Reviewed by Er. Inderjit Sharma (Retd.), Punjab PWD
Why a hydrometer? Sieves can only separate particles down to 75 µm. Silt and clay are much smaller, so they are measured by letting them settle in water and checking how fast they settle. This is the hydrometer (sedimentation) method.
What does it tell you? The percentage of silt and clay in a soil, which completes the grading curve below 75 µm. It is important for clayey subgrades, expansive soils, embankment cores and filter design.
This guide follows IS 2720 (Part 4): Grain Size Analysis (sedimentation by hydrometer).
The Idea: Big Particles Settle Faster
Apparatus Required
Hydrometer
Floating glass instrument; reads the density of the soil–water mix.
Two 1000 ml cylinders
One for the soil suspension, one as the control jar.
Thermometer & water bath
Keeps and measures the temperature (0.5°C accuracy).
Stopwatch
Times the readings from 30 seconds to 24 hours.
Balance, oven, sieves
0.01 g balance, oven, 4.75 mm and 75 µm sieves.
Dispersing agent
Keeps clay particles separate so they settle one by one.
Parts of the Hydrometer and Effective Depth
Procedure
Preparing the Suspension
- Oven-dry the soil passing the 75 µm sieve. Weigh 50 g for clayey soil or 100 g for sandy soil.
- Put it in an evaporating dish, add 100 ml of dispersing solution, warm gently for about 10 minutes, then stir for about 15 minutes.
- Pour it into the 1000 ml cylinder (rinse the dish with distilled water) and fill to exactly 1000 ml.
- Fill the control jar with the same amount of dispersing solution and distilled water (no soil).
Taking Readings
- Close the cylinder with a stopper and shake it end over end to mix the soil fully. Put it in the water bath and start the stopwatch.
- Gently put in the hydrometer and read at 0.5, 1, 2 and 4 minutes.
- Take the hydrometer out, rinse it and keep it in the control jar. Put it back (about 10 seconds before each reading) for readings at 8, 15, 30 minutes, 1, 2, 4 hours and finally 24 hours.
- Note the temperature of the suspension at each reading, and read the control jar at the same temperature.
Corrections (Made Simple)
👁️ Meniscus correction (Cm)
Muddy water hides the true reading, so we read the top of the meniscus. Cm = difference between top and bottom readings in clear water. Rh = R’h + Cm.
🌡️ Temperature & dispersant
The dispersant and temperature change the liquid’s density. The control jar reading at the same temperature gives this correction, which is subtracted.
📏 Effective depth (HR)
For each corrected reading, read HR (in cm) from the hydrometer’s calibration chart.
Formulas and Worked Example
μ = viscosity of water (poise) at the test temperature; Gs = specific gravity of soil; Gw = specific gravity of water at that temperature; HR in cm; t in minutes; Ws = dry weight of soil (g). If only the part passing 75 µm was tested, multiply N by (% passing 75 µm ÷ 100) to get % of the whole soil.
Reading at t = 30 min: R’h = 18.0, Cm = +0.5 → Rh = 18.5; control-jar correction = 3.0 → R = 15.5
HR from calibration chart = 13.0 cm
D = √[30 × 0.00851 ÷ (980 × 1.6835)] × √(13.0 ÷ 30) = 0.01244 × 0.6583 = 0.0082 mm (8 µm, silt size)
N = [100 × 2.68 ÷ (50 × 1.68)] × 15.5 = 3.190 × 15.5 = 49.5% finer than 0.0082 mm
Water Properties for the Calculation
| Temperature (°C) | Density of water (g/cm³) | Viscosity (poise) |
|---|---|---|
| 26 | 0.9968 | 0.00870 |
| 27 | 0.9965 | 0.00851 |
| 28 | 0.9963 | 0.00833 |
| 29 | 0.9960 | 0.00815 |
| 30 | 0.9957 | 0.00798 |
| 31 | 0.9954 | 0.00781 |
| 32 | 0.9951 | 0.00765 |
| 33 | 0.9947 | 0.00749 |
| 34 | 0.9944 | 0.00734 |
Try It: Hydrometer Calculator (One Reading)
Repeat for every reading (0.5 min to 24 h) and plot D against % finer on the same grading curve as the sieve analysis.
Precautions
🌡️ Steady temperature
Keep the suspension within ±2°C during the test, and the control jar within 1°C of it.
🤲 Don’t disturb
Put the hydrometer in and take it out slowly and gently, so the settling is not disturbed.
🧼 Clean hydrometer
Rinse the hydrometer after each reading and keep it in the control jar, so soil does not stick to it.
⏱️ Exact timing
Read at the exact times. Put the hydrometer in about 10 seconds early so it settles before the reading.
Practical Tips for Site Engineers
1. When to Ask for a Hydrometer Test
🧱 Clayey subgrades
When more than about 12% passes 75 µm, the fines decide how the soil behaves; knowing the clay content helps judge it.
⚠️ Expansive soils
High clay content along with high free swell or plasticity warns of swelling and shrinking (e.g. black cotton soil).
💧 Filters and drains
Filter design needs the full grading curve, including the fine sizes of the soil being protected.
2. Good Practice
- Plan the time: the test runs for 24 hours after preparation (plus soaking). Ask for it early, so it doesn’t hold up approvals.
- Test together: request sieve analysis, hydrometer, liquid limit and plastic limit together. The grading tells you how much is fine; the limits tell you how that fine soil behaves.
- Read the result simply: % finer than 2 µm = clay; % between 2 µm and 75 µm = silt. A soil with a lot of clay will be sticky, hold water and be hard to compact when wet.
- Quick site idea first: the jar test (shake soil in a glass jar of water and watch the layers settle) gives a rough idea of sand, silt and clay before the lab result arrives.
Key Takeaways
- The hydrometer method measures silt and clay (finer than 75 µm) by how fast particles settle in water.
- Use 50 g (clay) or 100 g (sandy soil) with 100 ml dispersant (33 g SHMP + 7 g sodium carbonate per litre).
- Read at 0.5, 1, 2, 4, 8, 15, 30 min, 1, 2, 4 h and 24 h; correct for meniscus and the control jar.
- D = √[30μ / (980(Gs − Gw))] × √(HR/t); N = [100Gs / (Ws(Gs − 1))] × R.
- On site: ask for it for clayey or expansive soils, plan for the 24-hour test, and read it together with LL and PL.
References: IS 2720 (Part 4): Methods of Test for Soils – Grain Size Analysis (hydrometer method); IS 1498: Classification and Identification of Soils for General Engineering Purposes.