Electrical Measuring Instrument & Machines

1. Electrical Measuring Instrument & Machines Trim iii A-Div

2. Introduction • Electrical measurements are used in physical, chemical, and biological research and in the chemical industries. television equipment, of aircraft, and of spacecraft. • Measurements of electrical quantities, such as voltage, impedance, current, AC frequency and phase, power, electric energy, electric charge, inductance, and capacitance. • Electrical equipment capable of converting nonelectrical quantities into electrical quantities.

3. Active electrical quantities • The measurement of “active” electrical quantities such as current and voltage. • Which characterize the energy state of a measured circuit, makes use of the direct action of these quantities on the measuring instrument and generally draws some amount of power from the circuit AMMETER, VOLTMETER , WATTMETER, ELECTRIC METER andFREQUENCY METER

4. Passive electrical quantities • The measurement of “passive” electrical quantities such as impedance and its complex components, inductance, and the tangent of the dielectric loss angle. • which characterize the electrical properties of a measured circuit, requires excitation of the circuit by an outside source of electric energy and measurement of the circuit’s response OHMMETER, MEGOHMMETER, INDUCTANCE METERS, CAPACITANCE METER and QUALITY FACTOR METER

5. AC & DC Measuring instruments • The techniques and instruments used for electrical measurements in DC circuits differ substantially from those used in AC circuits. AC Measuring Instruments: • In AC circuits, the choice of technique and instrument depends on the frequency, on the nature of the quantities’ variations, and on which values—instantaneous, effective, maximum, or average—of the varying electrical quantities are being measured. • measurements in AC circuits are made with electromagnetic, electrodynamics, induction, electrostatic, rectifier, and digital instruments and with oscillographs.

6. DC measuring instruments • Permanent-magnet instruments and digital measuring devices are the instruments most widely used for measuring DC circuits. • whereas Some of these instruments are used for measurements in both AC and DC circuits. • the values of measured electrical quantities the following ranges: current, from 10–16 to 105 amperes voltage, from 10–9 to 107 volts resistance, from 10–8 to 1016 ohms power, from 10–16 watt to tens of gigawatts, and AC frequency, from 10 –3 to 1012 hertz. 

7. GENERALIZED MEASUREMENT SYSTEM CONFIGURATION Input parameter sensor transducer amplifier Data processing Signal conditioning Output devices Transmission for telemetry Inverse transducer Data display Recorder Data storage LED ,LCD, CRT Magnetic tap

8. Sensor- primary sensing element , sense the input parameter or variable like pressure, velocity , acceleration etc and gives an output signal to the input signal. • Transducer- which converts any non electrical input to an electrical signal. • Amplifier- the low level signal from the transducer has to be amplified and brought to a useful level before it can be processed. • Signal conditioning- many times the transducer characteristics are nonlinear, I.e. the relationship b/w i/p and o/p is nonlinear especially with temperature transducer.

9. in such cases, signal conditioning like linearization is required. Thus signal conditioning could be linearization, scaling, filtering. • Data processing- the data is thus processed before it goes to the final output device. • Transmission- data can be transmitted over long distances by using a pair of wires , a coaxial cable, a fiber optical link or radio wave. • Output devices- the data could be displayed using an oscilloscope if it is a digital data ,we can have LED or LCD display.

10. Performance Characteristics Of Measuring Instruments Static characteristics • Accuracy • Precision • Error • Repeatability • Reproducibility • Reliability • Linearity • Sensitivity • resolution

11. Accuracy • Accuracy is a measure of how close the measured value is to the true value How close a measurement is to the actual or true value???? good accuracy true value poor accuracy true value

12. Precision The closeness of agreement between independent test results obtained under stipulated conditions How well several measurements agree with each other??? good precision poor precision

13. Accuracy and Precision What can you say about the accuracy and precision in each of the following: Good precision, poor accuracy Good precision, good accuracy

14. Error-Accuracy of the measured value is of prime importance, hence a knowledge of various likely error is essentials in order to obtain a reasonable estimate of the uncertainty in measurement. Error can be classified as: • Gross error: this class of errors mainly covers human mistake in reading instruments and recording and calculating measurement result. the responsibility of the mistake normally lies with the experimenter. • Systematic error: these type of error are divided into three categories— • Instrumental error • Environmental error • Observational error • Random error: it has been consistently found that experimental result show variation from one reading to another, even after all systematic error have been accounted for. • These error are due to a multitude of small factors which change or fluctuate from one measurement to another and due to surely to chance.

