measurements and instrumentation unit 1
Post on 22-Sep-2014
Embed Size (px)
MEASUREMENTS AND INSTRUMENTATION EC2351
Prepared By JhansiRani.R AP/ECE
BASIC MEASUREMENT CONCEPTSMeasurement systems Static and dynamic characteristics Units and standards of measurements Error analysis Moving coil meters Moving iron meters Multimeters Bridge measurements
Maxwell Hay Schering Anderson Wien bridge.R.JhansiRani AP/ECE
SIGNIFICANCE OF MEASUREMENT Importance
of Measurement is simply and eloquently expressed in the following statement of famous physicist Lord Kelvin: I often say that when you can measure what you are speaking about and can express it in numbers, you know something about it; when you cannot express in it numbers your knowledge is of meager and unsatisfactory kind
Measurement means, to monitor a process or a operation and using an instrument, express the parameter, quantity or a variable in terms of meaningful numbers. Measurement of a given parameter or quantity is the act or result of a quantitative comparison between a predefined standard and an unknown quantity to be measured. There are 2 basic requirements:The comparison standard is accurately defined and commonly accepted , and The procedure and the instrument used for obtaining the comparison must be provable.
EVOLUTION OF INSTRUMENTS.a) b) c)
Mechanical Electrical Electronic Instruments.MECHANICAL:
These instruments are very reliable for static and stable conditions. But their disadvantage is that they are unable to respond rapidly to measurements of dynamic and transient conditions.
ELECTRICAL: It is faster than mechanical, indicating the output are rapid than mechanical methods. But it depends on the mechanical movement of the meters. The response is 0.5 to 24 seconds. ELECTRONIC: It is more reliable than other system. It uses semiconductor devices and weak signal can also be detected.
Measuring instrument:It is defined as the device for determining the value or magnitude of a quantity or variable.
It is the one which is based on electronic or electrical principles for its measurement function.
ADVANTAGES OF ELECTRONIC MEASUREMENTMost of the quantities can be converted by transducers into the electrical or electronic signals. Electronic signals can be amplified, filtered, multiplexed, sampled and measured. Measured signals can be transmitted over long distance through cables or radio links, without any loss of information. Many measurements can be done simultaneously or in rapid succession. Electronic circuits can measure the events of very short duration Higher sensitivity, low power consumption and a higher degree of reliability are the important features of electronic instruments and measurements.
FUNCTIONAL ELEMENTS OF AN INSTRUMENTData Storage &playback element
Primary Sensing elementQuantity To be measured
Variable Conversion element
Variable manipulation element
Data Transmission element
observer Data presentation element
Data conditioning element
Primary Sensing Element: An element of an instrument which makes first contact with the quantity to be measured. In most cases a Transducer follows primary sensing element which converts the measurand into a corresponding electrical signal. Variable Conversion Element: output of the primary sensing element is in electrical form such as Voltage, Frequency.such an o/pt may not be suitable for the actual measurement system. (Ex: A/D converter)
Variable Manipulation Element: The level of the o/pt from the previous stage may not be enough to drive the next stage. Thus variable manipulation element manipulates the signal, preserving the original nature of the signal. Data Transmission Element: When the elements of the system are physically separated, it is necessary to transmit the data from one stage to other. This is achieved by the data transmission element. Data Presentation Element: The data is monitored, for analyzing purpose using data presentation element.(Ex: Visual display)R.JhansiRani AP/ECE
Moving coil senses current Magnets & coil convert current in coil to force Force is transmitted to pointer through mechanical links Pointer and scale presents the current value
Static characteristics: The set of criteria defined for the instruments, which are used to measure the quantities which are slowly varying with time or mostly constant, ie., do not vary with time is called static characteristics
Dynamic characteristics: when the quantity under measurement changes rapidly with time, it is necessary to study the dynamic relations existing b/w i/pt and o/pt which is expressed as differential equations The set of criteria defined based on such dynamic differential equation is called dynamic characteristics
Calibration is the process of making an adjustment or making a scale so that the reading of an instrument agree with the accepted and certified standard.
