analog measurement

1.  The measurement of a given quantity is essentially an act or
the result of comparison between the quantities and a
predefined standard.
 The device used for comparing the unknown quantity with the
unit of measurement or standard quantity is called a
Measuring Instrument.

2.  Direct method: compare the quantity directly with the primary
or secondary standard.
 Indirect method: in this method measurement involve the use
of instruments as a physical means of determining quantities
or variables.

3. Electrical instruments may be divided into two categories, that
are;
1. Absolute instruments,
2. Secondary instruments.
-Absolute instruments gives the quantity to be measured in
term of instrument constant & its deflection. e.g.
galvanometer
- In Secondary instruments the deflection gives the
magnitude of electrical quantity to be measured directly.
These instruments are required to be calibrated by
comparing with another standard instrument before putting
into use.

4. CLASSIFICATION OF SECONDARY INSTRUMENTS
 Secondary instruments can be classified into three types;
i. Indicating instruments
ii. Recording instruments
iii. Integrating instruments

5.  Indicating Instruments:
It indicate the magnitude of an electrical quantity at the time
when it is being measured. The indications are given by a
pointer moving over a graduated dial.

6.  Recording Instruments:
The instruments which keep a continuous record of the
variations of the magnitude of an electrical quantity to be
observed over a defined period of time.

7.  Integrating Instruments:
The instruments which measure the total amount of either
quantity of electricity or electrical energy supplied over a
period of time. For example energy meters

8.  In these types of instruments, pointer of the electrical
measuring instrument deflects to measure the quantity. The
value of the quantity can be measured by measuring the net
deflection of the pointer from its initial position. In order to
understand these types of instruments let us take an example
of deflection type permanent magnet moving coil ammeter .

9.  It has two permanent magnets
which are called the stationary
part of the instrument and the
moving part which is between
the two permanent magnets
that consists of pointer. Thus
the torque is proportional to the
current which is given by the
expression Td = K.I

10.  In opposite to deflection type of instruments, the null or zero
type electrical measuring instruments tend to maintain the
position of pointer stationary. They maintain the position of
the pointer stationary by producing opposing effect. Thus for
the operation of null type instruments following steps are
required:
 Value of opposite effect should be known in order to calculate
the value of unknown quantity.
 Detector shows accurately the balance and the unbalance
condition accurately.
 The detector should also have the means for restoring force.

11.  ACCURACY: Accuracy is the agreement between a
measured value and the true value.
 It is the closeness with which an instrument reading
approaches the true value of the quantity being measured.
 TRUE VALUE: The true value of quantity to be measured
may be defined as the average of an infinite number of
measured values.

12.  PRECISION : Precision also called reproducibility or
repeatability, the degree to which further measurements or
calculations show the same or similar results.
 Instrument precision is usually associated with the number of
digits displayed on the output, i.e., its resolution.

13. High accuracy, but
low precision
High precision,
but low accuracy

14.  STATIC ERROR: static error is defined as the difference
between the measured value and the true value of the quantity.
δa=Am-At
where δa=error
Am=measured value
At=true value

15.  DRIFT: if the quantity has perfect reproducibility the we can
say there is no drift
 Reproducibility is the closeness with which a given value may
be repeatedly measured.
 DEAD ZONE: it is defined as the largest change of input
quantity for which there is no output present.

16.  If we are making physical measurements, there is always error
involved. The error is notated by using the delta, Δ, symbol
followed by the variable representing the quantity measured.
 A simple way of looking at the error is as the difference
between the true value and the approximate value.
i.e: Error (e) = True value – Approximate value

17.  Limiting errors(guarantee errors)
 The accuracy of measuring instrument is guaranteed within a
certain percentage (%) of full scale reading
 E.g manufacturer may specify the instrument to be accurate at
±2 % with full scale deflection

18.  There are three types of errors
1. Gross errors
2. Systematic errors
3. Random errors

19.  cause by human mistakes in reading/using instruments
 may also occur due to incorrect adjustment of the instrument
and the computational mistakes
 cannot be treated mathematically
 cannot eliminate but can minimize
 Eg: Improper use of an instrument.
 This error can be minimized by taking proper care in
reading and recording measurement parameter.
 In general, indicating instruments change ambient conditions
to some extent when connected into a complete circuit.
 Therefore, several readings (at three readings) must be taken
to minimize the effect of ambient condition changes.

20.  Systematic error is caused by any factors that systematically
affect measurement of the variable across the sample.
 For instance, if there is loud traffic going by just outside of a
classroom where students are taking a test, this noise is liable
to affect all of the children's scores -- in this case,
systematically lowering them.
 There are 3 types of systematic error :-
(i) Instrumental error
(ii) Environmental error
(iii) Observational error

21. (i) Instrumental error
- inherent while measuring instrument because of
their mechanical structure (eg: irregular spring
tension, stretching of spring, etc)
- error can be avoid by:
(a) selecting a suitable instrument for the
particular measurement application
(b) apply correction factor by determining
instrumental error
(c) calibrate the instrument against standard

22. (ii) Environmental error
- due to external condition effecting the
measurement including surrounding area
condition such as change in temperature, humidity,
barometer pressure, etc
- to avoid the error :-
(a) use air conditioner
(b) sealing certain component in the instruments
(c) use magnetic shields

23. (iii) Observational error
- introduce by the observer
- most common : parallax error and estimation
error (while reading the scale)
- Eg: an observer who tend to hold his head too
far to the left, while reading the position of the needle
on the scale.

24. It occur due to unknown causes,
occur when all systematic error has accounted
- accumulation of small effect, require at high degree
of accuracy
- can be avoid by
(a) increasing number of reading
(b) use statistical means to obtain best
approximation of true value