15. Repeatability • Closeness of the agreement between the results of successive measurements of the same measure and carried out under the same conditions of measurement • Same (repeatability) conditions include: • the same measurement procedure • the same observer • the same measuring instrument, used under the same conditions • the same location • repetition over a short period of time

16. Reproducibility • Closeness of agreement between the results of measurements of the same measure and carried out under changed conditions of measurement • The changed conditions may include: • principle of measurement • method of measurement • observer • measuring instrument • reference standard • location • conditions of use • time

17. Linearity • A measure of how close is the output of an instrument to a straight line • The closeness to which a curve approximates a straight line.

18. Sensitivity • A measure of the change in instrument output that occurs when the measure and changes by a given amount • The ratio of the change in output magnitude to the change of the input which causes it after the steady state has been reached.

19. Sensitivity to disturbance • Temperature • Humidity • Electromagnetic interference • Radiation • Acceleration • Shock • Vibration

20. Resolution • The least interval between two adjacent discrete details which can be distinguished one from the other. Classic definition based on analog output instruments: • The smallest change of the magnitude of the measure and that produces a minimum observable output of the instrument the instrument's output is digital: • More and more modern instruments have digital outputs because of the wide usage of computer • The "resolution" of the digital output can be a very small number, but this is not the resolution of the instrument (e.g. what is the "resolution" of a 32-bit IEEE floating point number?) • The resolution of an digital output instrument should be limited by the front end rather than the digital computation

21. Dynamic characteristics Input which are not static but are dynamic in nature i.e the input is vary with the time. • Speed of response:- it is defined as the rapidity with which a measurement system respond to change in the measured quantity. • Measuring lag:- it is the delay in the response of a measurement system to changes in the measured quantity has occurred. • Fidelity:- it is defined as the degree to which a measurement system indicate to change in the measured quantity without any dynamic error. • Dynamic error:- it is the difference between the true value of the quantity changing with the time and the value indicate by the measurement system if no static error is assumed.

22. Basic electrical principle • Electricity. It's everywhere. It's in your house, in the computer on which you are reading this, in the air, in your body. You get the point. It also plays an important integral part of sound systems. From the microphone to the loudspeaker, everything is electrical, and you need to know a few things about it. • Voltage is defined as the electric charge between two points. • In engineering terms, voltage is the potential energy of a bunch of electrons. • Current, measured in Amperes, is the flow of electric charge; we can say that current is the rate of flow of electrons, but technically it is the opposite flow that we call current-- as electrons move in one direction, holes are left behind, and appear to flow in the opposite direction.

23. Principle of Voltmeter • A voltmeter is a device that measures the voltage passing between any two points, typically used to check electric circuits for power. • A voltmeter, also known as a voltage meter, is an instrument used for measuring the potential difference, or voltage, between two points in an electrical or electronic circuit. • Some voltmeters are intended for use in direct current (DC) circuits; others are designed for alternating current (AC) circuits. • A basic analog voltmeter consists of a sensitive galvanometer (current meter) in series with a high resistance. The internal resistance of a voltmeter must be high. • A digital voltmeter shows voltage directly as numerals. Some of these meters can determine voltage values to several significant figures.

25. Voltage and Current The switch is closed making a complete circuit so current can flow. Voltage but No CurrentThe switch is open so the circuit is broken and current cannot flow.

