Contents

# PRECISION AND ACCURACY

The performance of a measuring instrument is represented by the terms precision and accuracy. A good instrument must be precise and accurate.

## PRECISION

Precision of an instrument is the extent to which the instrument repeats its result while making repeat measurement on the same unit of product.

It is the repeatability of the measuring process. It refers to the repeat measurement for the same unit of product under identical condition. It indicates to what extent the identically performed measurement agree with each other.

If the instrument is not precise it will give widely varying results for the same dimension when measured again and again. The set of observations will scatter about the mean.

The scatter of these measurement is designated as (= the standard deviation) it is used as an index of precision. The less the scattering the more precise is the measurement. Thus lower the value of the more precise is the measurement.

## ACCURACY

Accuracy of an instrument is the extent to which the average of a long series of repeat measurement made on the same unit of product differs from the true value of the product.

The difference between the true value and the measured value is known as error of measurement.

It is practically difficult to measure exactly the true value. Therefore a set of observation is made whose mean value is taken as the true value of the quality measured,

The distinction between precision and accuracy is represented with the help of following figures.

## FACTORS AFFECTING ACCURACY OF A MEASURING SYSTEM

The accuracy of an instrument depends on 5 basic elements (SWIPE)

S- Standard

W- Workpiece

I- Instrument

P= Person

E- Environment

### 1. Standard

Normally the measuring instrument is calibrated with a standard are at regular interval. The standard may be affected by

Coefficient of thermal expansion

Stability with time

Elastic properties

Geometric compatibility

Position etc

### 2. Work piece

The following factors affect the accuracy

Cleanliness surface finish etc.

Surface defects

Hidden geometry

Thermal equalization etc

### 3. Instrument

The inherent characteristics of the instrument which affect the accuracy are

Scale error

Effect of friction backlash hysteresis etc

Deformation while handling heavy w/p

Calibration error

### 4. Person

The factors responsible for accuracy are

Training skill

Sense of precision appreciation

Ability to select measuring instrument & standard

Attitude towards personal accuracy achievement

Planning for measurement technique to have minimum just with consistent in precision.

### 5. Environment

The environmental factor are:

Temperature press humidity

Clean surrounding and minimum vibration