Measurement instruments Static and dynamic characteristics Calibration Systematic Errors, Random Errors, Total measurement of Errors

2. Measurement instruments
Static and dynamic characteristics
Calibration
Systematic Errors, Random Errors, Total measurement of
Errors
Presentation on Scientific and industrial instrumentation

3. Classification of measuring instruments

4. Mechanical, Electrical and Electronics Instruments:
Mechanical instrument: Mechanical instruments are very reliable for static and
stable conditions. As they use mechanical parts these instruments cannot
faithfully follow the rapid changes which are involved in dynamic instruments.
But they are cheaper in cost and durable.
Electrical Instruments: When the instrument pointer deflection is caused by the
action of some electrical methods then it is called an electrical instrument. The
time of operation of an electrical instrument is more rapid than that of a
mechanical instrument.
Electronic Instruments: Electronic instruments use semiconductor devices. They
are very fast in response. With the use of electronic devices, a very weak signal
can be detected by using pre-amplifiers and amplifiers.

5. Absolute / Primary Instruments and Secondary Instruments
Electrical Measuring Instruments are divided into its two types which are
Absolute instruments / Primary instruments and Secondary instruments.
Measuring instruments are classified into its many types and its
classification is shown below:

6. Absolute / Primary Instruments
These are the instruments which give the value of the quantity measured
according to the constants of the instrument and its deflection. Primary
instruments are mostly used in standard laboratories and as a standardising in
various institutions and research organizations.
Secondary Instruments
Secondary instruments are the instruments in which the value of the electrical
quantity is measured and is determined from the deflection of the instruments.
These instruments provide the magnitude of the electrical quantity which is to be
measured.

7. Analog and Digital instrument:
Analog instrument: The signals of an analog unit vary in a continuous fashion
and can take on an infinite number of values in a given range. Fuel gauge,
ammeter and voltmeters, wristwatch, speedometer fall in this category.
Digital Instruments: Signals that vary in discrete steps and that take a finite
number of different values in a given range are digital signals and the
corresponding instruments are of digital type. Digital instruments have some
advantages over analog meters, in that they have high accuracy and high speed
of operation. Digital multimeter is an example for the digital instrument.

8. Deflection and Null Output Instruments:
In a deflection-type instrument, the deflection of the instrument
indicates the measurement of the unknown quantity. The measurand
quantity produces some physical effect which deflects or produces a
mechanical displacement in the moving system of the instrument.
Permanent Magnet Moving Coil (PMMC), Moving Iron (MI), etc., type
instruments are examples of this category.
In Null type instruments, a zero or null indication leads to the
determination of the magnitude of the measurand quantity. The null
condition depends upon some other known conditions. These are
more accurate and highly sensitive as compared to deflection-type
instruments. A dc potentiometer is a null- type instrument.

9. Integrating Instruments
These instruments measure the total amount of electricity or electric energy
supplied to it with a part of time. These instruments measure the electricity in
ampere-hours. These instruments provide readings with the product of
electricity and time. These instruments include ampere-hour meters and energy
meters in its category.
Recording instruments
It keeps record of the variations in the magnitude of an electricity with the
passage of time. These instruments are used in power houses for the
measurement of electric current, voltage, electric power etc.
Indicating Instruments
Indicating instruments indicate the magnitude of an electrical quantity which
is to be measured with respect to time. These instruments are categorized
into ammeters, voltmeters, watt meters, frequency meters, power factor
meters etc.

11. Static characteristics of instrument:
The static characteristics of a measuring instruments are concerned only with the
steady-state reading that instrument settles douent.
 Accuracy and inaccuracy(measurement uncertainty)
 Precision/repeatability/reproducibility
 Tolerance
 Range or span
 Linearity
 Sensitivity of measurement
 Threshold
 Resolution
 Sensitivity of disturbance
 Hysteresis effect
 Dead space

12. Accuracy: The accuracy of an instrument is a measure of how close the output reading
of the instrument is to the correct value.
Inaccuracy is the extent to who is a reading might be wrong and if often quoted as a
percentage of the full scale reading of an instrument.
Precision:Precision is a term that describes an instruments degree of freedom from
random errors.
Repeatability: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 a
observer, same location and same conditions of use maintained throughout.
Reproducibility:The closeness of output readings for the same input when there are
changes in the method of measurement, observer, measuring instrument, location,
conditions of use and time of measurement.
Range or span:The minimum and maximum value of a quantity that the instrument is
designed to measure.

