Measuring Instruments

Transcription

1 Issue No. 1 Page 1 of 24 Measuring Instruments A measuring instrument is a device for measuring a physical and electrical quantity. Measurement Categories:- Direct: Measuring the desired quantity `face-to-face', as it were. For example: - using a ruler to measure the width of a bench. Indirect: Measuring a second quantity from which the 1st may be indirect. For example:- measuring the color of the sun and inferring its surface temperature. Types of measuring instrument:- 1. Physical quantity measuring instrument 2. Electrical quantity measuring instrument Ruler Physical Quantity Measuring Instrument A ruler, sometimes called a rule or line gauge, is an instrument used in geometry, technical drawing, printing as well as engineering and building to measure distance or rule straight lines. The ruler is a straightedge which may also contain calibrated lines to measure distances. Section:- industrial electrical machine drive

2 Issue No. 1 Page 2 of 24 Protractor Protractor:-A device used to measure angles. Degrees: Measuring Angles: - We measure the size of an angle using degrees. Example: Here are some examples of angles and their degree measurements. Acute Angles:-An acute angle is an angle measuring between 0 and 90 degrees. Example: Obtuse Angles: - An obtuse angle is an angle measuring between 90 and180 degrees. Example: Section:- industrial electrical machine drive

3 Issue No. 1 Page 3 of 24 Right Angles:-A right angle is an angle measuring 90 degrees. Examples: Complementary Angles: - Two angles are called complementary angles if the sum of their degree measurements equals 90 degrees. Example:-These two angles are complementary. Supplementary Angles: - Two angles are called supplementary angles if the sum of their degree measurements equals 180 degrees. Example:-These two angles are supplementary. Vertical Angles: - For any two lines that meet, such as in the diagram below, angle AEB and angle DEC are called vertical angles. Angle BEC and angle AED are also vertical angles. Vertical angles have the same degree measurement. A B 110 E D C Section:- industrial electrical machine drive

4 Issue No. 1 Page 4 of 24 Rectangles 3 Area = 4 x 3 Area = 12 square units Area = 12 units 2 Changing this into a formula (rule) to find the area Height Area = Base x Height A = BH Base Triangle Height Base Height Base is12cm Base is 8 cm Area = Base X Height 2 A = BH/2 (OR) ½BH A = 8 X 12 2 A = 48 cm 2 Section:- industrial electrical machine drive

5 Issue No. 1 Page 5 of 24 Parallelogram Area = Base x Height A = BH Height Base Notice that the base and height have remained the same, so: area is base x height Trapezium b a Area = a x (b + c) 2 C Section:- industrial electrical machine drive

6 Issue No. 1 Page 6 of 24 The Vernier & Micrometer caliper From your experience in using a ruler and meter stick, it is evident that we are very much limited by our eyesight. How can we hope to measure accurately something as small as 0.01 cm or smaller? For such measurements, the lines on the instrument would be so close that they would run into each other. Two devices have been invented to make these fine measurements possible. They are Vernier Caliper and Micrometer Caliper. 1. Vernier Caliper The vernier caliper is a slide-type caliper used to take inside, outside, and depth measurements. The vernier caliper has two metric scales and two English scales (See Figure 1 below). Figure 1 Consider the metric scales. One of them is fixed and located on the lower part of the beam. It is divided into centimeters and subdivided into millimeters. The other metric scale is called the vernier metric scale and is the lower scale on the slide. This scale is made with nine millimeters contains ten divisions (See Figure 2 below). Figure 2 Each division on the vernier scale equals 0.9 of a division on the fixed scale. The part of the reading from the vernier scale is in tenths of a millimeter, which means that the precision of the instrument is 0.1 mm or 0.01 cm. In practice the object to be measured is placed between jaws and the vernier is placed against the end of the object as shown in Figure 3. By sight we can see that the object is larger than 7.7 cm and smaller than 7.8 cm. In order to find the second decimal place we look at the vernier scale and find the calibration line that just lines up with a line on the fixed scale is the eighth line from zero line. Therefore the dimension of the object being measured is Section:- industrial electrical machine drive

