuring and control
system is specified by response to certain standard test inputs
• Step input- abrupt change from one steady value to another ,will give the transient response
• Ramp input –which varies linearly with time, will give the ramp response
• Sine wave input – will give the frequency response
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Terms of performance of measurement
•True value
•Measured value
•Nominal value
•Static error
•Relative static error
•Accuracy
•Precision
•Sensitivity
•Reliability
•Hysterisis
•Resolution
•Response time
•Life time
•Frequency response
•Switching time
•Bandwidth
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True value
•The real numerical unit.
•It is almost impossible to obtain in practice.
For example:
Light speed = 299792458.63… m/s
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Measured value
•Value indicated by an instrument.
•It should always follow by its uncertaintyin measurement
For example:
l = (3.5± 0.1) cm
R = (102.5 ± 0.2) Ω
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Nominal value
•Value of the quantity specified by the manufacturer
•It follows by toleranceFor example:
l = 3.5cm ± 10%
R = 10k Ω ± 5%
(True value is between 9.9k Ω and 10.1k Ω)
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Static error
•The different between the measured value and the true value of the quantity.
δA = Am - At
δA = static error
Am = measured value
At = true value
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Relative Static error
•The ratio of static error to true value
εr= δA/ At
εr= (Am - At )/ At
εr = relative static error
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Accuracy
•Closeness with which an instrument reading approaches the true value of the quantity measured
Example:
Reading from instrument A, l = 3.82cm
Reading from instrument B, l = 3.91 cm
True value, l = 3.90cm
Conclusion: Instrument B is more accurate.
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Precision
•It is a measure of reproducibility of the measurement
•It composed of 2 characteristics:
a) Conformity
b) Number of significant figures
Instrument A, l= 3.82, 3.82, 3.81, 3.82…
Instrument B, l = 3.82, 3.84, 3.83, 3.80…
Conclusion: Instrument A is more precise.
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Accuracy and Precision
True value, l = 1.50mm
Instrument A, l = 1.475mm
Instrument B, l = 1.49mm
Conclusion:
Instrument A is more precise
Instrument B is more accurate
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Accuracy and Precision
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Sensitivity
The ratio of the magnitude of the output signal or response to a change the magnitude of input signal.
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ExampleA wheastone bridge requires a change of 7Ωin an unknown arm of the bridge to produced a change in deflection of the galvanometer.
Determine the sensitivity
magnitude of output responseMagnitude of input
Sensitivity =
= 3mm7Ω
= 0.429 mm/Ω
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Hysterisis A phenomenon which depicts/shows the difference in output when loading and unloading
Cause: friction, backslash,elastic deformation, magnetic and thermal effects.
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Resolution/ DiscriminationThe smallest increment in input which can be detected with certainty by an instrument
Example:
A mercury thermometer react every 0.5°Cof changes of ambient temperature.
This thermometer won’t have any reaction if the changes of temperature is 0.4°C
And it only move a step if the changes of temperature is 0.6°C
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Response timeThe period of time which from it sensing till it reach to steady state condition.
Example:
A mercury thermometer react every 0.5°Cof changes of ambient temperature which require 1.5s to settle.
If the temperature change rapidly every 1s, then this thermometer will never could gives a proper value.
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Frequency response
The minimum time that an instrument can sense an instantaneous changes.
Example:
our eyes cannot see light photon because the rapidly moving of photons is much more faster than our brain processing time.
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Switching timeThe best on-off time for a switching device which is distortion free.
Example:
For a device which require 2s to charge up and 3s to discharge. Hence the switching time must be always larger than 5s.
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BandwidthA range of frequency that can sense by an instrument.
Example:
The bandwidth of our ears is from 20Hz to 20kHz. Any sound that outside this range is undetectable.
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Others
• Range – the limit within which the input can vary-resisted thermometer can be quoted to have range of -200 to + 800 degree celcius
• Dead band or dead space is used for range where there is no output
• Stability –ability of system to give the same output when used to measure a constant input over a period of time.
• Dynamic characteristics – are characteristics of measurement that are time dependent
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Reliability
A period of an instrument that maintain its accuracy and precision.
Example:
After two years of using an instrument…
Accuracy of Instrument A drop 1%
Accuracy of Instrument B drop 5%
Conclusion: Instrument A is more reliable
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Requirement • Fitness of purpose – measurement to predefined
accuracy • Calibration – comparing output with the standards -Company standard-National standards calibration records normally include: Reference
number, calibrations data, calibration result, frequency of calibration,, repair and modification information , limitation
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National Standards• National standards are defined my international agreement maintain by
national establishment like national physics laboratory in great Britain, and national bureau of standards in the US.
Primary standards Mass:• Length• Time• Current• Temperature• Luminous intensity• Amount of substance
Supplementary standards:• Plane angle• Solid angleSafety system
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Error Calculation• Accuracy can be stated in terms of errors
introduced
• Percentage error = Indicated value – True value . X . 100%Maximum scale value
%100xvalueTrue
valueTruevalueIndicated −
Precision is used to specify the closeness of output result when a measuring device is subjected to the same input on a number of occasions
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EXAMPLEA measuring system consist of a transducer , an
amplifier and a recorder, with an individual sensitivities as follows :
• Transducer sensitivity 0.2 mV /0 C• Amplifier gain 2.0 V/mV• Recorder sensitivity 5.0 mm /V• Determine the overall system sensitivity• K= K1 x K2x K3• = 0.2 mV/0C x 2.0V/mVx 5.0 mm/V• =2.0 mm/0 C
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EXAMPLEA 0 to 10 bar pressure gauge was found to have an error of ±
0.15 bar when calibrated by the manufacturer .Calculate (a) the percentage error of the gauge and (b) the possible error asa percentage of the indicated value when reading of 2.0 bars was obtained in a test .
Percentage error = 0.15/10. x 100 = ± 1.5%Possible error = ± 0.15%∴ error at 2.0 bars = 0.15/10. x 100 =± 7.5%
• The gauge is therefore more unreliable at the lower end of its range, and alternative gauge with a more suitable range should be used .
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EXAMPLE• For a general measuring system where the errors in
the transducer, signal conditioner , and recorder are ±2 % , ± 3%,and ±4 % respectively, calculate the maximum possible system error and the probable or root- sum –square error.
• Maximum possible error = ± (2+3+4)% =±9%• Root –sum-square error = ± √(22+32+42)%• = √ 29% =± 5.4%• Thus the error is possibly as large as ± 9% but
probably not larger than ± 5.4%.