05/03/2023 1

INSTRUMENTATION & MEASUREMENTS

Some important terms in instrumentation field

Engr. Fida Hussain

5th term BE (Electrical)

SOME IMPORTANT TERMS There are some important terms in instrumentation

for which it is necessary to be familiar with,

1. Accuracy: “It is degree of closeness to the actual value of measurand It is the ability of instrument to measure actual or true value.”

2. Precision:

“It is consistency or repeatability of measurement”.

It depends on conformity and significant figures.

Example of accuracy & precision

3. Conformity:“The ability instrument to show every fractional part of the quantity being measured.”

Example: We have a resistor of 2385692 ohms, we are measuring it with a ohmmeter it is measuring true value but observer is taking that valueconsistently as 2.4 MΩ due to limitation of scale

4. Significant Figures

“Significant figures convey the actual information about the magnitude and measurement precision of the quantity.”

Example:

Resistance of 50Ω may be closer to 51Ω or 49Ω if there are 2 significant figures but, if it is specified as 50.0Ω then it may be closer to 50.1 Ω or 49.9 Ω.

More the significant figure greater will be the precision.

5. Sensitivity

“The smallest change in measuring quantity to which instrument will respond”

It is expressed as;

Problem from book example # 1.3 (U.A Bakshi)

inputin Changeoutputin ChangeySensitivit

6. Threshold:

“If the measuring quantity is slowly varied form zero & onwards the output does not change until some minimum value of input is exceeded, that minimum value is called threshold”.

Below threshold value instrument will be in dead space.

Range of an instrument means max & min

values in which it operate or measure.

7. Resolution:

“It is smallest increment of the quantity being measured which can be detected by the instrument with certainty”.Resolution is the fineness to which an instrument can be read

Problem from book, example # 1.4 (U.A Bakshi)

8. Bandwidth:

“The bandwidth of an instrument relates to the maximum range of frequency over which it is suitable for use”.

9. Linearity:

“Means behavior of output is linear with input.

9. Stability: “Ability of an instrument to retain its performance throughout its useful life”.

10.Zero drift:

“Drift is the gradual shift in the instrument indication over an extended period during which the value of the input variable does not change”.

Zero drift means deviation in instrument output with time from its zero value, when measurand is constant.

Solved example # 1.1 from book (U.A Bakshi)

05/03/2023 10

INSTRUMENTATION & MEASUREMENTS

Errors in measurement

Engr. Fida Hussain

5th term BE (Electrical)

Error:

Accuracy is the most important characteristic of an instrument and it is measured in terms of error.

“The algebraic difference between measured value of the quantity and its true value is called “absolute static error”.

Where At is the true value & Am is the measured value.

tm AAe

This absolute error does not provide useful information about accuracy.

For example an error of is negligible when the true value is 1000V, but it will be very significant when the true value is 5V.

Hence the term relative or percentage error is specified and is given as

V1

%100e %

valuetrue valuetrue- valuemeasured

valuetrueerror absolutee

r

r

t

tm

t

tm

AAA

AAA

Percentage error

The relative percentage error and the accuracy can be mathematically related as.

Static Correction:The algebraic difference between true value and the measured value.

accuracy percentage % 100A aaccuracy relative A

11

t

tmr A

AAeA

mt

mt

ASCAeSCAASC

Solved problem example no. 1.2