27. Voltage: • Voltage is a measure of the energy carried by the charge. Strictly: voltage is the "energy per unit charge". • The proper name for voltage is potential difference or p.d. for short, but this term is rarely used in electronics. • Voltage is supplied by the battery (or power supply). • Voltage is used up in components, but not in wires. • We say voltage across a component. • Voltage is measured in volts, V.Voltage is electric potential energy per unit charge, measured in joules per coulomb ( = volts). • Voltage is measured with a voltmeter, connected in parallel.

28. Types: • An oscilloscope can be used to measure low voltages; the vertical displacement corresponds to the instantaneous voltage. • Oscilloscopes are also excellent for the measurement of peak and peak-to-peak voltages in AC and RF applications. • Voltmeters for measuring high potential differences require heavy-duty probes, wiring, and insulators. • In computer practice, standard lab voltmeters are adequate because the voltages encountered are moderate, usually between 1 V and 15 V. • Cathode-ray-tube (CRT) monitors operate at several hundred volts. A typical lab voltmeter can indicate these voltages, but CRT units should be serviced only by qualified technicians because the voltages are high enough to be lethal.

29. Current principle • An ammeter is a measuring instrument used to measure the electric current in a circuit. • Electric currents are measured in amperes (A), hence the name. Instruments used to measure smaller currents,in the milliampere or microampere range, are designatedasmilliammetersor microammeters. • It must be placed in series with the measured branch, and must have very low resistance to avoid significant alteration of the current it is to measure.

30. Types: • The D'Arsonvalgalvanometer is a moving coil ammeter. It uses magneticdeflection, where current passing through a coil causes the coil to move in a magnetic field.  Linear Scale due to Moving Coil

31. Moving iron ammeters • Moving iron ammeters use a piece of iron which moves when acted upon by the electromagnetic force of a fixed coil of wire. This type of meter responds to both direct and alternating currents (as opposed to the moving coil ammeter, which works on direct current only). Non-linear Scale Due to Moving Iron

32. Continue • In a hot-wire ammeter, a current passes through a wire which expands as it heats. Although these instruments have slow response time and low accuracy, they were sometimes used in measuring radio-frequency current. • Digital ammeter designs use an analog to digital converter (ADC) to measure the voltage across the shunt resistor; the digital display is calibrated to read the current through the shunt. • A picoammeter, or pico ammeter, measures very low electrical current, usually from the picoampere range at the lower end to the milliampere range at the upper end. Picoammeters are used for sensitive measurements where the current being measured is below the theoretical limits of sensitivity of other devices, such as Multimeters.

33. Current: • Current is the rate of flow of charge. • Current is not used up, what flows into a component must flow out. • We say current through a component. • Current is measured in amps (amperes), A. • Current is measured with an ammeter, connected in series. To connect in series you must break the circuit and put the ammeter acoss the gap, as shown in the diagram. • The symbol I is used for current in equations. 

35. What is Current? • Current is the flow of charge from a voltage source • 1 Ampere (“Amp”) = Flow of 1 Coulomb/sec +++

36. Metal wires (conductors) How Does Current Flow? Current can only flow through conductors +++ Currentflow

37. Plastic material (insulators) When Does Current NOT Flow? Current cannot flow through insulators +++ No currentflow

38. What is Current? • Electricity flows when electrons travel through a conductor. • We call this flow “current.” • Only some materials have free electrons inside. glass rubber oil asphalt fiberglass porcelain ceramic quartz (dry) cotton (dry) paper (dry) wood plastic air diamond pure water YES! silver copper gold aluminium iron steel brass bronze mercury graphite dirty water concrete Conductors: NO! Insulators: No free electrons = No current

39. Application of Ammeter/Voltmeter

40. Wattmeter: • Power in an electric circuit is the product (multiplication) of voltage and current, • A wattmeter is a device used to measure how much electrical power a circuit is producing, expressed in watts. • Wattmeters measure electrical currents in three different values. These are watts, volts and amperes. • A wattmeter is hardly over required in a d.c circuit because power (P = VI) can be easily determined from voltmeter and ammeter readings. • However, in an a.c circuit, such a computation is generally speaking impossible. It is because in an a.c circuit, power (P = VI Cos θ) • depends not only on voltage and current but also on the phase shift between them.

