Explain Absolute and Secondary instruments???

Explain Absolute and Secondary instruments???

Electrical Measurements

Types of electrical measuring instruments:
Measurements involve the use of instruments as a physical means of data mining quantities or variables.
             
The history of development of electrical measuring instruments is divided into three categories.

• Mechanical measuring instruments

• Electrical measuring instruments

• Electronic measuring  instruments

In this article we will discuss only types of electrical measuring instruments. Electrical methods of indicating the output of the detectors are more Rapid than mechanical methods. It is unfortunate that an electrical system normally depends upon a mechanical meter movement as indicating device. This mechanical movement has some inertia and therefore these instruments have a limited time response.

Electrical measuring instruments are classified into two categories. They are
1) Absolute instruments
2) Secondary instruments

1) Absolute instruments : These absolute instruments give the magnitude of the quantity under measurement in terms of physical constants of the instrument.These absolute instruments are standard and are used laboratories. Examples of these instruments are tangent galvanometer Rayleigh's current balance. These absolute instruments have a very good article in wiki pedia.

2) Secondary instruments : These instruments are so constructed that the quantity being measured can only be measured by observing the output indicated by the instrument. The instruments are calibrated by comparison with an absolute instruments or another secondary instruments which has already been calibrated against an absolute instrument.
                 Working with absolute instruments for routine work is time consuming since every time a measurement is made , it takes a lot of time to compute the magnitude up the quantity under measurement. Therefore secondary instruments are most commonly used. Absolute instruments are seldom used except in standards institutions while Secondary instruments find usage almost in every sphere of measurement.A voltmeter, a glass thermometer and pressure gauge are examples of secondary instruments.

These secondary instruments are classified into 3 types :
1) Indicating instruments
2) Recording instruments
3) Integrating instruments

1) Indicating instruments : These instruments will give instantaneous values of the quantity to be measured. Scale and pointer mechanism are invalid. PMMC, MI, dynamometer, wattmeter, frequency meter, power factor meter are examples.

2) Recording instruments : These instruments records the value to be measured or observed. Recorded over a graph paper by a light weight pen.These are used to observe load variation continuously. Examples are recording voltmeter, recording wattmeter, storage oscilloscope.

3) Integrating instruments : These instruments add the measured value to the existing value. These instruments will give the total electricity consumed over a peroid of time. Energy meter or KWhr meter ,kVARh meter and ampere hour meter.

               Depending on the way electrical instruments present result of measurements they can be classified into 2 major categories.

1) Deflection type instruments
2) Null type instruments

1) Deflection type instruments : Deflection type instruments In these deflection types instruments, the deflection of the instrument provides a basis for determining the quantity under measurement. The measured quantity produces some physical effect with deflects are produces a mechanical displacement of the moving system of the instrument. An opposing effect is built in the instrument which tries to oppose the deflection or the mechanical displacement of the moving system. The opposing effect is closely related to the deflection or mechanical displacement which can be directly observed. The opposing effect is so designed that its magnitude increases with the increase of reflection or mechanical displacement of the moving system caused by the quantity under measurement. The balance is achieved when opposing effect equals to cause producing the deflection or mechanical movement. The value of the measured quantity can then be inferred from the deflection or mechanical displacement at the point of balance.This is how deflection type instruments work.

2)Null type instruments : In these null type instruments, a zero or null Indication leads to determination of the magnitude of measured quantity. In contrast to deflection type instruments, a null type instrument attempts to maintain the deflection at zero by suitable application of an effect opposing that generated by the measured quantity. 

Therefore for the  operation of a null type instruments the following are required:

• The effect produced by the measured quantity

• The opposing effect whose value is accurately known. This is necessary in order to determine the numerical value of the measured quantity accurately

• A detector which detect the null conditions that is a device which indicates zero deflection when the effect produced by the measured quantity is equal to the effect produced by the opposing quantity. The detector should be capable of displaying unbalance i.e., a condition when the effect producer by the measured quantity is not equal to the opposing effect.

There is another way in which electrical measuring instruments may be classified. This classification is based on the functions they perform. 

The three main functions are explained below:
1) Indicating function : Instruments and Systems use different kinds of methods for supplying information concerning the variable quantity under measurement. Most of the time this information is obtained as a deflection of a pointer of a measuring instrument. In this way , the instrument performs a function which is known as indicating function.

2) Recording function : In many cases the instrument makes a written record usually on paper, of the value of the quantity under measurement against time or against some other variable.Thus this electrical instrument performs a recording function. For example, a potentiometric type of recorder used for monitoring temperature records the instantaneous values of temperatures on a strip chart recorder.

3) Controlling function : This is one of the most important functions especially in the field of industrial control processes. In this case, information is used by the instrument or the system to control the original measured quantity.Thus there are three main groups of electrical instruments. The largest group has the indicating function. Next in line is the group of instruments which have both indicating and recording functions. The last group falls into a special category and performs all the three functions i.e., indicating, recording and controlling.

