Determining the specific gravity of soil using a pycnometer involves careful measurement of masses and volumes using a specialized glass container. Here's a detailed breakdown of the process:
Understanding the Pycnometer
A pycnometer is a glass flask with a stopper that has a capillary tube. This tube allows for the removal of excess liquid when the stopper is inserted, ensuring a precise volume measurement. This is crucial for specific gravity calculations.
Step-by-Step Procedure
Here's how to determine the specific gravity of soil using a pycnometer, incorporating information from the provided reference:
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Preparation:
- Clean and dry the pycnometer thoroughly.
- Determine and record the mass of the empty, dry pycnometer, Wp (in grams).
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Soil Sample Introduction:
- Take a known mass of dry soil, Ws (in grams), and carefully place it into the pycnometer.
- Determine and record the combined mass of the pycnometer and the dry soil, Wps (in grams).
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Adding Water:
- Fill the pycnometer with distilled water heated to a temperature of 110˚C ± 5˚C (as per the reference). Ensure the soil is completely submerged and that no air bubbles are trapped.
- Insert the pycnometer stopper, ensuring any excess water flows out through the capillary tube.
- Determine and record the weight of the pycnometer, water, and soil combined, Wpsw (in grams).
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Temperature Measurement:
- Insert the thermometer into the water in the pycnometer (as per the reference) and read and record the temperature, Ti, to the nearest whole degree Celsius. Note, the reference mentions the temperature of distilled water to be 110˚C ± 5˚C when it's filled which is very high. It is very unlikely that this temperature is reached when the water and soil are inside the Pycnometer, The temperature needs to be recorded during this step as the temperature of the water in the pycnometer is likely lower than the initial water.
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Water-Filled Pycnometer Weight (Reference Point):
- Empty the pycnometer completely and then refill it with only distilled water, heated to the same temperature Ti as measured earlier.
- Determine and record the weight of the pycnometer when completely filled with water, Wpw (in grams).
Specific Gravity Calculation
The specific gravity of soil (Gs) is calculated using the following formula:
Gs = (Ws ) / (Ws + Wpw - Wpsw)
Where:
- Ws = Mass of dry soil (g) = Wps - Wp
- Wpw = Mass of pycnometer filled with water (g)
- Wpsw = Mass of pycnometer, soil, and water (g)
Key Considerations
- Temperature Control: The temperature of the water is important because the density of water changes with temperature, influencing the calculation of specific gravity, as mentioned in the reference.
- Air Bubbles: Ensure all air bubbles are removed to avoid inaccurate volume measurements. Gentle agitation can help release trapped air.
- Accuracy: Precision in weighing and temperature measurements is crucial for accurate results.
- Reference Temperature: All measurements should ideally be taken at or very close to a reference temperature for water density corrections. The reference temperature usually is 20°C
Summary Table of Values
| Variable | Description |
|---|---|
| Wp | Mass of the empty pycnometer (g) |
| Ws | Mass of the dry soil (g) |
| Wps | Mass of pycnometer and dry soil (g) |
| Wpsw | Mass of pycnometer, soil and water (g) |
| Ti | Temperature of the pycnometer (˚C) |
| Wpw | Mass of pycnometer filled with water (g) |
By following these steps and carefully measuring the relevant weights, you can effectively determine the specific gravity of soil using a pycnometer.