41. Therefore, a wattmeter is necessary for a.c power measurement.The wattmeter shows a reading which is proportional to the product of the current through its current coli, the p.d across its potential or pressure coil and cosine of the angle between this voltage and current. • The “wattmeter” is an indicating type instruments, generally used for power measurement of the electrical circuit . • A wattmeter consists of (1) a low resistance current coil which is inserted in series with the line carrying the current and (ii) a high resistance pressure coil which is connected across the two points whose potential difference is to be measured. • The wattmeter require polarity markings so that the current in the stationary coils will be in the correct direction relative to the current in the movable coil

43. Wattmeter • The fixed coil of the wattmeter is connected in series with the circuit (load), and the moving coil is connected across the line. • When line current flows through the fixed coil of a wattmeter, a field is set up around the coil. The strength of this field is in phase with and proportional to the line current. • The moving coil of the wattmeter generally has a high-resistance resistor connected in series with it. The purpose for this connection is to make the moving-coil circuit of the meter as purely resistive as possible. • As a result, current in the voltage circuit is practicallyin phase withline voltage. Therefore, when voltage is impressed on the voltage circuit, current is proportional to and in phase with the line voltage.  I V

44. Types: • There are two principle types of wattmeter viz: i. Dynamometer Wattmeter – for both d.c and a.c power ii. Induction Wattmeter – for a.c power only. • A special meter movement designed especially for power measurement is called thedynamometer movement, and is similar to a D'Arsonval or Weston movement in that a lightweight coil of wire is attached to the pointer mechanism. • However, unlike the D'Arsonval or Weston movement, another (stationary) coil is used instead of a permanent magnet to provide the magnetic field for the moving coil to react against. • The moving coil is generally energized by the voltage in the circuit, while the stationary coil is generally energized by the current in the circuit. • A dynamometer movement connected in a circuit looks something like this:

45. Electrodynamometer

46. Electrostatic Voltmeter • It is thus seen that the force of attraction is proportional to the square of the potential difference applied, so that the meter reads the square value (or can be marked to read the rms value). One of the direct methods of measuring high voltages is by means of electro-static voltmeters. For voltages above 10 kV, generally the attracted disc type of electrostatic voltmeter is used. When two parallel conducting plates (cross section area ‘A’ and spacing ‘s’) are charged q and have a potential difference V, then the energy stored in the is given by

47. Electrostatic Voltmeter Electrostatic voltmeters of the attracted disc type may be connected across the high voltage circuit directly to measure up to about 200 kV, without the use of any potential divider or other reduction method. [The force in these electrostatic instruments can be used to measure both a.c. and d.c. voltages]. The right hand electrode forms the high voltage plate. The centre portion of the left hand disc is cut away and encloses a small disc which is movable and is geared to the pointer of the instrument. The range of the instrument can be altered by setting the right hand disc at pre-marked distances. The force of attraction F(t) created by the applied voltage causes the movable part-to which a mirror is attached-to assume a position at which a balance of forces takes place. An incident light beam will therefore be reflected toward a scale calibrated to read the applied voltage magnitude.

48. Electrostatic Voltmeter Absolute Electrostatic Voltmeter • Advantages: • Low loading effect • Active power losses are negligibly small • Voltage source loading is limited to the reactive power needed to charge the system capacitance.(i.e., For 1V Voltmeter- Capacitance is few Pico farad) • Voltages upto 600kV can be measured. • Disadvantage: • For constant distance ‘s’, F α V2, the sensitivity is small. This can be overcome by varying the gap distance d in appropriate steps.

50. Energy meter • An electricity meter or energy meter is a device that measures the amount of electric energy