7 Issue No. 1 Page 7 of 24 Figure 3 Digital Electronic Caliper Can provide readings to resolution of.0005 in. or 0.01 mm at touch of button No rack, pinion or glass scale Can connect to Statistical Process Control (SPC) equipment for inspection purposes Measurements Inside diameter Outside diameter Step Depth 2. Micrometer Caliper The micrometer caliper is used to make very fine measurements beyond the hundredths of a centimeter. As its name implies, distances are measured to m or 10-6 m (recall the SI prefix for an order of magnitude of 6 is micro) which is equal to cm. This device uses the uniformity in the spacing of threads on a bolt. If a nut is threaded on the bolt and the bolt is rotated one complete evolution, the end of the bolt will have moved a linear distance equal to the width of a thread. If instead of a nut, we attach a rotating scale as well as place a calibrated line (also Called the fixed scale) along the length of the bolt, then it becomes possible to measure small fractions of a rotation (and small fractions of the width of a thread). Section:- industrial electrical machine drive

8 Issue No. 1 Page 8 of 24 Figure 4 As shown in Figure 4 the basic parts of a micrometer are labeled. The object to be measured is placed between the anvil and spindle. Turn the thimble until the object fits snugly. Do not force the turning of the thimble, since this may damage the very delicate threads on the spindle located inside the thimble. Some calipers have a ratchet, which helps protect the instrument by not allowing the thimble to turn when forced. The barrel is graduated in millimeters and it also has graduations in halves of millimeters, which are indicated by the lower set of graduations on the barrel. The threads on the spindle are made so it takes two complete turns of the thimble for the spindle to move precisely one millimeter. The head (rotating scale) is divided into fifty equal divisions each division indicating 0.01 mm, which is the precision of the instrument. Since our eye can still estimate another decimal place between marks on the rotating scale (or 0.001mm, which is m), this device is called a micrometer. Rules for reading a micrometer in millimeters: 1. Find the whole number of mm in the measurement by counting the number of mm graduations on the barrel to the left of the head. 2. Find the decimal part of the measurement by reading the graduation on the Rotating scale that is most nearly in line with the centerline on the barrel, and multiply this reading by If the head is at or immediately to the right of the half mm graduation, add 0.50 mm to the reading on the rotating scale. 3. Estimate one more decimal place. 4. Add the numbers found in steps above. Example: Read the measurement on the micrometer below. Section:- industrial electrical machine drive

9 Issue No. 1 Page 9 of 24 Figure 5 Steps Reading 1. The barrel reading is 8.00 mm 2. The rotating scale reading is 0.65 mm (Note that the head is 3. The estimate decimal place is mm Past the half mark.) 4. The total measurement is mm Wire Gauge A wire gauge is a measurement of how large a wire is, either in diameter or cross sectional area. This determine the amount of electric current a wire can safely carry as well as its electrical resistance and weight per unit of length. Section:- industrial electrical machine drive

10 Issue No. 1 Page 10 of 24 Mathematics Fraction Measurements and calculation Definition: A fraction is a quantity that cannot be represented by a whole number. A fraction is an ordered pair of whole numbers, the A numerator B denominator The denominator tells us how many congruent pieces the whole is divided into, thus this number cannot be 0. The numerator tells us how many such pieces are being considered. Examples: How much of a pizza do we have below? We first need to know the size of the original pizza. The blue circle is our whole. If we divide the whole into 8 congruent pieces, The denominator would be 8.We can see that we have 7 of these pieces. Therefore the numerator is 7, and we have of pizza 7/8 Percentage Definition: - A percentage is a special fraction. The % says, per hundred. For example, 47%, is simply another way of writing the fraction 47/100 Percentage Fraction 7% 7/100 19% 19/100 83% 83/100 So to write a percentage as a fraction, you simply put it over 100 like this: 24% = 24/100 Section:- industrial electrical machine drive

11 Issue No. 1 Page 11 of 24 Copy and complete this chart Percentage Fraction Percentage Fraction 23% 23/100 80% 24% 84% 55% 12% 20% 1/5 48% 17% 71% 78% 36% To change a fraction into a decimal, you divide the bottom number (the denominator) into the top number (Numerator). For example, to change ¾ into a decimal, you would divide the 3 by the 4. ¾ Remember that a percentage is a fraction always has 100 as its denominator So, to change a percentage into a decimal, you always divide by 100 For example, 35% as a fraction is 35/100 so to change this into a decimal, you divide 35 by % = 0.35 Copy and complete this chart Percentage Decimal Decimal Percentage 27% % % % % % 0.07 Change the fraction into a decimal (23 32 = ) Change the decimal into a % ( X 100 = 71.9%) Section:- industrial electrical machine drive

