Lecture 02 The accuracy of measuring instrument is described with certain percentage (%) . E.g manufacturer may specify the instrument to be accurate at 2 % . If input measured quantity to the instrument is steadily increased the output reading varies in the manner shown in curve A If the input is then steadily decreased the output varies in the manner shown in curve B Non coincidence between the loading and unloading curves is known as hysteresis Hysteresis is exhibited by instruments that contain springs and electrical windings formed around an iron core EE220 Instrumentation & Measurement 3 EE220 Instrumentation & Measurement 4 Instrument System are divided into different types based on accuracy, performance and application Active and Passive Instruments Null-Type and Deflection-Type Instruments Analogue and Digital Instruments EE220 Instrumentation & Measurement 5 Active Instrument requires external source of power. Either a battery or an external voltage source. Examples of active components include transistors, LED lights, and opamps or other IC chips EE220 Instrumentation & Measurement 6 Passive Instrument requires no external source of power Pressure Gauge EE220 Instrumentation & Measurement 7 Pressure Gauge is a Deflection Type Instrument Value of quantity measured displayed in terms of movement of pointer Calibration of spring is difficult therefore Deflection type instrument is less accurate More convenient to use EE220 Instrumentation & Measurement 8 Dead Weight Pressure Gauge is a Null Type Instrument, works on the principle that P= F/A Weights are put on top of piston until the downward force balances the fluid pressure Weights are added until the piston reaches a datum level known as the null point Accuracy depends upon calibration of weights Calibration of weights is easier therefore null type instruments are more accurate Inconvenient to use, for calibration purposes only EE220 Instrumentation & Measurement 9 Analogue Instruments give a continuously varying output as the quantity being measured changes ◦ Deflection Type pressure Gauge Digital Instruments give output that varies in discrete quantities EE220 Instrumentation & Measurement 10 Standard quantity of any physical quantity Global standard, the International System of Units(SI). History of uniform systems/standards dates back to Bronze age. Fundamental units Derived units Go through table 2-3 of your book … Conversion of units Standards of measurement Electrical standards ◦ ◦ ◦ ◦ International Primary Secondary Working ◦ ◦ ◦ ◦ ◦ Ampere Resistance Voltage Capacitance Inductance brief.. Do not exceed four pages… Be concise and Ampere’s Law DC Electromegnatism… Devices use to measure, indicate or both are called meters… Why Meters….?? ◦ Whilst Troubleshooting, testing, or repairing equipment you need various meters … ◦ To check for proper circuit voltages, currents, resistances, and to determine if the wiring is defective. You may be able to connect these test instruments to a circuit and take readings. A stationary, permanent-magnet, movingcoil meter is the basic meter movement used in most measuring instruments used for servicing electrical equipment. When current flows through the coil, a resulting magnetic field reacts with the magnetic field of the permanent magnet and causes the movable coil to rotate. Greater the intensity of current, stronger the magnetic field. Stronger the magnetic field , greater the rotation of the coil. Example: Galvanometer. A galvanometer is an instrument for detecting and measuring electric current. The term "galvanometer", in common use by 1836, was derived from the surname of Italian electricity researcher Luigi Galvani Galvanometers were the first instruments used to detect and measure electric currents. All galvanometers are based upon the discovery by Hans C. Oersted that a magnetic needle is deflected by the presence of an electric current in a nearby conductor. The extent to which the needle turns is dependent upon the strength of the current. The early moving-magnet form of galvanometer had the disadvantage that it was affected by any external magnetic field. Disadvantage: Mechanical fault in spring In the first galvanometers, a freely turning magnetic needle was hung in a coil of wire; in later versions the magnet was fixed and the coil made movable. Modern galvanometers are of this movablecoil type and are called d'Arsonval galvanometers (after Arsène d'Arsonval, a French physicist) Further improvement included replacement