Ruchi Gajjar
 A.
K. Sawhney - A course in Electrical and electronics measurement
and Instrumentation, Dhanpatrai & Sons
 It is necessary to have a permanent record or state of a
phenomenon being investigated
 Recorders, often measure – flow, force, pressure, temperature, I,
V, Power, etc
 A recorder, thus records electrical and non-electrical quantities
as a function of time.
 Recording requirements:
 Method by which data is recorded
 If sys = analog, Analog recording is to be used
If sys = digital, then digital recording is to be used
Recorders
Graphic
Strip Chart
Galvanometer
Null type
X-Y
Analog
Digital
Oscillographic
Magnetic Tape
1. Marking with ink filled stylus
 Most common
Advantage:
 Ordinary paper can be used
 Low cost
 Operation over a wide range of recording speeds possible
 Little friction between stylus tip and paper
Disadvantage:
 Ink splatters at high speed
 Patches at low speeds
 Clogs when stylus at rest
2. Marking with heated stylus
 Writes on special paper
 Overcomes the difficulties encountered in ink writing system
 It melts a thin, white wax like coating on a black paper base
Advantage:
 Reliable
 Offers high contrast trace
Disadvantage:
 Cannot be used for recording certain processes which produce
heat which indirectly effects recording
3.With pressure sensitive paper
 If chart made from a pressure sensitive paper, a simple recording
process is possible
 A V-shaped pointer is passed under a chopper bar which presses
pen into paper once/second -> making a series of marks into the
special paper
Disadvantage:
 System -> not purely continuous and hence is suitable for
recording some slowly varying quantities
4. Electric stylus marking
 Paper with special coating which is sensitive to current
 When current conducted from the stylus to paper -> a trace
appears on the paper
Advantage:
 Wide range of marking speed
 Low stylus friction
 Long stylus life
Disadvantage:
 Cost = high
5. Electrostatic stylus
 Uses a stylus which produces a high voltage discharge thereby
producing a permanent trace on an electro-sensitive paper
6. Optical marking
 Uses a beam of light to write on a photosensitive paper
 This method allows higher frequency to be recorded and permits a
relatively large chart speed with good resolution
Disadvantage:
 Cost = high
 Paper must be developed before a record as writing process ->
photographic
 Therefore, method not suitable for instantaneous monitoring
required
2 types for producing graphic
representation
1. Curvilinear System
 Stylus – mounted on central pivot
and moves through an arc which
allows a full-width chart marking
 For full-range, lines drawn on chart –
curved
 Used on many records with PMMC
galvanometer actuating stylus filled
with ink
Disadvantage:
 Tracing on charts is difficult to
analyse because of curved time
base line
2. Rectilinear System
 Line at constant time is perpendicular to time axis
 Therefore, system produce straight line across width of the chart
1. Galvanometer type
 Operates on deflection principle
 Deflection
is
produced
by
galvanometer which produces a
torque on account of current passing
through the coil
 Current is proportional to quantity
being measured
2. Null- type
 Change in input produced by signal from sensors or transducers
upsets the balance of measuring circuit of the recorders
 As a result, error signal -> generated that operates some device which
restores balance and brings system to null condition
 Amount of movement of this balance restoring device is the indication
of magnitude of error signal
 Direction of movement is an indication of direction of quantity being
measured
 Types of null recorders:
1. Potentiometric
2. Bridge
3. LVDT
2.1 Potentiometric
 Disadvantage of galvanometric recorders -> low input impedance and
limited sensitivity
 Therefore to overcome the input impedance problem, we amplify
difference between input term and display or indicating instrument
 Accuracy -> improved if input signal is compared with a reference
voltage by using a potential circuit
 Error signal = difference between input signal and reference voltage,
is amplified and used to energise field coil of dc motor
 A wiper is connected to dc motor.
 This wiper moves over potentiometer in appropriate direction to
require magnitude of error signal to obtain balance
 Wiper comes to rest when unknown signal voltage is balanced as
voltage of potentiometer
2.1 Potentiometric
Single point recorders:
 Recording instrument can
combination of both
be entirely mechanical, electrical or a
 Graphic record shows variation of measured variable with time
 Instrument which records only one value -> single point recorder
Multi point recorders:
 Records several inputs
 Strip chart recorders – measure variations of quantity wrt time
 X-Y recorders – gives graphic record of relationship between two
variables
e.g. emf plotted as a function of another emf
–> one self-balancing potentiometer control position of paper while other
control position of pen, i.e. one moves in X-direction and the other in Ydirection