LECTURER PROF.Dr. DEMIR BAYKA AUTOMOTIVE ENGINEERING LABORATORY I

FUNDAMENTAL CONCEPTS IN MEASUREMENT AND EXPERIMENTATION

MEASUREMENT ERRORS AND UNCERTAINTY

THE “ERROR” IN A MEASUREMENT IS THE DIFFERENCE BETWEEN THE MEASURED VALUE AND THE TRUE VALUE OF THE “MEASURAND”

SINCE THE TRUE VALUE IS UNKNOWN THE DIFFERENCE BETWEEN THE TRUE VALUE AND MEASURED VALUE CAN ONLY BE ESTIMATED

THE ESTIMATION OF THE DIFFERENCE BETWEEN THE TRUE VALUE AND THE MEASURED VALUE IS CALLED UNCERTAINTY

TYPES OF ERRORS 1. GROSS ERRORS 2. SYSTEMATIC (FIXED) ERRORS (BIAS) 3. RANDOM ERRORS

1. GROSS ERRORS LARGELY HUMAN ERRORS DUE TO MISREADING INSTRUMENTS INCORRECT ADJUSTMENT OR IMPROPER USE OF INSTRUMENTS COMPUTATIONAL MISTAKES

2. SYSTEMATIC (FIXED) ERRORS (BIAS) THESE ERRORS ARE DUE TO INSTRUMENTS OR THEIR ENVIRONMENTS

TYPICAL INSTRUMENT CHARACTERISTICS WHICH LEAD TO SYSTEMATIC ERRORS ARE FRICTION IRREGULAR SPRING TENSION IMPROPER CALIBRATION

TYPICAL ENVIRONMENTAL BASED SYSTEMATIC ERRORS ARE EFFECT OF CHANGES IN SURROUNDING TEMPERATURE HUMIDITY BAROMETRIC PRESSURE MAGNETIC OR ELECTRICAL FIELDS

ENVIRONMENTAL BASED SYSTEMATIC ERRORS CAN BE MINIMISED BY PROPER CONDITIONING OF THE ENVIRONMENT ISOLATING OR SHIELDING

IN GENERAL SYSTEMATIC ERRORS CAN BE OVERCOME BY APPLYING CORRECTION FACTORS AFTER DETERMINING THE AMOUNT OF ERROR BY CALIBRATING THE INSTRUMENTS OR

EXAMPLE : A THERMOMETER IS CALIBRATED AND THUS MARKED AT THE FACTORY. THIS CALIBRATION MAY BE DONE BY EITHER FULLY OR PARTIALLY IMMERSING THE THERMOMETER INTO THE CALIBRATION ENVIRONMENT. IF A FULL IMMERSION THERMOMETER (ROOM THERMOMETER) IS PARTIALLY IMMERSED IN A FLUID THEN A STEM CORRECTION WILL BE NECESSARY

3. RANDOM ERRORS THESE ERRORS ARE MOSTLY DUE TO UNKNOWN AND RANDOMLY OCCURRING CAUSES THEY ARE DIFFICULT TO DETERMINE AND PREDICT THEY ARE DEALT WITH BY STATISTICAL METHODS

CALIBRATION BY CALIBRATION THE STATIC RESPONSE OF AN INSTRUMENT IS DETERMINED DURING CALIBRATION ALL INPUTS TO THE MEASUREMENT SYSTEM ARE KEPT CONSTANT EXCEPT THE MEASURAND WHICH IS VARIED IN A CONTROLLED MANNER

CALIBRATION A CALIBRATION STANDARD SHOULD, IF POSSIBLE, BE ABOUT 10 TIMES MORE ACCURATE THAN THE INSTRUMENT BEING CALIBRATED

ACCURACY DEGREE OF CLOSENESS OF MEASUREMENTS TO THE TRUE VALUE OF THE MEASURAND ACCURACY IS DETERMINED BY COMPARISON WITH CALIBRATED VALUES

ACCURACY THE ACCURACY OF AN INSTRUMENT IS EXPRESSED AS : *ABSOLUTE ACCURACY *RELATIVE ACCURACY RELATIVE ACCURACY IS DEFINED WITH RESPECT TO *ACTUAL READING *FULL SCALE READING OF THE INSTRUMENT

PRECISION (REPEATABILITY) THIS IS THE DEGREE OF AGREEMENT BETWEEN REPEATED MEASUREMENTS

ACCURACY AND PRECISION

RESOLUTION IS A MEASURE OF THE SMALLEST CHANGE IN THE INPUT SIGNAL THAT THE MEASUREMENT SYSTEM CAN DETECT

THRESHOLD STARTING FROM INPUT SIGNAL EQUAL TO ZERO, IF THIS SIGNAL IS SLOWLY INCREASED, THERE WILL BE SOME MINIMUM SIGNAL LEVEL BELOW WHICH NO OUTPUT CHANGE CAN BE DETECTED

HYSTERESIS STARTING FROM ZERO INPUT, IF A SIGNAL IS SLOWLY INCREASED UP TO A CERTAIN VALUE AND THEN REDUCED SLOWLY DOWN AGAIN THEN HYSTERESIS IS THE CHANGE IN THE RESPONSE OF THE MEASUREMENT SYSTEM

HYSTERESIS

SPAN THIS IS NORMALLY ACCEPTED AS THE INPUT SIGNAL RANGE THAT THE MEASUREMENT SYSTEM WILL MEASURE. EXAMPLE THERMOMETERS USED BY DOCTORS HAVE A SPAN OF 7 C RANGING FROM 35 C TO 42 C

DYNAMIC RANGE THIS IS THE SPAN OF AN INSTRUMENT EXPRESSED IN TERMS OF RATIO OF THE HIGHEST AND LOWEST VALUES OF THE MEASURAND.

SENSITIVITY THE SENSITIVITY OF AN INSTRUMENT IS THE RATIO OF THE LINEAR MOVEMENT OF THE OUTPUT TO THE CHANGE IN THE MEASURED VARIABLE qiqi qoqo

SENSITIVITY

ZERO DRIFT AND SENSITIVITY DRIFT THIS IS A VARIATION IN THE OUTPUT OF A MEASUREMENT DEVICE WHICH IS NOT CAUSED BY ANY CHANGES IN THE INPUT SIGNAL THIS IS USUALLY CONSIDERED SEPARATE FROM THE DRIFT RESULTING FROM VARIATIONS IN AMBIENT TEMPERATURE

ZERO DRIFT AND SENSITIVITY DRIFT

LINEARITY IF AN INSTRUMENT IS SUPPOSED TO BE LINEAR, THE LINEARITY GIVES THE INDICATION OF THE MAXIMUM DEVIATION OF ANY CALIBRATION POINTS USUALLY FROM A LEAST SQUARES BEST STRAIGHT LINE FIT THROUGH THE CALIBRATION DATA

LINEARITY

INDEPENDENT LINEARITY

PROPORTIONAL LINEARITY