12 Issue No. 1 Page 12 of 24 Electrical quantity measurement Electrical measuring instruments are a type of device that measures the electrical quantity. E.g. Voltage, current, resistance, power. R: - the resistance of the conductor in units of Ohms V: - the potential difference measured across the resistance in units of Volts I: - the current through the resistance in units of Amperes Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them I V R Ohm s Law I = V /R V = I R R = V/I Voltage: - is the force or pressure that pushes the electrons to flow through a conductor. SI unit - volt, Symbol - V, v Current: - is the flow of electrons or movement of electrons through a conductor. SI unit - Ampere Symbol - A, i Resistance: - the opposition to the flow of electrons. SI unit - ohm (Ω) Symbol - R, r

13 Issue No. 1 Page 13 of 24 Galvanometer Electrical Quantity Measuring Instrument A galvanometer is a type of sensitive ammeter: an instrument for detecting electric current. It is an analog electromechanical actuator that produces a rotary deflection of some type of pointer in response to electric current flowing through its coil in a magnetic field. Galvanometer was the first instrument used to detect measure and indicate direction of electric current. Galvanometer mechanisms are divided into moving magnet and moving coil galvanometers Ohmmeter An ohmmeter is an electrical measuring instrument that measure electrical resistance, the opposition to an electric current. Micro-ohmmeter makes low resistance measurement. Mega-ohmmeter measure large values of resistance. The SI unit of resistance is ohm(ω) The first ohmmeter was based on meter movement known as a ratio meter. These were similar to galvanometer type movement. The total resistance can be known by ohm s law R = V/I

14 Issue No. 1 Page 14 of 24 Voltmeter A voltmeter is an instrument used for measuring electrical potential difference (voltage) between two points in an electric circuit. There are two types of voltmeter 1. Analog: - voltmeter moves a pointer across a scale in proportion to the voltage of the circuit. 2. Digital: - voltmeter gives a numerical display of voltage by use of an analog to digital converter. Digital voltmeter can be made with high accuracy typically better than 1%. Voltmeter operates on the electrostatic principle uses the mutual repulsion b/n two charged plates to deflect a pointer attached to a spring. To know the total voltage in the circuit by ohm s law V = IR Ammeter An ammeter is electrical measuring instrument used to measure the electrical current in a circuit, the flow of electrons in the circuit. Electrical current are measured in amperes (A), therefore the SI unit of current is ampere. Hence the name instruments used to measure smaller current, in the milliampere, or microampere range, are designated as milli-ammeters or micro-ammeters. To know the total current in the circuit by ohm s law I = V/R There are many types of ammeters 1. Moving coil ammeters: - the D arsonval galvanometer is a moving coil ammeter. It uses magnetic deflection, where current passing through a coil causes the coil to move in a magnetic field 2. Electrodynamic ammeter: - an electrodynamic movement uses an electromagnet instead of the permanent magnet of the D arsonval movement. This instrument can respond to both alternating and direct current (AC, DC) and true RMS for AC. 3. Moving iron ammeters: - uses a pieces of iron which movies when acted upon by the electromagnetic force of a fixed coil of wire. This type of meter responds to both (AC,

15 Issue No. 1 Page 15 of 24 DC) the moving iron ammeters were invited by Austrian engineer (fried rich Drexler in 1884). 4. Hot-wire ammeters: - in this 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. This also measure true RMS for an applied AC current. 5. Digital ammeters: - in this ammeter the result reading is in the form of digital number. Designs uses a shunt resistor to produces a calibration voltage proportional to the current flowing. 6. Integrating ammeters: - there is also a range of devices referred to as integrating ammeters. In these ammeters the current is summed over time, giving as a result the product of current and time; which is proportional to the energy transferred with that current. This can be used for energy meters (watt-hour meters) or for estimating the charge of battery or capacitor. 7. Pico-ammeter: - measures very low electrical current, usually from the Pico ampere range at the lower end to the milliamp range at the upper end. Pico ammeters are used for sensitive measurements where the current being measured is below the theoretically limits of sensitivity of other devices, such as multimeters. Application The majority of ammeters are either connected in series with circuit carrying the current to be measured (for small frictional amepers), or have their shunt resistor connected similarly in series. In either case, the current passes through the meter or (mostly) through its shunt. They must not be connected to a source of voltage; they are design for minimal burden, almost a short circuit.

16 Issue No. 1 Page 16 of 24 Oscilloscope An oscilloscope, previously called oscillograph, is a type of electronic test instrument that allows observation of constantly varying signal voltage, usually as a two-dimensional graph of one or more electrical potential difference using the vertical or Y-axis, plotted as a function of time (horizontal X-axis). Oscilloscopes are used to observe the change of an electrical signal over time. An oscilloscope displays voltage waveforms Oscilloscopes display the waveform of a signal and allow quantities such as phase to be measured.