What are the Characteristics of Static and Dynamic Electrical Measuring Instruments??

What are the Characteristics of Static and Dynamic Electrical Measuring Instruments??

Electrical Measurements

The performance characteristics of electrical measuring instruments can be divided into two categories:

1) Static characteristics
2) Dynamic characteristics

1) Static characteristics of Electrical Measuring Instruments :
Some applications involve the measurement of quantities that are either constant or vary slowly with time. Under these circumstances it is possible to define a set of criteria that gives a meaningful description of quality of measurement without interfering with dynamic descriptions that involve the use of differential equations. These criteria are called static characteristics.

The main static characteristics are

Accuracy: It is the closeness with which an instrument reading approaches  the true value of the quantity being measured. Thus accuracy of a measurement means conformity to truth.It the important static characteristic of electrical measuring instruments.
               
Accuracy can be specified in terms of inaccuracy or limits of errors and can be expressed in the following ways:

a.Point accuracy : This is the accuracy of the instrument only at one point on its scale.The specification of this accuracy does not give any information about the accuracy at other points on the scale or in the words,does not give any information about the general accuracy of the instrument.

b.Accuracy as percentage of scale range : When an instrument has uniform scale, it's accuracy may be expressed in terms of scale range.

c.Accuracy as percentage of true value : The best way to conceive the idea of accuracy is to specify it in terms of the true value of the quantity being measured within +0.5% or  -0.5% of true value.


Precision : It is a measure of the reproducibility of the measurements i.e., given a fixed value of quantity, precision is a measure of the degree of agreement  within a group of measurements. The term precise means clearly or sharply defined. As an example of the difference in meaning of the two terms accuracy and precision, suppose that we have an ammeter which possesses high degree of precision by virtue of its clearly legible, finely divided, distinct scale and a knife edge pointer with mirror arrangements to remove parallax. It is also the important static characteristic of electrical measuring instruments.
              Let us say that its readings can be taken to 1/100 of an ampere. Now every time we take a reading, the ammeter is as precise as ever, we can take readings down to 1/100 of an ampere and the readings are consistent and clearly defined. However, the readings taken with this ammeter are not accurate, since they do not confirm to truth on account of its faulty zero adjustment.

Stability : The ability of a measuring system to maintain standard of performance over prolonged periods of time. Zero stability defines the ability of an instrument restore to zero reading after the input quantity has been brought to zero,while other conditions remain the same.

Resolution : If the input to an instrument is increases slowly from some arbitrary non-zero value, it will be observed that the output of the instrument does not change at all until there is a certain minimum increment in the input. This minimum increment in what is input is called resolution of the instrument. Thus, the resolution is defined as the smallest incremental of the input quantity to which the measuring system responds. This is the third most important static characteristic of electrical measuring instruments.
          Resolving power or discrimination power is defined as the ability of the system to respond to small changes of the input quantity. One of the major factors influencing the resolution of an instrument is how finely its output scale is subdivided. If the input to an instrument is increased very gradually from zero value, there will be some minimum value of input below which no output change can be observed or detected. This minimum value of input defines the threshold of the instrument.

Threshold : If the instrument input is increased very gradually from zero there will be some minimum value below which no output change can be detected. This minimum value defines the threshold of the instrument. In specifying threshold, the first detectable output change is often described as being any noticeable measurable change.

Drift : It is a slow variation in the output signal of a transducer or measuring system which is not due to any change in the input quantity. It is primarily due to changes in operating conditions of the components inside the measuring system. The drift is noticeable as zero drift and sensitivity drift.
            Zero drift is a deviation observed in the instrument output with time from the initial value, all the other measurement conditions are constant. This may be caused by a change in component values due to variation in ambient conditions or due to aging. Typical units by which zero drift is measured are volts per °C in the case of a voltmeter affected by changes in ambient temperature. This is often called the zero drift coefficient related to temperature changes.

Repeatability : It is the characteristic of precision instruments. It describes the closeness of output readings when the same input is applied repetitively over a short period of time, with the same measurement conditions, same instrument and observer, same location and same conditions of use maintained throughout. It is affected by internal noise and drift. It is expressed in percentage of the true value. Measuring transducers are in continuous use in process control operations and the repeatability of performance of the transducer is more important than the accuracy of the transducer, from considerations of consistency in product quality.

Reproducibility : It is the closeness with which the same value of the input quantity is measured at different times and under different conditions of usage of the instrument and by different instruments. The output signals and indications are chequed for consistency over prolonged periods and at different locations. Perfect reproducibility ensures interchangeability of instruments and transducers.

Dead Zone : It is the largest change of input quantity for which there is no output of the instrument. For instance, the input applied to the instrument may not be sufficient to overcome the friction and will, in that case not move at all.
           It is due to either static friction(stiction), backlash or hysteresis. Dead zone is also known as dead band or dead Space. All elastic mechanical elements used as primary transducers exhibit effects of hysteresis, creep and elastic after- effect to some extent.

