Precision of measuring instruments

Before learning about precision and accuracy, it’s necessary to understand the term “measurement”: –

Measurement is necessary to comprehend the external world, and we have developed a sense of measurement over millions of years of life. Measures necessitate the use of tools that provide scientists with a numerical value. The issue here is that the result of any measurement made with any measuring instrument is subject to some degree of uncertainty. This degree of indeterminacy is referred to as an error. Precision and accuracy are two critical factors to consider when taking measurements. These terms refer to the degree to which a measurement approaches a known or accepted value.

About Precision

Precision refers to the closeness of two or more measurements to one another. Precision is not synonymous with accuracy. Precision is occasionally subdivided into the following:

• Repeatability

A variation occurs when identical conditions are maintained, and repeated measurements are taken over a short period.

• Reproducibility

Variation occurs when the same measurement procedure is applied to different instruments and operators over extended periods.

About Accuracy

• Accuracy refers to an instrument’s ability to measure a value accurately. In other words, it refers to the degree to which a measured value is similar to a standard or actual value. Accuracy is achieved through the use of small readings. The small reading reduces the calculation’s error. The system’s accuracy is classified into three categories:
• Point Accuracy

The precision of an instrument at a single point on its scale is referred to as point accuracy. It is critical to note that this accuracy provides no information about the instrument’s overall accuracy.

• Accuracy as Percentage of Scale Range

The uniform scale range determines a measurement’s accuracy. 

• Accuracy as Percentage of True Value

The accuracy of instruments is determined by comparing their measured values to their actual values. Instrument accuracy is ignored up to 0.5% from the actual value.

The following factors cause uncertainty in a measurement:

• Limitations of the measuring device 
• the skill of the person making the measurement.
• Irregularities in the object being measured

Precision vs Accuracy

Precision

• It is a measure of the measurement’s reproducibility.
• The term precise refers to something that is clearly or sharply defined.
• Precision is unachievable.
• Precision is determined by various factors and necessitates that it comprises sophisticated analytical techniques.
• Precision is required but not sufficient for accuracy.

Accuracy

• It is the degree to which the quantity being measured is close to its value.
• Measurement accuracy equates to conformity to reality.
• Accuracy may be enhanced.
• Accuracy is contingent upon the use of straightforward analysis techniques.
• Precision requires accuracy but is not sufficient.

Factors affecting the Measuring System’s accuracy and precision

The fundamental components of an accuracy evaluation are the five components of a measuring system, which include the following:

• Factors affecting the calibration standards.
• Factors affecting the workpiece.
• Factors affecting the instrument’s inherent characteristics.
• Factors affecting the individual
• Environmental factors.

Factors affecting the Standard include the following: It may be impacted by the following:

• Coefficient of thermal expansion
• Calibration interval
• Properties of elasticity
• Compatibility with geometry

The following factors affect the instrument’s inherent characteristics: 

Adequate amplification for accuracy.

Friction, backlash, hysteresis, and zero drift errors.

When heavy workpieces are measured, deformation occurs during handling or use. 

Errors in calibration.

Repeatable and comprehensible.

The geometry of contact for both the workpiece and the Standard.

Individuals are influenced by various factors, including their training and abilities.

• Capacity to select measurement instruments and standards.
• Ability to choose measurement instruments and standards 
• Attitude toward one’s accuracy accomplishments.
• Planning measurement techniques that are economical and conform to precision requirements, for example.

Environmental Factors:

• Temperature, relative humidity, and so forth. A clean environment and minimal vibration enhance precision. 
• Adequate illumination.
• Equilibration of temperature between Standard, workpiece, and instrument.
• Thermal expansion effects caused by light-induced heat Thermostats, sunlight, and people. Additionally, manual handling may result in thermal expansion.
• Accuracy can be increased only after all sources of error associated with the above five elements of the measuring system are analysed, and corrective actions are taken.

Methods for Reducing Measuring Error

• Verify the accuracy of all measurements. For instance, duplicate all data entries on two worksheets and compare them.
• Verify that your formulas are accurate.
• Ascertain that observers and data collectors are appropriately trained.
• Make the measurement using the most precise instrument available.
• Conduct the measurements in a controlled environment.
• Pilot-test your instrumentation. For instance, convene a focus group and inquire about the ease of comprehending the questions.
• Multiple measures for the same construct are acceptable. For instance, if you’re conducting a depression screening, use two particular questionnaires.

Precision measuring instruments come in a variety of configurations.

Vernier calliper:

A Calliper is a tool that is used to determine the internal and external dimensions of objects, as well as linear measurements. When things are placed between the jaws, it provides excellent accuracy. The Vernier calliper is made up of a marked scale called a Vernier scale attached to a slider. This slider includes a pointer for taking in-depth dimensions.

Micrometre Screw Gauge: 

It features a U-shaped frame with an attached sleeve. The rotation of the thimble is converted to linear movement of the spindle. The main scale and vernier scale are displayed on the sleeve and thimble.

Dial Indicators: 

Dial Indicator examples include measuring the movement of a car body during bending moments, testing and inspecting the waviness of a machined component’s surface. In both cases, small linear displacements must be precisely measured. Dial indicators mounted on a magnetic stand provide precise deflection measurements for the components under study. Additionally, they are available in an electronic format.

Height Gauge and Surface Plate:

A height gauge mounted on a surface plate can be used to find heights accurately. A height gauge operates similarly to a vernier calliper. It is available in all three configurations: a vernier scale, a dial gauge, or a digital display. The surface plate is a solid granite plate placed horizontally on the ground. On the gauge, a sharp-pointed scriber is provided for marking the object. The thing to be measured for height is placed on the surface plate. The scriber can be adjusted up and down using an adjusting screw.

Summary

• The accuracy of a measured value refers to the degree to which a measurement is accurate. Uncertainty in measurement estimates the margin of error between the measurement result and this value.
• The precision of measured values refers to the degree to which repeated measurements agree.
• A measuring tool’s precision is proportional to the size of its measurement increments. The more precise the device, the smaller the measurement increment.
• Significant figures convey information about a measuring tool’s precision.
• When multiplying or dividing measured values, the final result cannot have more decimal places than the least precise value.