Specific Gravity Determination
Specific Gravity of Soil Solids
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Test Definition and Importance
Specific Gravity (Gs): The ratio of the mass of soil solids per unit volume to the mass of distilled water per unit volume at the same temperature. Typical value for bentonite ranges between 2.40 - 2.80.
Gs = ρsρw = W2 - W1(W4 - W1) - (W3 - W2)
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Test Procedure (Pycnometer Method)
1
Pycnometer Preparation
Clean, dry pycnometer (50-500 mL) is weighed. A stoppered system should be used.
2
Dry Sample Addition
Bentonite sample dried in oven at 105±5°C (5-100g) is carefully transferred to the pycnometer.
Sample Amount: 5-10g (small pycnometer) / 50-100g (large pycnometer)
3
Water and Air Removal
Distilled water is added (2/3 full). Air bubbles are removed using a vacuum pump (10 min).
Critical: Air bubbles must be completely removed! Otherwise, results will be low.
4
Temperature Equilibrium and Filling
At 20±5°C, water level is filled to the mark. External surface is dried.
5
Water-Only Measurement
Pycnometer is emptied, cleaned, and filled with distilled water only at 20±5°C.
6
Temperature Recording
Test temperature is recorded (°C). If different from 20°C, temperature correction factor is applied.
°C
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Calculation and Formula
ASTM D854 / BS 1377-2 Specific Gravity Formula:
Gs
=
W₂ - W₁
Dry Sample Weight
(W₄ - W₁) - (W₃ - W₂)
Displaced Water Weight
W₁ Pycnometer + Stopper (g)
W₂ Pycnometer + Stopper + Dry Sample (g)
W₃ Pycnometer + Stopper + Sample + Water (g)
W₄ Pycnometer + Stopper + Water Only (g)
Live Calculation Robot
Dry Sample (W₂-W₁):
--
g
Total Water Capacity (W₄-W₁):
--
g
Remaining Water (W₃-W₂):
--
g
Displaced Water:
--
g
÷
Specific Gravity (Gs):
Error
Temperature Corrected Gs (@20°C):
Error
⚠️ W₄ must be > W₁
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Specific Gravity Data Sheet
Weighing Values and Calculation
| Parameter | Symbol | Value (g) | Notes |
|---|---|---|---|
| Pycnometer + Stopper | W₁ | Dry, clean | |
| + Dry Sample | W₂ | Dried at 105°C | |
| + Sample + Water | W₃ | 20±5°C, at meniscus level | |
| + Water Only | W₄ | At same temperature | |
| Temperature | T | °C | |
| Specific Gravity (Gs) | Error | ||
| Temperature Corrected Gs | -- | ||
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Bentonite Specific Gravity Criteria
Sodium Bentonite
2.60 - 2.80
API 13A / OCMA
--
Calcium Bentonite
2.40 - 2.60
API 13A
--
Foundry Sand
2.50 - 2.70
TSE 5360
--
Cat Litter
2.30 - 2.50
Domestic Market
--
Precision: The difference between two parallel test results should be ≤ 0.03 (BS 1377-2). Repeatability limit: ±0.02.
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Required Equipment
- ☑ Pycnometer (50-500 mL, stoppered)
- ☑ Precision balance (0.01g / 0.001g accuracy)
- ☑ Oven (105°C ± 5°C)
- ☑ Vacuum pump or aspirator
- ☑ Thermometer (0.1°C accuracy)
- ☑ Desiccator cooler
- ☑ Distilled water (de-aerated)
- ☑ Funnel and glass rod
- ☑ Water bath (for temperature control)
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Temperature Correction Factors (K)
| Temperature (°C) | ρw (g/cm³) | K Factor |
|---|---|---|
| 18 | 0.9986 | 1.0007 |
| 19 | 0.9984 | 1.0003 |
| 20 | 0.9982 | 1.0000 |
| 21 | 0.9980 | 0.9997 |
| 22 | 0.9978 | 0.9994 |
| 23 | 0.9976 | 0.9991 |
| 24 | 0.9973 | 0.9988 |
| 25 | 0.9970 | 0.9984 |
Correction: Gs₂₀°c = K × GsT
⚠️ Critical Warnings and Error Sources:
- Air Bubbles: The most common source of error. Vacuum application time must be sufficient (min. 10 min).
- Temperature Equilibrium: W₃ and W₄ measurements must be taken at the same temperature (±2°C).
- Meniscus Reading: Water level should be read at eye level, using the bottom of the meniscus as reference.
- Dry Sample: Must be dried to constant weight at 105°C (min. 4 hours).
- Hydrated Clay Minerals: High temperature may cause water loss; drying at 80°C is recommended.