1764 words - 8 pages

Pharos University

EE212_ELECTRICAL MEASUREMENT & Interface Prepared By: Dr. Sahar Abd El Moneim Moussa

Dr. Sahar Abd El Moneim Moussa

1

CH1_ ERRORS IN MEASUREMENTS

Dr. Sahar Abd El Moneim Moussa

2

INTRODUCTION

Measurement involves using an instrument as a physical means of determining a quantity or variable.

A measuring device: a device used for obtaining a value which nearly represents an unknown variable or quantity.

The advancement of science and technology leads to the progress in the measuring techniques.

Dr. Sahar Abd El Moneim Moussa

3

Classification

of Measuring Instruments:

1- Mechanical Instruments:

•

These are the ...view middle of the document...

Which are common for measurements of physical quantities like length, mass and time.

Disadvantages are:

•

i- limited accuracy due to human factors. Ii- Less accuracy

Dr. Sahar Abd El Moneim Moussa

7

Methods

of Measurements: (cont.)

1- Indirect Method: • The unknown quantity under measurement is determined via the use of measurement systems as follows:

•

Advantages are:

i- High accuracy & sensitivity with lower cost ii- Measurement of non-physical quantities

Dr. Sahar Abd El Moneim Moussa 8

Basic

definitions:

1- True Value: At It is the actual value of a variable to be measured 2- Measured Value: Am It is the value measured by a specified instrument 3- Static (Absolute) Error: (A) • It is the difference between measured value (Am ) and true value ( At ). • It is the deviation of the measured value from the true value. A= Am -At

Dr. Sahar Abd El Moneim Moussa 9

Basic

definitions: (cont.)

4- Static Correction: (C)

•

It is the correction to be added to the measured value to give the true value

C =-At= At -Am

Dr. Sahar Abd El Moneim Moussa

10

Basic

definitions: (cont.)

5- Relative Error: (r)

•

The Static Error is expressed as a fraction of the full scale deflection or the true value r (as a fraction of true vale) =( Am - At ) / At r (as a fraction of true vale) =( Am - At ) / f.s.d r %(as a fraction of true vale) = (A / At) x 100%

r %(as a fraction of true vale) = (A / f.s.d) x 100%

Dr. Sahar Abd El Moneim Moussa 11

Example 1: A meter reads 127.5 V and the true value of the voltage is 127.43 V, determine the: i- Static error ii- Static correction for this instrument. Solution: The Static Error: A = Am – At = 127.5 – 127.43 = + 0.07 V

Static correction: C = – A = – 0.07 V

Dr. Sahar Abd El Moneim Moussa

12

Example 2: A voltmeter has a true value of 1.5V. An analog indicating instrument with a scale 0-2.5 shows a voltage of 1.46V. What are the values of absolute error and correction. Express the error as a fraction of the true value and f.s.d. Solution:

It will Solved in the lecture

Dr. Sahar Abd El Moneim Moussa

13

Basic

definitions: (cont.)

6- Limiting Error:

•

It is the specification given to an instrument which guarantees that a full scale deflection will be within a certain percentage of a full-scale reading. However, the readings that are less than full scale has a larger limiting error. Therefore it is the limit of the deviation from the specified value specified by manufacturer

•

Dr. Sahar Abd El Moneim Moussa

14

Example 3: A 300 V voltmeter is specified to be accurate within 2% at full scale. Calculate the limiting error when the instrument is used to measure a 120 V source and 60 V. Comment. Solution: The limiting error at full scale is 2% The magnitude of the limiting error at full scale is = 300 x ( 2/100 ) = 6 V The limiting error at 120 V = 6/120 x 100 = 5% The limiting...

Tap into the world’s largest open writing community