Types of Precision Measuring Instruments Used in Industry
With a need to make quality products which meet design specified tolerances, a large number of firms, research and development centers, and school and college laboratories use measuring instruments that have high accuracy and precision. The special branch of science that deals with such instruments is known as metrology. Here we will look at the features and use of certain well known precision measuring instruments used for linear measurement.
1. Vernier Caliper: It is an instrument used to measure internal and external dimensions of an object with a great accuracy. It is provided with inward jaws and outward jaws to facilitate the measurement. A screw clamp is provided that can lock the position of the vernier scale, so that the instrument can be moved without disturbing the reading. A manually operated vernier caliper has a main scale (in millimeters or inches) and a sliding vernier scale attached to the movable jaw, as shown in the figure.
The term "least count" is used to define the accuracy of a vernier caliper. It is the ratio of the smallest division on the main scale to the total number of divisions on vernier scale. So when we measure a dimension of an object using a vernier caliper, the formula for final dimension is,
Final Dimension=Main scale reading + (Vernier scale reading x Least count)
Disadvantage: It is very delicate to choose the vernier scale reading, which is the line on the vernier scale, exactly coinciding to a line on the main scale. Reading accuracy thus depends on the operator’s skill and eyesight.
Nowadays, vernier calipers with digital displays or dial gauges are available. These reduce the complexity of choosing the coincidence of main scale and vernier scale and calculating the final reading as in manually operated vernier caliper. But due to less cost, manually operated vernier calipers are more popular.
2. Micrometer Screw Gauge: To measure diameter of thin wire or thickness of thin metal sheets with accuracy, micrometer screw gauge is very useful. It has U-shape frame with fixed sleeve. Rotating thimble movement is converted to linear movement of spindle. Main scale and vernier scale are displayed on sleeve and thimble respectively.