Question

Hydrometer readings are corrected for:
(A) Temperature correction
(B) Meniscus correction
(C) Dispersing agent correction
(D) Pressure correction
Choose the correct answer from the options given below:

• A. (A) and (B) only.
• B. (C) and (D) only.
• C. (A), (B) and (C) only.
• D. (B), (C) and (D) only.

Hint:

Think of what changes between the calibration condition (clear water, fixed temperature) and the actual test suspension.

Answer 1

The Correct Option is C

Step 1: The hydrometer method is used in soil mechanics to determine the particle size distribution of the fine fraction of soil (silt and clay) by measuring the density of a soil suspension at different times as particles settle, based on Stokes' law.
Step 2: A dispersing (deflocculating) agent such as sodium hexametaphosphate is added to the suspension so fine particles stay separated; since the hydrometer is calibrated assuming dispersed particles in plain water, a dispersing agent correction (C) is required to remove the agent's own contribution to the suspension's density.
Step 3: The hydrometer is calibrated at a standard temperature (usually 27°C), but the actual test is often run at a different room temperature, changing the viscosity and density of the fluid, so a temperature correction (A) is required.
Step 4: Because soil suspensions are opaque, the reading must be taken at the top of the meniscus instead of the true (invisible) liquid surface, so a meniscus correction (B) is required.
Step 5: A pressure correction (D) is not part of the standard hydrometer analysis, since the hydrometer floats freely under atmospheric pressure with no defined pressure adjustment step. So the valid corrections are (A), (B) and (C) only, option 3.