Characteristics of Measurement Systems To choose the one most suited to a particular measurement application, we have to know the system characteristics. The The perfor performan mance ce charact characteris eristic ticss may be broadl broadly y divide divided d into into two group groups, s, namely namely ‘static’ ‘static’ and ‘dynamic’ characteristics Static characteristics
The performance criteria for the measurement of quantities that remain constant, or vary only quite slowly. Dynamic characteristics characteristics
The relationship between the system input and output when the measured quantity (measured) is varying rapidly. n practice, the characteristics of the one group may well influence the characteristics of the other. n order to access overall instrument performance, however, the two groups of characteristics are normally studied separately and then a semi!quantitative superposition is carried out. Accuracy
This is the closeness with which the measuring instrument can measure the ‘true value’ of the measured under stated conditions of use, i.e. its ability to ‘tell the truth’. The accuracy of an instrument is quantified by the difference of its readings and the one given by the ultimate or primary standard. "ccuracy depends on inherent limitations of instrument and shortcomings in measurement process. Precision
#recision is defined as the ability of instrument to reproduce a certain set of readings within given accuracy. #recision describes an instrument’s degree of random variations in its output when measuring a constant quantity. #recision depends upon repeatability. Repeatability
$epeatability is defined as ability of instrument to reproduce a group of measurements of same measured quantity, made by same observer, using same instrument, under same conditions. #rec #recisi ision on is ofte often n conf confus used ed with with accu accurac racy y. %igh %igh preci precisi sion on does does not not impl imply y anyth anythin ing g abou aboutt measurement accuracy. ac curacy. Accuracy Precision
"ccuracy "ccuracy represents degree degree of correctness correctness of the measured measured value w.r. w.r.t. t. true value. "ccuracy "ccuracy of instrument depends on systematic errors. #recision represents degree of repeatability of several
independent measurements of desired input at the same reference conditions #recision of instruments depends on factors that cause random or accidental errors. Resolution (Discrimination)
t is the minimum change or smallest increment in the measured value that can be detected with certainty by the instrument. t can be least count of instrument. Dead Space
Threshold &ead space' Threshold is defined as the range of different input values over which there is no change in output value. Tolerance
Tolerance is a term that is closely related to accuracy and defines the maimum error that is to be epected in some value. Tolerance describes the maimum deviation of a manufactured component from some specified value Range
The range of an instrument defines the minimum and maimum values of a quantity that the instrument is designed to measure. Linearity
This is the closeness to a straight line of the relationship between the true process variable and the measurement. i.e. deviation of transducer output curve from a specified straight line. inearity is usually reported as non!linearity, which is the maimum of the deviation between the calibration curve and a straight line positioned so that the maimum deviation is minimi*ed. Sensitiity of measurement
The sensitivity of measurement is a measure of the change in instrument output that occurs when the quantity being measured changes by a given amount. Reliability
$eliability is the probability that a device will adequately perform (as specified) for a period of time under specified operating conditions. +ome sensors are required for safety or product quality, and therefore, they should be very reliable. !nstrument Drift
t is defined as the variation of output for a given input caused due to change in sensitivity of the instrument due to certain interfering inputs like temperature changes, component instabilities, etc. #rime sources occur as chemical structural changes and changing mechanical stresses. &rift is a comple phenomenon for which the observed effects are that the sensitivity and offset values vary.
"ysteresis
areful observation of the output'input relationship of a block will sometimes reveal different results as the signals vary in direction of the movement. -echanical systems will often show a small difference in length as the direction of the applied force is reversed. The same effect arises as a magnetic field is reversed in a magnetic material. This characteristic is called hysteresis. %ysteresis is defined as the magnitude of error caused in the output for a given value of input, when this value is approached from opposite directions i.e. from ascending order / then descending order. auses are backlash, elastic deformations, magnetic characteristics, frictional effects (mainly). %ysteresis can be eliminated by taking readings in both direction and then taking its arithmetic mean. #ac$lash
t is defined as the maimum distance or angle through which any part of mechanical system may be moved in one direction without causing motion of net part. an be minimi*ed if components are made to very close tolerances.