Backlash : The maximum distance or angle through which any part of mechanical system may be moved in one direction without applying appreciable force or motion to the next part in a mechanical sequence.

Hysteresis : Hysteresis is phenomenon which depicts different output effects when loading and unloading whether it is a mechanical system or any electrical system or any other system. Hysteresis is the difference in the readings of an instrument, which fixed value of the input signal, which depends on whether that input value is approached from increasing or decreasing values of input. That is upscale and down scale deflections do not coincide when the measurement is made of the same value by method of symmetry. The non coincidence between the loading and unloading curves is known as hysteresis.

Linearity : It defines the proportionality between input quantity and output signal. If the sensitivity is constant for all values from zero to full scale value of the measuring system, then the calibration characteristic is linear and is a straight line passing through origin. If it is an indicating or recording instrument the scale may be made linear. In case there is a zero error the characteristic assumes the form of equation given by y=mx+c where y is output,x is input,m is slope and c is intercept.

              Linearity is the closeness of the calibration curve of a measuring system to a straight line. If an instruments calibration curve for desired input is not a straight line, the instrument may still be highly accurate. In many applications, however, linear response is most desirable.

Range or Span : Span and range are the two terms that convey the information about the lower and apa calibration points. The range of indicating instruments is normally from zero to full scale value and the Span is simply the difference between the full scale and lower scale value. But same instruments operate under a bias so that they start reading, for example,voltages from 5V to 25V only. The zero of these instruments is suppressed from indication by means of a bias. In such case, the scale range is said to be from 5V to 25V and the scale span is 25-5 i.e.,20V.

Bias : Bias describes a constant error which exits over the full range of measurement of an instrument. The error is normally removable by calibration.

Tolerance : It is a term which is closely related to accuracy and defines the maximum error which is to be expected in some value. While it is not, strictly speaking,a static characteristic of measuring instruments, it is mentioned here because the accuracy of some instruments, is sometimes quoted as a tolerance figure. Tolerance, when used correctly, describe the maximum deviation of a manufactured component from some specified value. Electric circuit components such as resistors have tolerances of perhaps 5%.

Dynamic characteristics of Electrical Measuring Instruments : 
Measurement systems having inputs dynamic in nature, the input varies from instant to instant, so does the output.The behaviour of the system under such conditions is dealt by the dynamic response of the system and its dynamic characteristics of electrical measuring instruments are given below:

Dynamic error : It is the difference of true value of the quantity changing with the time the value indicated by the instrument provided static error is zero. Total dynamic error is the phase difference between input and output of the measurement system.

Fidelity : It is the ability of the system to reproduce the output in the same form as the input. In the definition of fidelity any time lag or phase difference is not included. Ideally a system should have 100% fidelity and the output should appear in the same form as the input and there is no distortion produced by the system. Fidelity needs are different for different applications.

Bandwidth : It is the range of frequencies for which its dynamic sensitivity is satisfactory.For measuring systems, the dynamic sensitivity is required to be within 2% of its statics sensitivity.
                For other physical systems, electrical filters electronic amplifiers, the above criterion is relaxed with the result that their bandwidth specification extend to frequencies at which the dynamic sensitivity is 70.7 % of that at zero or the mid- frequency.

Speed of response : It refers to its ability to respond to sudden changes of amplitude of input signal. It is usually specified as the time taken by the system to come close to steady state conditions, for a step input function. Hence the speed of response is evaluated from the knowledge of the system performance under transient conditions and terms such as time constant, measurement lag, settling time and dead time dynamic range are used to convey the response of the variety of systems, encountered in practice.This is the important dynamic characteristics of electrical measuring instruments.

Time constant : It is associated with the behaviour of a first order system and is defined as the time taken by the system to reach 0.632 times its final output signal amplitude. System having small time constant attains its final output amplitude earlier than the one with larger time constant and therefore, has higher speed of response.

Measurement lag : It is defined as the delay in the response of an instrument to a change in the measurand. This lag is usually quite small but it becomes quite significant where high speed measurements are required.

Measurement lag is of two types. In retardation type, 

• The response of the instrument begins immediately after a change in the measurand has occurred. 
• In time delay type, the response of the system begins after a delay time after application of the input.

Settling time : It is the time required by the instrument or measurement system to settle down to its final steady state position after the application of the input. Fo portable instruments, it is the time taken by the pointer to come to rest within - 0.3% to +0.3% of its final scale length while for panel type instruments, it is the time taken by the pointer to come to rest within -1% to +1% of its final scale length. Smaller settling time indicates highest speed of response.Settling time is also dependent on the system parameters and varies with the condition under which the system operates.This is also the important dynamic characteristics of electrical measuring instruments.