of fine wire suspension with a pivot, and provided restoring torque and electrical connections through spiral spring. If a pointer is attached to the moving coil so that it passes over a suitably calibrated scale, the galvanometer can be used to measure quantitatively the current passing through it. Such calibrated galvanometers are used in many electrical measuring devices. Permanent magnet moving coil and errors due to PMMC…???? ◦ Its part of your assignment 1… brief and simple… U-shaped permanent magnet. Movable coil. A pointer. A scale. Coil is suspended between the poles of the magnet. Coil twist in response to the interaction of the applied current through the coil and the magnetic field of the permanent magnet. When the driving force of the coil current is removed, the restoring force provided by the spring returns the coil to its zero position Let B be the strength of the magnetic field, n be the number of turns in the coil, A be the area of the coil and I be the current flowing through the coil. The Torque will be Restoring torque is given by Where C is the restoring constant and is the final deflection. At steady state or equilibrium position, Which in turn, ◦ And ◦ Where G is the galvanometer Constant and A= I = ….? B = …? T = …? = radians N = …. N =100 A = 0.0012 I = 0.02 A B = 0.05 T T = ??? Assignment 1 Used to measure electric current through the circuit. Modified form of galvanometer. How? ◦ Galvanometer is used for small currents.. ◦ What if large value of current??? We add an additional resistor…known as Shunt Resistor What does the word shunt means..?? Shunt is a name given to low resistance/resistor. Placed parallel to Galvanometer… why??? Alternate path.. Shunt will be ◦ Small…why? Ammeter sensitivity is the amount of current necessary to cause full scale deflection (maximum reading) of the ammeter. The smaller the amount of current, the more "sensitive" the ammeter…..why??? Ampere or milliampere…?? How to calculate shunt….? Let be the maximum current that can be passes through the galvanometer. galvanometer resistance = Shunt Resistance = Total current = Current through Shunt resistance = Current through galvanometer = Then voltage drop at galvanometer is Since shunt resistance is parallel to galvanometer so Current through shunt is By 1 and 2 Current is larger than Let n be the factor by which then… is greater than Putting the above value in 3 Note:In several books = and = Shunt resistance = ? ◦ When = 1 mA, = 100 Ω, = 10mA ◦ Ideally the shunt resistance should have value??? ◦ What will happen when you connect the ammeter parallel to the circuit??? Several values of shunt resistors and a rotary switch are used to select the desired range of current to measure. An Aryton shunt uses a combination of resistors. + + R1 R2 R3 R4 Rm _ S _ Multi range Ammeter 42 Consider Recall equation (5)… Using the above equation, we can derive for the unknown quantities and in general we have Ammeter inserted in series. Always set the range to the highest scale and then reduce as needed. Observe polarity. Better quality analog meters include a mirror along the scale. This eliminates parallax error. Besides current, Voltage is also essential quantity to be measured … Galvanometer is further altered … Voltmeter used to measure the potential difference between any two points in a circuit… Connected in parallel to the circuit.. No alteration/drawing the current flowing through the circuit. Current should be negligible through the voltmeters. So the resistance of the voltmeter should be high. The series resistor is called a "multiplier" resistor To determine the value of ‘multiplier’ resistor Consider the above figure… If is the voltage across the voltmeter, is the current through voltmeter, is the resistance through Galvanometer and is the multiplier resistance then Ideally, Multiplier resistance should be….??? R = 10 Ω = = 50 mA Show how it can be adopted to measure voltage upto 750 V ? We can also find the value of multiplier resistance by determining the sensitivity of the meter. Sensitivity is given by S and is determined by Also… Sensitivity or A DC voltmeter can be converted into a multi range voltmeter by connecting a number of resistors (multipliers) in series with the meter movement. R1 R2 R3 R4 Im V1 + V2 V3 Rm V4 _ Multirange voltmeter 53 For extension of simple voltmeter… we utilize the product of voltmeter sensitivity and the given range followed by subtraction of internal/meter resistance from the product … In other words…