17 Issue No. 1 Page 17 of 24 Clamp Meter In electrical and electronics engineering a current clamp or current probe is an electrical device having two jaws which open to allow clamping around an electrical conductor. This allows properties of the electric current in the conductor to be measured, without having to make physical contact with the wire Taco Meter Tachometer is used for measuring rotational speed Can be used to measure speed of a rotating shaft Can also be used to measure flow of liquid by attaching a wheel with inclined vanes Tachometers can be classified on the basis of data acquisition contact or noncontact types They can also be classified on the basis of the measurement technique time based or frequency based technique of measurement They can also be classified as analog or digital type

18 Issue No. 1 Page 18 of 24 Comparison between Analog and Digital Tachometers Analog Tachometer Has a needle and dial type of interface No provision for storage of readings Cannot compute average, deviation, etc. Digital Tachometer Has a LCD or LED readout Memory is provided for storage Can perform statistical functions like averaging, etc. Contact Type The tachometer has to be in physical contact with the rotating shaft Preferred where the tachometer is generally fixed to the machine Generally, optical encoder / magnetic sensor is attached to shaft of tachometer Non-Contact Type The tachometer does not need to be in physical contact with the rotating shaft Preferred where the tachometer needs to be mobile Generally, laser is used or an optical disk id attached to rotating shaft and read by a IR beam or laser Analog Tachometers These are generally the ones that display the speed of car. The interface is pointer and button arrangement Watt Meter The wattmeter is an instrument for measuring the electric power (the rate of electrical energy) in watts of any given circuit. The traditional wattmeter is an electrodynamics instrument. The device consist of a pair of fixed coils, known as current coil, and a movable coil known as the potential (voltage) coil. The current coil is connected in series with the circuit, while the potential (voltage) coil is connected in parallel. A current flowing through the current coil generates an electromagnetic field around the coil. The strength of this field is proportional to the line current and in phase with it. The potential (voltage) coil has, as a general rule, a high-value resistor connected in series with it to reduce the current that flow through it. The result of this arrangement is that on a DC circuit, the deflection of the needle is proportional to both the current and the voltage, thus conforming to the equation

19 Issue No. 1 Page 19 of 24 P = VI For AC power, current and voltage may not be in phase, owing to the delaying effects of circuit inductance or capacitance. On an AC circuit the deflection is proportional to the average instantaneous product of voltage and current, thus measuring true power, P = VI cosθ Here cosθ represents the power factor which shows that the power Transmitted may be less than the apparent power obtained by multiplying the reading of a voltmeter and ammeter in the same circuit. Insulation Tester (megger) The insulation resistance (IR) test (also commonly known as megger) is a spot insulation test which uses an applied DC voltage. (typically either 250vdc, 500vdc or 1000vdc for low voltage equipment) (<600vdc, 2500vdc and 5000vdc for higher voltage equipment) To measure insulation resistance in KΩ, MΩ, GΩ. The insulation resistance result would be infinite. The measured resistance is intended to indicate the condition of the insulation or dielectric b/n two conductivity parts, where the higher the resistance, the better the condition of the insulation.

20 Issue No. 1 Page 20 of 24 Frequency Meter Frequency meter is a device for measuring the repetition per unit of time (customarily a second) of a complete electromagnetic waveform. For counts per unit of time, the SI unit for frequency is the HERTZ (Hz), 1Hz means that an event repeats once per second. A previous name for this unit was cycles per second (cps).a traditional unit of measure used with rotating mechanical device is revolutions per minute, abbreviated RPM. 60 RPM equals one hertz. The period, usually denoted by T is the duration of one cycle, and is the reciprocal of the frequency T = 1/f The SI unit of period is the second For periodic wave, frequency has an inverse relationship to the concept of wavelength; simply frequency is inversely proportional to wave length λ (lamda). The frequency is equal to the phase velocity υ of the wave divide by the length λ of the wave: F = υ/ λ The audible frequency range for humans is typically given as being b/n about 20Hz and 20000Hz (20 KHz). Some dogs breeds can perceive vibration up to 60,000 (60KHz). In Europe, Africa, Austria, southern South America, most of Asia, and Russia, the frequency of the alternating current in household electrical outlets is 50Hz. Where as in North America and northern South America, the frequency of the alternating current in household electrical outlets is 60Hz. Other type of frequency is angular frequency w is defined as the rate of change of angular displacement θ (during rotation) or the rate of change of the phase of a sinusoidal wave form do /dt =w = 2πf Angular frequency is commonly measured in radians per second (rad/s) Lux Meters Lux meters Measures illumination (light) levels at Offices Industrial plants Streets All other (work) places Body

