A pycnometer is used to determine

Question

Prepp Team · Accepted Answer

Understanding the Pycnometer and Its Uses in Soil Testing

A pycnometer is a specialized glass flask used primarily in laboratories to determine the density or specific gravity of liquids or solids, particularly fine-grained materials like soil or aggregates. Its design includes a stopper with a fine capillary tube, allowing for precise volume measurements by filling the flask completely and ensuring no air bubbles are trapped.

In geotechnical engineering and materials science, the pycnometer is a fundamental tool for characterizing certain properties of soil and aggregate samples. The question asks what properties a pycnometer is used to determine.

Determining Water Content Using a Pycnometer

The pycnometer method is one way to determine the water content of a soil sample, especially for fine-grained soils where oven drying might alter properties or when a relatively quick method is needed (though oven drying is standard). The method involves:

Weighing the empty, clean pycnometer ($W_1$).
Adding the wet soil sample to the pycnometer and weighing ($W_2$).
Filling the pycnometer with water up to the mark with the soil inside and weighing ($W_3$).
Emptying, cleaning, and filling the pycnometer with water only up to the mark and weighing ($W_4$).
Oven-drying the soil sample and weighing the dry soil ($W_d$).

The weight of water in the original wet sample is $W_w = (W_2 - W_1) - W_d$. The water content ($w$) is then calculated as:

$\text{Water Content} (w) = \frac{W_w}{W_d} = \frac{(W_2 - W_1) - W_d}{W_d}$

Alternatively, if you know the specific gravity of solids, you can determine water content from $W_1, W_2, W_3, W_4$ without explicit oven drying within this procedure context, by relating volumes and weights using specific gravity.

Determining Specific Gravity of Soil Solids Using a Pycnometer

The pycnometer is widely used to accurately determine the specific gravity of soil solids ($G_s$). Specific gravity is the ratio of the density of the soil solids to the density of water at a specified temperature. The method involves:

Weighing the empty, clean pycnometer ($W_1$).
Adding a known weight of oven-dried soil sample to the pycnometer and weighing ($W_2$). Let $W_d = W_2 - W_1$.
Adding de-aired water to the pycnometer containing the soil, filling it up to the mark, and weighing ($W_3$).
Emptying, cleaning, and filling the pycnometer with water only up to the mark and weighing ($W_4$).

The weight of water displaced by the soil solids is conceptually $(W_4 - W_1) - (W_3 - W_2)$. This represents the volume of soil solids multiplied by the density of water. The specific gravity ($G_s$) is calculated as:

$G_s = \frac{\text{Weight of dry soil}}{\text{Weight of equal volume of water}} = \frac{W_d}{(W_4 - W_1) - (W_3 - W_2)}$

Or more commonly formulated using the weights:

$G_s = \frac{W_2 - W_1}{(W_4 - W_1) - (W_3 - W_2)} = \frac{W_d}{W_4 - W_1 - (W_3 - W_2)}$

Where:

$W_1$: Weight of empty pycnometer
$W_2$: Weight of pycnometer + dry soil
$W_3$: Weight of pycnometer + dry soil + water
$W_4$: Weight of pycnometer + water

Analyzing the Options

Let's look at the given options:

Option 1: Voids ratio and dry density - Voids ratio and dry density are calculated parameters. Dry density can be found from bulk density and water content. Voids ratio requires specific gravity and dry density or water content and specific gravity. The pycnometer doesn't directly measure these.
Option 2: Water content and void ratio - As discussed, water content can be determined. However, void ratio is calculated, not directly measured by the pycnometer.
Option 3: Specific gravity and dry density - Specific gravity of solids is determined. Dry density is calculated, not directly measured by the pycnometer.
Option 4: Water content and specific gravity - Both water content and specific gravity of soil solids are properties that can be directly determined using the pycnometer test procedure as outlined above.

Therefore, the pycnometer is used to determine water content and specific gravity.

Soil Property	Pycnometer Use
Water Content ($w$)	Can be determined using the pycnometer method, particularly useful when standard oven drying is not preferred or possible.
Specific Gravity of Solids ($G_s$)	Standard and accurate method for determining the specific gravity of soil or aggregate solid particles.
Dry Density ($\rho_d$)	Calculated from bulk density and water content or other properties; not directly measured by pycnometer.
Voids Ratio ($e$)	Calculated from specific gravity, water content, and/or dry density; not directly measured by pycnometer.

Revision Table: Key Soil Properties and Determination Methods

Soil Property	Definition	Common Determination Method(s)
Water Content ($w$)	Mass of water / Mass of dry solids	Oven Drying Method, Pycnometer Method, Speedy Moisture Tester
Specific Gravity ($G_s$)	Ratio of density of solids to density of water	Pycnometer Method
Bulk Density ($\rho$)	Total mass / Total volume	Core Cutter Method, Sand Replacement Method, Water Displacement
Dry Density ($\rho_d$)	Mass of dry solids / Total volume	Calculated from Bulk Density and Water Content ($\rho_d = \frac{\rho}{1+w}$)
Voids Ratio ($e$)	Volume of voids / Volume of solids	Calculated from $G_s$, $w$, and $\rho_d$ ($e = \frac{w G_s}{S}$ where S is degree of saturation, or $e = G_s \frac{\rho_w}{\rho_d} - 1$)

Additional Information on Soil Testing and Pycnometer

Understanding soil properties is crucial in geotechnical engineering for designing foundations, earthworks, and other civil engineering structures. Various laboratory and field tests are performed to determine these properties.

Pycnometer Accuracy: The accuracy of the pycnometer test for specific gravity relies on careful procedures, including ensuring the soil sample is fully de-aired when water is added. Air bubbles can significantly affect the volume measurement.
Temperature Effects: The density of water changes with temperature. Specific gravity calculations often require measuring the temperature of the water during the test and using the corresponding water density or a standard reference temperature (commonly $4^\circ C$ or room temperature like $20^\circ C$ or $27^\circ C$ depending on standards).
Soil Type: While the pycnometer method for specific gravity is applicable to most soils, it is particularly useful for fine-grained soils (silts and clays) and also used for coarse-grained soils (sands and gravels) and aggregates.
Relationship between Properties: Soil properties like water content, specific gravity, void ratio, porosity, and densities are interconnected through fundamental phase relationships. Determining a few key properties allows for the calculation of others. The pycnometer provides two essential inputs for these calculations.