What is meant by mechanical measurements?

Measurement is a term that refers to analyzing a manufactured component regarding the degree of accuracy for dimensions, tolerances, geometric profile, roundness, flatness, smoothness, etc. Measurement always involves comparing the manufactured component or the prototype with a standard specimen whose dimensions and other parameters are assumed to be perfect and do not undergo changes with respect to time.
Precisely in mechanical engineering the branch that deals with the application of scientific principles for measurements is known as metrology. The domain of metrology in general deals with various measurements like mechanical, chemical, thermodynamic, physical, and biological measurements. In mechanical engineering, the measurements are limited to mechanical specific such as length, mass, surface profile, flatness, roundness, viscosity, heat transfer, etc.

Terms applicable to mechanical measurements

Readability

Readability refers to the ability of a measurement instrument to accurately provide measurement values, whether the value being measured is small or large. It also signifies the range of values an instrument can measure.

Least count

Least count refers to the minimum value that can be measured with a particular measuring instrument. For instance, the least count of a Vernier caliper is denoted by the ratio between the least divisions on the main scale and the number of divisions on the Vernier scale.

Range of an instrument

The range of a mechanical instrument is simply the minimum value to the maximum value an instrument can measure.

Sensitivity

Sensitivity refers to the response capability of an instrument. For instance, the sensitivity of an instrument intended for vibration measurement is denoted as, the ratio between the pointer movement on the scale of the instrument and the change in the magnitude of physical quantity causing the vibration.

Repeatability

Repeatability is the ability of a measuring instrument to repeat the same results or provide consistent results over long use of the instrument.

Hysteresis

Hysteresis results due to the presence of internal losses in an instrument. For a dial and pointer kind of instrument intended for measurement of pressure, the primary cause of losses is due to the presence of static and dynamic friction between the pointer and the pointer pivot. Due to the friction, the pointer doesn't fully return to its initial position when the quantity measured is removed. As a result, it shows an error.

Accuracy

Accuracy denotes the degree of closeness of an instrument value with the true value of the quantity being measured. It is always mentioned in percentage.

Precession

Precession refers to how closely the given instrument provides the same readings repeatedly without significant deviations.

Response time

Response time denotes the ability of an instrument to detect even a minor change in the physical quantity being measured in less time. Instruments intended for the measurement of values related to temperature and pressure changes for live monitoring purposes must have less response time.

Important mechanical measuring instruments

Different mechanical measuring instruments are used based on different applications. Starting from simple mechanical instruments to expensive sophisticated instruments are used in various domains of engineering including heat and temperature measurements, fluid measurements, elasticity, dynamic measurements, etc.

Vernier calipers

Vernier calipers are the measuring instruments designed for measuring linear dimensions. The instrument can also measure the internal and external diameters of a circular cross-section. The instrument is characterized by two jaws, one is fixed and the other is free to slide linearly. The movable jaw is operated by a small knob provided at the bottom, this knob is preferably operated by the thumb of the user.

The Vernier calipers have two scales, one is the main scale and the other is the Vernier scale. The divisions of the main scale and the Vernier scale are in millimeters.

Resolution of Vernier calipers is also termed as Vernier reading. Vernier reading denotes the smallest value that can be measured by the instrument. The metric Vernier calipers have a resolution of 0.02 mm to 0.05 mm, while the imperial Vernier calipers have a resolution of 0.001 inches.

Dial gauge

A dial gauge is used to measure the flatness and amount of inclination of an object or the workpiece. Dial gauges are also intended to measure the roundness of a cylindrical specimen. The minimum value that a dial gauge can measure is limited to 0.01 mm.

A typical dial gauge has a vertical stylus whose motion is constrained in the vertical direction. One end of the stylus directly remains in contact with the workpiece being measured. The other end has a mechanism having a torsional spring and a pointer. The linear movements of the stylus is converted to rotational motion in the pointer.

Micrometers

Micrometers are external diameter measuring instruments of cylindrical workpieces. These instruments are precession instruments and are more accurate than Vernier calipers.

These instruments have a knob and a scale over the handle to take measurements. These instruments are also known as screw gauges.

Instruments used in fluid applications

Viscometer

Viscometers are laboratory instruments that intend to measure the viscosity of a fluid and its related flow properties. Viscometers work on the principle of viscometry.

Flowmeter

Flow meters consists of a small rotating element that remains suspended on the fluid whose flow rates are required to be measured. The rotating element gets lifted due to the applied force of the flowing fluid. The height of lift is proportional to the rate of flow.

Venturimeter

Venturimeter is an instrument that applies Bernoulli's principle. Venturi meters are primarily used to measure the flow discharge of fluid.

Context and Applications

This topic is primarily taught in various undergraduate and postgraduate degree courses like-

• Bachelors in Technology (Mechanical engineering, Civil engineering)
• Masters in Technology (Mechanical engineering, Civil engineering)
• Bachelors of Science in Physics

Practice Problems

Q1. Which of the following is a precession instrument?

a. Vernier caliper

b. Micrometer

c. Dial gauge

d. Viscometer

Answer: Option b

Explanation: Micrometers are precession measuring instruments that measure the external diameters of a cylindrical specimen.

Q 2. Which of the following is true for a Vernier caliper?

a. It can measure external diameter.

b. It can measure internal diameter.

c. It is well suited for linear measurements.

d. All of these

Answer: Option d

Explanation: Vernier calipers are the measuring instruments that can accurately measure both internal and external diameters of cylindrical workpieces and are well suitable for linear measurements.

Q 3. Which of the following instrument converts the linear motion of the stylus into rotary motion?

a. Flow meter

b. Venturi meter

c. Dial gauge

d. Roto meter

Answer: Option c

Explanation: The dial gauge has a vertical stylus whose linear motion is converted to rotary motion.

Q 4. Which among the following instruments uses Bernoulli's principle?

a. Dial gauge

b. Viscometer

c. Micrometer

d. Venturimeter

Answer: Option d

Explanation: The Venturi meter uses Bernoulli's principle for the determination of discharge of a flowing fluid.

Q 5. Which of the following properties of the instrument gives error in measurement due to internal losses?

a. Hysteresis

b. Precession

c. Accuracy

d. Response time

Answer: Option a

Explanation: Hysteresis refers to the internal losses, like dynamic and static friction in an instrument which may lead to measurement errors.