calibration and recorders
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The document discusses instrument calibration, emphasizing the comparison method and the use of digital multimeters (DMMs) as standard instruments. It outlines the calibration process for various devices, detailing accuracy ratios, adjustment procedures, and the use of x-y recorders and plotters for graphical representation of data. Key aspects include the precision required for calibration and the general workings of recorders used in measuring variables.
calibration and recorders
- 1. Unit V Calibration & Recorders Unit V: Instrument calibration: Comparison method, digital mult-imeters as standard instrument, calibration, instrument Recorders: X-Y recorders, plotters 4/21/2017 1 NEC 403 Unit V by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad
- 2. Instrument Calibration • Calibration is a process of comparison between a known measurement (the standard) and the measurement using instrument under test. • Typically, the accuracy ratio of the standard to measuring device should be 10:1 or more. However, accuracy ratio of 4:1 is acceptable in most cases. • Bench type digital meters are sufficiently accurate to calibrate other digital/ analog meters. • Potentiometer can be employed as calibrator for low level DC ammeter and voltmeter calibration. 4/21/2017 2 NEC 403 Unit V by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad
- 3. Instrument Calibration Comparison Method 4/21/2017 3 NEC 403 Unit V by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad • Any instrument can be calibrated by comparing its reading with a more accurate instrument (“Standard”), reading the same quantity. • A dc voltmeter can be calibrated with using comparison method by placing a standard meter in parallel and applying a dc voltage. • The dc voltage applied is adjusted in steps and reading of two meters is noted (calibration table), which helps in identifying whether the instrument confirms to specified accuracy or not) • Adjustments (zero control, calibration adjustments) should be done before calibration Variable DC supply Standard DC Voltmeter DC Voltmeter to be calibrated
- 4. Wattmeter Calibration Comparison Method 4/21/2017 4 NEC 403 Unit V by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad Variable DC supply Standard DC Ammeter Variable DC supply Standard DC Voltmeter Current Coil Voltage Coil R Current Limiting resistor Wattmeter to be calibrated I V Exact power P=V.I
- 5. digital Multi-meters as Standard 4/21/2017 5 NEC 403 Unit V by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad • Best accuracy of analog instruments are 2% of full scale. • DMM has high accuracy. Typical DMM accuracy is (0.5% + 1 digit). Thus for 100.0 V reading by a 3 ½ digit DMM accuracy is 0.6%. • Typical high performance DMM accuracy is (0.1% + 1 digit) and is sufficiently high to calibrate DMM’s. • Typical bench type DMM accuracy is (0.01% + 1 digit) and is sufficiently high to calibrate high performance DMM’s.
- 6. digital Multi-meters as Standard 4/21/2017 6 NEC 403 Unit V by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad • Calibration of Dc Voltmeter using DMM or DC calibrator require a precise DC source. • Precision variable DC voltage source can be made using constant current source, zener diode and amplifier as shown in the circuit + - R5 -VEE VCC Q1 R1 R3 R2 D1Vz + - Iz VS + - Constant Current Source Zener diode R4 Vo + - Amplifier 5 4 1 R R VV zo
- 7. X-Y Recorders 4/21/2017 7 NEC 403 Unit V by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad • X-Y recorder gives graphic record of relationship of two variables. • Ex. V-I characteristics of a diode, speed torque characteristics of motor, stress strain characteristics, hysteresis characteristics • Input impedance of X-Y recorder typically varies between 11 K to 1M depending on the voltage range selected. • One limitation of X-Y recorder is that they operate for low frequency
- 8. X-Y Recorders 4/21/2017 8 NEC 403 Unit V by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad • A X-Y recorder use a stationary sheet of chart paper and records by moving the pen which can move in X & Y direction simultaneously. • Pen holder slides along a movable pen carriage • Movement of pen carriage is controlled by two servomotors (one for each dimension) through pulley & pulley card arrangement • Each servomotor is controlled by two inputs. • Pen down/ up is controlled using a solenoid arrangement.
- 9. X-Y Recorders 4/21/2017 9 NEC 403 Unit V by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad Pen Movable Pen Carriage Idler Pulley Pulley Card AttenuatorY input Balance circuit Reference source Amplifier To Servo motor AttenuatorX input Balance circuit Reference source Amplifier To Servo motor
- 10. Plotters 4/21/2017 10 NEC 403 Unit V by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad • A plotter is used to print vector graphics with high print quality • Single sheet is roller fed back and forward as required while the pen is moved horizontally to produce the plot or drawing • Hence vertical movement of paper and horizontal movement of pen produce the required drawing • Plotter is used for same applications as that of X-Y recorder • Plotters are used to draw technical diagrams • Plotters may use analog or digital printing HP 7090A plotter