Question

Which of the following corrections are applied to hydrometer readings during sedimentation analysis?
A. Temperature correction
B. Meniscus correction
C. Humidity correction
D. Moisture correction
E. Deflocculating agent correction
Choose the correct answer from the options given below:

• A. B, C and D only
• B. A and D only
• C. A, B, C and D only
• D. A, B and E only

Hint:

Remember the signs of the hydrometer corrections:
- Meniscus correction (\( C_m \)) is always positive (\( + \)).
- Dispersing agent correction (\( C_d \)) is always negative (\( - \)).
- Temperature correction (\( C_t \)) can be positive (\( + \)) or negative (\( - \)).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Hydrometer analysis is a laboratory method used to determine the grain-size distribution of fine-grained soils (silts and clays) passing through a No. 200 sieve.
This analysis is based on Stokes' Law, which governs the rate of sedimentation of spherical particles in a liquid medium.

Step 2: Detailed Explanation:
The raw hydrometer reading (\( R_h \)) taken during sedimentation analysis must be corrected for experimental factors that alter the density of the water or affect the visibility of the hydrometer scale.
Three key corrections are applied to the observed reading:
1. Meniscus Correction (\( C_m \)): Hydrometer scales are calibrated to be read at the bottom of the meniscus of a clear liquid.
However, soil suspensions are opaque, making it necessary to read the scale at the top of the meniscus.
Because the scale values increase downward, this introduces a systematic error.
A positive correction (\( C_m \)) must be added to the observed reading to obtain the correct value. This matches statement B.
2. Temperature Correction (\( C_t \)): Hydrometers are calibrated at a specific standard temperature (typically \( 20^\circ\text{C} \) or \( 27^\circ\text{C} \)).
If the test temperature is higher than the calibration temperature, the liquid expands, reducing its density, which causes the hydrometer to sink deeper (lower reading).
In this case, a positive correction is applied.
If the test temperature is lower, a negative correction is applied. This matches statement A.
3. Deflocculating (Dispersing) Agent Correction (\( C_d \)): A deflocculating agent is added to the suspension to prevent clay particles from clumping together.
This agent increases the density of the liquid, causing the hydrometer to float higher (higher reading).
A negative correction (\( C_d \)) must be subtracted from the observed reading to account for this effect. This matches statement E.
Humidity and moisture corrections are not used in hydrometer sedimentation analysis.
Therefore, the correct corrections are A, B, and E.

Step 3: Final Answer:
The corrections applied are A, B, and E only.