LESSON 24. Measurement-methods-Instrument and classification of instruments-static and dynamic characteristics of instrument.General measurement system-Functional elements of a measurement system-
Measurement of a given quantity is essentially an act or result of comparison between the quantity (whose magnitude is unknown) and a predefined standard.
Two basic requirements
Standard used for comparison should be accurately defined.
The apparatus used and the method adopted must be provable.
Methods of measurement
• Direct methods – unknown quantity directly compared against a standard.
• Indirect methods –direct measurement is not always possible, they are inaccurate and less sensitive. Hence this method is followed.
An instrument may be defined as a device for determining the value or magnitude of a quantity or variable.
Development phases of an instrument
1) Mechanical instrument
2) Electrical instrument
3) Electronic instrument.
Classification of instruments
Absolute instruments – these give magnitude of the quantity under measurements in terms of physical constants of the instrument.
Secondary instruments –quantity being measured can only be measured by observing the output indicated by the instrument calibrated by comparison with an absolute instrument /secondary instrument.
Generalized input-output configuration
In the design and or use of measuring instruments a number of methods for nullifying or reducing these interfering and modifying inputs are available (For eg. Use of (i) feed back / closed lap system reduces these errors (ii) filtering also reduces these errors.
Applications involve the measurement of quantities that are constant or vary only quite slowly. Under these conditions, it is possible to define a set of performance criteria that give a meaningful description of the quality of measurement without becoming concerned with dynamic description.
Dynamic relations between the instrument input and output must be examined, generally by the use of differential equation. Performance criteria based on these dynamic relations constitute the dynamic characteristics.
Analog and digital modes of operation
Signals that vary in a continuous fashion and take on infinity of values in any given range are called analog signals.
Signals which vary in discrete steps and thus take up only finite different values in a given range are called digital signals.
Readability: indicates the closeness with which the scale of the instrument may read.
Least count: the smallest difference between two indications that can be defected on the instrument scale.
Sensitivity: ratio of the linear movement of the painter on the instrument to the change in the measured variable causing this motion.
An instrument said to exhibit hysteresis when there is a difference in readings as whether the value of the measured quantity is approached from above / below. Hysteresis may be the result of mechanical friction, magnetic effects elastic deformation or thermal effects.
Accuracy of an instrument indicates the deviators of the reading from a known input. Precision of an instrument indicates its ability to reproduce certain reading with a given accuracy.
In a system having both inputs and outputs suppose a steper instantaneous input signal is applied to a system. In general there will be a slight delay in the output response and this delay is called the ‘rise time’ or delay of the system.
Distortion is a very general term that may be used to describe the variation of a signal from its true form. Depending on the system, the distortion may result from either poor frequency response or poor phase shift response.
In al electrical circuits proper care must be taken to avoid impedance mistaking.
Null type device attempts to maintain deflection at zero by suitable application of an effect opposing that generated by the measured quantity. Deflection instrument –
Accuracy depends on the calibrator of the spring null instrument it depends on the accuracy of the standard weights
Measurement system consists of a transducing element which converts the quantity to be measured in an analogous form.
The analogous signal is then processed by some intermediate means and is then fed to the end devices which present the results of the measurement.
General measurement system
Instruments classification according to applications.
Monitoring of processes and operation
Control of processes and operations
Experimental engineering analysis
A.Monitoring of processes and operation
- monitoring the function indicates the conditions.
Eg. : Thermometer, barometers and anemometer, electric meter, water meter, garmete.
B.Control of processes and operations
- component of an automatic control system.
Eg. : Thermostatic control, Air conditioner, thermocouples, accelerometers, altimeters.
C.Experimental Engineering analysis
(i) Theoretical methods
(ii) Experimental methods
Functional elements of an instrument or a measurement system
- to describe both the operations and the performance of measuring instruments
- in terms of functional elements of instrument system
- performance is defined in terms of static and dynamic characteristics.
Primary Sensing element
Receives energy from the measured medium and produces on output depending in some way on the measured quantity.
Variable conversion element
To convert the physical or any variable to another more suitable variable while preserving the information content of the original signal.
Variable manipulation system / element
To perform the intended task, an instrument may require a signal represented by some physical variable be manipulated in some way. Ie. Change in numerical value according to some definite rule with physical preservation of the nature of the variable. The manipulation element may appear / may not appear / elsewhere in the chain.
Data transmission element
For transmitting the data from one to another, when the functional elements of an instrument are actually physically separated.
Data Presentation Element
- Should be recognizable by one of the human senses. An element that performs the ‘translation’ function is called as data presentation element.
Data storage / playback function
For storing the data and get back the data