Instrumentation and measurement, important points

Precision is a measure of reproductability in measurement. 3 types of galvanometer sensitivity – current, voltage, megohm. Current sensitivity is ratio between deflection(in mm) and current (in micro amphere). Voltage sensitivity is ratio between deflection (in mm) and voltage ( in milli volt). Megohm is numerically equal to current sensitivity. Megohm sensitivity is the megohms required in series with galvanometer to produce a single scale deflection when 1 v is applied.Instrumental error can be reduced by correction factors. Static error is of 3 types – gross error, systematic error and random error

Gross error – due to human mistakes

Systematic error – inherent in measuring instrument. It is of 3 types – observational, environmental, instrumental

Random error- treated mathematically

Observational error – parallax error

Absolute error = true error – measured error

% error = absolute error *100/true error

Ammeter is always connected in series with circuits, Resistance of ammeter should be low, for that number of turns in operating coil is low. Voltmeter is connected parallel with circuit. Resistance of voltmeter should be high, for that high resistance is placed series with the operating coil. To enhance the measuring capacity of ammeter and voltmeter shunts and multipliers are used. Shunt is placed parallel with the instrument to reduce amount of current passing through the instrument

Manganin or constantan is used as shunt, it is present with in the box instrument with a length of constant temperature resistance, alternatively external shunt with low resistance is available. Temperature resistance of shunt and instrument should be low and nearly identical. Resistance of shunt should not vary with time

Shunt should have low thermal emf. Shunts are placed with in the instrument if the current is less than or equal to 200A otherwise it is placed outside. Shunt should carry current without excess temperature rise . When shunt is used with ammeter, then measured current is sum of the shunt current and full scale deflection current of ammeter

To maintain current division in shunt and instrument constant for all frequency, impedance ratio of instrument and shunt should be constant, for that time constant (L/R) of shunt and instrument should be same. Multiplier is a high resistance connected series with galvanometer to avoid damage due to excess current. Power indicated by dc circuit is sum of the power consumed by load and ammeter/voltmeter

Voltmeter/ammeter is used in dc circuit and wattmeter is used in ac circuits for power estimation. Two types of wattmeter- induction type and electrodynamometer type

Low resistance – 1ohm and below 1 ohm

Medium resistance – 1 to 10,000 ohm

High resistance – 10,000 +

Measurement of low resistance – ammeter voltmeter method, kelvin’s double bridge method, potentiometer method. kelvin’s double bridge method is a modified wheatstone bridge and it have increased accuracy. kelvin’s double bridge method have second set ratio arms hence its name is called as double bridge

When secondary turns delivering maximum current to instrument then the product of secondary V & I is called rated burden. two types of errors in CT & PT – ratio error & phase angle error. Ratio error due to energy component of excitation current and phase angle error due to magnetic component of excitation current. CT have ring or circular shaped core. In CT coils are wounded closely to avoid secondary leakage flux and separately wounded

To reduce errors in CT reduce ampere-turns of secondary less than primary. PT is used to measure high voltages with low range voltmeters. In PT supply and load is connected parallel with primary. Errors in PT are less than CT. Ratio error in PT can be reduced by increasing small amount of secondary turns, the increased turns compensate voltage drop in PT. In PT windings are not wounded separately (one over the other). Errors in PT can be controlled by reducing no load current. To reduce core loss in PT good quality core material is used

Transducers are also called pick up. Transducer consist of two elements – sensing and transduction element. Inverse transducer – electrical energy into mechanical energy, eg : piezoelectric crystal. Strain gauge is used as secondary transducer. Gauge factor of a strain gauge is 1 + 2v

Element used in strain gauge should have high gauge factor, low resistance temperature coefficient, linear characteristics, good frequency response and high resistance . 3types of strain gauges – wire, foil and semiconductor strain gauge

Inductive transducer is used to convert linear motion into electrical signal. In inductive transducer a plate is placed at right angles to the axis of coil any displacement in coil will result inchange in inductance. LVDT- linear variable differential transformer is widely used as inductive transducer. LVDT consist of 1 primary and 2 secondary windings. When iron core in null position of LVDT then 2 secondary windings have same potential so output (difference of secondaries) is zero. LVDT have infinite resolution, linear chara, low power consumption, constant output impedance (advantages). LVDT performance is affected by temperature, vibration, stray magnetic fields

Measuring of pressure/force is done by bellows, diaphragms, bourden tube,Bellow have corrugated shape and provides high force and wide pressure range. Diaphragms are made of metal alloys like bronze, beryllium, copper, stainless steel. Diaphragm have high accuracy and good response. Bourden tube is curved or twisted metallic tube and made up of long thin walled cylinder. Measuring fluid flows – hot wire anemometer, using thermometer, nucleonic meter, turbine , magnetic flow meter

Turbine flow meter is used to measure flow of gas of very low flow rate or liquid gases. Magnetic pick up is used to sense speed of rotation of turbine flow meter. Magnetic flow meter uses faradays law of electromagnetic induction. Induced emf in magnetic flow meter is linear and directly proportional to volumetric flow. Magnetic flow meter can handle both low flowing and high flowing liquids

Magnetic flow meter is expensive, heavy and in large size. Instruments used to measure pressure – mechanical, electro-mechanical, piezo electric, electronic. Torque is measured using strain gauge method, where strain gauges are mounted to each other at an angle of about 45 degree to each other. In strain gauge method strain bridge method is used where two gauges have tensile stresses and other two have compressive stresses

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