Dynamic Characteristics Speed of response
t is defined as the rapidity with which a measurement system responds to changes in the measured quantity. Measuring lag
0very system requires its own time to respond to the changes in input. This time is called as lag. t is the retardation or delay in the response of a measurement system to changes in the measured quantity. The measuring lags are of two types1 Retardation type
n this case the response of the measurement system begins immediately after the change in measured quantity has occurred. "s soon as there is a changes in the measured quantity, the measurement system begins to respond. Time delay lag
n this case the response of the measurement system begins after a dead time after the application of the input. 2idelity1 t is defined as the degree to which a measurement system indicates changes in the measured quantity without dynamic error. %idelity
t is defined as the degree to which a measurement system is capable of faithfully reproducing the changes in input, without any dynamic error. Dynamic error
t is the difference between the true value of the quantity changing with time / the value indicated by the measurement system if no static error is assumed. t is also called measurement error.
&or$ing and Parts of Domestic 'as Meter
" gas meter is a speciali*ed flow meter, used to measure the volume of fuel gases such as natural gas and propane 3as meters are used at residential, commercial, and industrial buildings that consume fuel gas supplied by a gas utility 3ases are more difficult to measure than liquids, as measured volumes are highly affected by temperature and pressure 3as meters measure a defined volume, regardless of the pressuri*ed quantity or quality of the gas flowing through the meter Temperature, pressure and heating value compensation must be made to measure actual amount and value of gas moving through a meter
Diaphragmbellos Meters These are the most common type of gas meter, seen in almost all residential and small commercial installations. 4ithin the meter there are two or more chambers formed by movable diaphragms. 4ith the gas flow directed by internal valves, the chambers alternately fill and epel gas, producing a near continuous flow through the meter. "s the diaphragms epand and contract, levers connected to cranks convert the linear motion of the diaphragms into rotary motion of a crank shaft which serves as the primary flow element. This shaft can drive an odometer!like counter mechanism or it can produce electrical pulses for a flow computer. &iaphragm gas meters are positive displacement meters.
Process Medium ,atural 'as
Primary Sensing *lement
Diaphragm
+ariable Conersion *lement
Leers
+ariable Manipulation *lement
Cran$ Shaft
-bserer
Data Transmission *lement 'ear Syatem
Data Presentation *lement Dial Meter
Static Characteristics
f we have a thermometer in a room and its reading shows a temperature of 567, then it does not really matter whether the true temperature of the room is 89.:7 or 56.:7. +uch small variations around 567 are too small to affect whether we feel warm enough or not. ;ur bodies cannot discriminate between such close levels of temperature and therefore a thermometer with an inaccuracy of <6.:7 is perfectly adequate. f we had to measure the temperature of certain chemical processes, however, a variation of 6.:7 might have a significant effect on the rate of reaction or even the products of a process. " measurement inaccuracy much less than <6.:7 is therefore clearly required. "ccuracy of measurement is thus one consideration in the choice of instrument for a particular application. ;ther parameters such as sensitivity, linearity and the reaction to ambient temperature changes are further considerations. These attributes are collectively known as the static characteristics of instruments, and are given in the data sheet for a particular instrument. t is important to note that the values quoted for instrument characteristics in such a data sheet only apply when the instrument is used under specified standard calibration conditions. &ue allowance must be made for variations in the characteristics when the instrument is used in other conditions. The various static characteristics are defined in the following paragraphs.
Accuracy The accuracy of an instrument is a measure of how close the output reading of the instrument is to the correct value. n practice, it is more usual to quote the inaccuracy figure rather than the accuracy figure for an instrument. naccuracy is the etent to which a reading might be
wrong, and is often quoted as a percentage of the full!scale reading of an instrument. f, for eample, a pressure gauge of range 6=86 bar has a quoted inaccuracy of <8.6> (<8> of full! scale reading), then the maimum error to be epected in any reading is 6.8 bar. This means that when the instrument is reading 8.6 bar, the possible error is 86> of this value. 2or this reason, it is an important system design rule that instruments are chosen such that their range is appropriate to the spread of values being measured, in order that the best possible accuracy is maintained in instrument readings. Thus, if we were measuring pressures with epected values between 6 and 8 bar, we would not use an instrument with a range of 6=86 bar. The term measurement uncertainty is frequently used in place of inaccuracy.