Characteristics of electrical measuring instruments:
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 the 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
i) 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.
ii)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.
iii)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.
iV)Resolution:
If the input to an instrument 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.
V)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.
vi)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 ageing.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.
vii)Repeatability:
It is the characteristic of precision electrical measuring 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 the performance of the transducer is more important than the accuracy of the transducer, from considerations of consistency in product quality.
viii) 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 checked for consistency over prolonged periods and at different locations. Perfect reproducibility ensures interchangeability of instruments and transducers.
ix) 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 dead Space. All elastic mechanical elements used as primary transducers exhibit effects of hysteresis, creep and elastic after- effect to some extent.
X)Backlash:
The maximum distance or angle through which any part of the mechanical system may be moved in one direction without applying appreciable force or motion to the next part in a mechanical sequence.
XI)Hysteresis:
Hysteresis is a 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 the method of symmetry.The non-coincidence between the loading and unloading curves is known as hysteresis.
Xii)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 the 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 the 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.
Xiii)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.
Xiv)Bias:
Bias describes a constant error which exits over the full range of measurement of an instrument. The error is normally removable by calibration.
XV)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%.
2) 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:
1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.For 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 the highest speed of response.Settling time is also dependent on the system parameters and varies with the conditions under which the system operates.This is also the important dynamic characteristics of electrical measuring instruments.