In Indian Standard (IS) test sieves used for particle-size analysis, the “mesh number” is defined as the aperture size expressed to the nearest decamicron (0.01 mm).\nApproximately how many micrometres is the aperture width for an IS sieve of mesh number 50?

Introduction / Context:
IS (Indian Standard) sieves are widely used in chemical engineering and materials processing for particle size classification. This question checks your understanding of the definition of IS “mesh number” and how to convert it into a practical aperture size in micrometres for calculations or lab work.

Given Data / Assumptions:

• IS mesh number is equal to the aperture size expressed to the nearest decamicron.
• 1 decamicron = 0.01 mm = 10 micrometres.
• Required: aperture for mesh number 50 in micrometres.

Concept / Approach:
The mapping is direct: mesh number = aperture (in decamicrons). Convert decamicrons to millimetres and then to micrometres. Remember 1 mm = 1000 micrometres. This avoids confusion with “mesh per inch,” which is a different convention used in ASTM/BS standards.

Step-by-Step Solution:

Verification / Alternative check:
If the sieve were specified by “openings per inch,” the number would not match this value. Because the question explicitly uses the IS definition (aperture expressed in decamicrons), the direct conversion to 0.50 mm (500 μm) is correct.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing IS “mesh number” with “mesh per inch”; omitting the conversion 1 mm = 1000 μm; or misreading decamicron as micrometre can all lead to errors by a factor of 10 or 20.

Final Answer:
500