Note: if the device is repaired, aged or modified then recalibration is carried out.
Accuracy Precision Resolution Error Sensitivity Threshold Reproducibility Zero drift Stability Linearity
ACCURACY:DEGREE OF CLOSENESS WHICH THE INSTRUMENT READINGAPPROACHES THE TRUE VALUE OF THE QUANTITY TO BE MEASURED. IT INDICATES THE ABILITY OF AN INSTRUMENT TO INDICATE TRUE VALUE OF THE QUANTITY.
A) ACCURACY AS % OF FULL SCALE READING:IF THE INSTRUMENT HAVE UNIFORM SCALE, THEN ACCURACY IS EXPRESSED AS % OF FULL SCALE READING. ACCURACY IS 0.1% FOR FULL SCALE OF 50 UNITS MEANS 0.05 UNITS ERROR IS PRESENT IN ANY MEASUREMENT. ACCURACY IS 0.2% FOR FULL SCALE OF 25 UNITS MEANS 0.05 UNITS ERROR
THUS AS READING DECREASES ERROR IS MORE AND LEADS MISLEADING.
B) ACCURACY AS % OF TRUE VALUE:
Best method for specifying accuracy. It is specified in terms of true value of the quantity being measured. Eg: 0.1% of true value. As the reading gets smaller error also gets reduced. Hence accuracy is better.
C) Accuracy as % of scale span:
Maximum point on scale -Minimum point on scale is scale span. For range 25-225, Scale span is 200 If accuracy is 0.2% of span then, error is 0.4 units in any measurement.
D) Point Accuracy
It is specified at only one point of scale.R.JhansiRani AP/ECE
is the measure of consistency or repeatability of measurement. It denotes the closeness with which individual measurements are departed or distributed about the average of numbers of measured values. High precision may not have high accurate
Types:conformity Number of significant figures.
Conformity: Error created due to limitation of scale reading is a precision error.Ex: resistor of value 2385692 is read as 2.4M.
Significant figures: Precision is obtained from number of significant figures.Ex: 110 ohms can be specified as 109 or 111 thus 3 significant figures. If it is specified as 110.0 then it may be 110.1 or 109.9 Thus there are 4 significant figures. Greater the significant figure greater is the precision.R.JhansiRani AP/ECE
Error:The algebraic difference between the indicated value and the true value of the quantity to be measured is called an error. Error of 1 ut is negligible when measure in order of 1000 ut Error of 1 ut is significant when measure in order of 5 ut e = At Am , where e error (or) absolute error Am measured value of quantity At true value of quantity Note: instead of specifying absolute error, the relative or percentage of error is specified.R.JhansiRani AP/ECE
absolute error True value
Sensitivity: The ratio of the change in output of an instrument to a change in the value of the quantity to be measured.
Note: if the calibration curve is linear, then sensitivity of the instrument is the slope of the calibration curve.
Reciprocal of sensitivity is called inverse sensitivity or deflection factor. unit: sensitivity mm/A, mm/, counts/V etc; Deflection meter - A/mm, /mm, V/counts etc; Sensitivity should be high, to achieve this the range of the instrument should not exceed the value to be measured.
Resolution means smallest measurable input change. Threshold:If the i/pt is slowly varied from zero, the o/pt does not change until some minimum value of the i/pt is exceeded. This minimum value of the i/pt is called threshold.
Threshold is the smallest measurable i/pt.
LINEARITYTHE CLOSENESS TO WHICH A CURVE APPROXIMATES A STRAIGHT LINE.
DEFINITION: IT IS DEFINED AS THE MAXIMUM DEVIATION OF THE ACTUAL CALIBRATION CURVE (O/PT) FROM THE IDEALIZED ST.LINE, EXPRESSED AS A % OF FULL SCALE READING OR A % OF THE ACTUAL READING.
drift: The deviation in the instrument output with time from its zero value, when the variable to be measured is a constant.
It is the degree of closeness with which a given value may be repeatedly measured.Reproducibility and repeatability are a measure of the closeness with which