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 Analog Digital Graphic Oscillographic Magnetic Tape Strip Chart X-Y Galvanometer Null type
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 Y- direction