21 Issue No. 1 Page 21 of 24 Multimeter A multimeter or a multitester also known as a VOM (volt-ohmmeter) is an electronic measuring instrument that combines several measurement function in one unit. A typical multimeter would include basic features such as the ability to measure voltage, current and resistance. A multimeter is a combination of a multi-range AC, DC meters. Multimeter can measure many quantities some of them are:- Voltage AC,DC Inductance Current AC, DC Temperature Resistance Continuity Capacitance Diode Frequency Transistor

22 Issue No. 1 Page 22 of 24 Maintain Measuring Instruments Sudden shock (dropping) can cause instrument to malfunction and will void warranty. Keep measuring faces clean and dry during use. Coat measuring faces with rust preventing oil after use and return to protective case. Electronics on electronic measuring instruments should not come in contact with fluids (oil, water etc.) Warranty void. Do not use other electronic devices on or near electronic precision instruments. This will damage the chip and void Warranty. Magnetic fields, Electronic fields or excessive humid conditions will cause malfunction and unexpected errors. This will void warranty. Batteries should be removed when instrument is not in use for long periods. This will eliminate battery corrosion causing instrument to malfunction and void warranty. Do not store electronic instruments exposed to direct sunlight, high temperatures, excessive moisture or volatile gasses. This will void warranty. Flashing LCD visible = Replace with new battery. No display on LCD = Replace battery or send to instrument specialist for servicing. LCD doesn t change when moving slider = Remove battery clean and replace after 24 hours. Inconsistent accuracy = Dirt in sensor = Send to instrument cleaning specialist and have instrument cleaned/serviced. Always clean the battery with a soft cotton cloth when replacing and avoid any contact with fingers etc. which will leave a Moist residue on the battery causing poor contact. Only use silver oxide batteries SR44 (1.5V) on mark line electronic precision instruments. Working temperature = 5 to 40 degree Celsius. Storage temperature = -20 degree Celsius to 40 degrees Celsius. Humidity influence = relative humidity <80% without influence. Maximum measuring speed on electronic instruments = 1.5me/sec, 60in/sec. Keep slides and measuring scales clean and dry using cotton fabric gently. Do not use volatile liquids such as petrol, acetone, and other organic solutions. This will void warranty.

23 Issue No. 1 Page 23 of 24 SI Units International System These fundamental units are prescribed in the SI (System International, also called the International System of Units) and mandated by the ISO (International Standards Organization). These SI units are: SI Unit Prefixes N O Name Metric Prefixes Power of Ten 1 yocto y zepto z atto a femto f pico p nano n micro µ milli m centi c deci d deca da hecto h kilo k mega M giga G tera T peta P exa E zetta Z yotta Y SI Unit Prefixes for Length Name Symbol factor Giga meter Gm 10 9 mega meter Mm 10 6 kilometer km 10 3 decimeter dm 10-1 centimeter cm 10-2 millimeter mm 10-3 micrometer μm 10-6 nanometer nm 10-9 Pico meter pm 10-12

24 Issue No. 1 Page 24 of 24 Quantity Unit Symbol Length meter m Mass kilogram kg Temperature kelvin K Time second s Amount of Substance mole mol Luminous Intensity candela cd Electric Current ampere a Volume cubic meter m 3 Density kilograms per cubic meter kg/m 3 Speed meter per second m/s Newton kg m/ s 2 N Energy Joule (kg m 2 /s 2 ) J Pressure Pascal (kg/(ms 2 ) Pa The next table lists some of the units of measurement useful to Electronic Engineers. Quantity Unit Symbol Value Frequency hertz Hz or H s 1 Velocity meter/second m.s 1 Acceleration meter/second-squared m.s 2 Force newton N kg.m.s 2 Energy joule J N.m Power watt W J.s 1 Electric Current ampere A Electric Charge coulomb C A.s Voltage volt V J.C 1 Electric Field volt/meter V.m 1 Resistance ohm Ω V.A 1 Resistivity ohmmeter Ω.m Conductance Siemens S Ω 1 Conductivity Siemens/meter S.m 1 Capacitance farad F C.V 1 Magnetic Flux Linkage weber Wb V.s Magnetic Flux Density tesla T Wb.m 2 Inductance henry H Wb.A 1