13. Linearity:Output reading of an instrument is linearly proportional to the quantity being
measured.
Output reading
Measured Quantity
Gradient=sensitivity of
measurement

14. Sensitivity of measurement:A measure of the change in instrument output that occurs
when the quantity being measured being by a given amount.
Scale deflection
Value of measured producing deflection
Threshold:The minimum level of input for which the change in the instrument output
reading is large enough magnitude to be detectable.
Absolute value
Percentage of full scale reading
Resolution:The lower limit on the magnitude of the change in the input measured
quantity that produces an observable change in the instrument output.
Sensitivity of disturbance:As variations occur in the ambient temperature pressure etc,
certain static instrument characteristics changes and the sensitivity to disturbance is a
measure of the magnitude of change.
Sensitivity=

15. Dead space:The range of different input values over who is there is no change in output
value.
Hysteresis effect:

16. Dynamic characteristics: The dynamic characteristics of a measuring instrument
describe its behaviour between the time a measured quantity changes value and the
time when the instrument output attains a steady value in response.

17. Define calibration and explain difference methods of calibration
All measuring instruments are to prove themselves their ability to measure reliably
and accurately . For this the results of measurement are to be compared with
higher standards which are traceable to national or international standards.
The procedure involved is termed as calibration. Calibration is thus a set of
operations that establish the relationship between the values that are indicated by
the measuring instrument and corresponding known values of measured.

18. Primary calibration:
If the instrument is calibrated against primary standards, then the calibration
is called primary calibration. After the primary calibration, the instrument can
be used as a secondary calibration instrument.

19. Secondary calibration:
The secondary calibration instrument is used as secondary for further calibration of
other devices of lesser accuracy. This type of instruments are used in general
laboratory practice as well as in the industry because they are practical calibration
sources.

20. Secondary calibration can further be classified into two types.
1. Direct calibration
2. Indirect calibration
Direct calibration:
Direct calibration with a known input source is in general of the same order of
accuracy as primary calibration. So, the instrument which are calibrated directly
are also used as secondary calibration instruments.

21. Indirect calibration:
This procedure is based on the equivalence of two different devices adopting
same similarity concept.

22. Measurement Error
The measurement error is defined as the difference between the true or actual
value and the measured value. The true value is the average of the infinite number
of measurements, and the measured value is the precise value.
Types of Errors in Measurement:
The error may arise from the different source and are usually classified into the
following types. These types are
1.Gross Errors
2.Systematic Errors
3.Random Errors

24. 1. Gross Errors
The gross error occurs because of the human mistakes. For examples consider the person
using the instruments takes the wrong reading, or they can record the incorrect data. Such
type of error comes under the gross error. The gross error can only be avoided by taking the
reading carefully.
Two methods can remove the gross error. These methods are :
# The reading should be taken very carefully.
# Two or more readings should be taken of the measurement quantity. The readings are
taken by the different experimenter and at a different point for removing the error.

25. 2.Systematic Errors
Systematic errors in experimental observations usually come from the measuring instruments.
The systematic errors are mainly classified into three categories.
2 (i) Instrumental Errors
2 (ii)Environmental Errors
2 (iii) Observational Errors
2 (i) Instrumental Errors:
These errors mainly arise due to the three main reasons.
#Inherent Shortcomings of Instruments
#Misuse of Instrument
#Loading Effect

26. 2 (ii) Environmental Errors
These errors are due to the external condition of the measuring devices. Such types of errors mainly occur due to
the effect of temperature, pressure, humidity, dust, vibration or because of the magnetic or electrostatic field
2(iii) Observational Errors
Such types of errors are due to the wrong observation of the reading. There are many sources of observational
error. For example, the pointer of a voltmeter resets slightly above the surface of the scale.
3. Random Errors
Random errors in experimental measurements are caused by unknown and unpredictable changes in the
experiment. These changes may occur in the measuring instruments or in the environmental conditions.
Examples of causes of random errors are:
#electronic noise in the circuit of an electrical instrument,
#irregular changes in the heat loss rate from a solar collector due to changes in the wind.