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Geotechnical Analysis Tool · IS 2720 (Part 4) : 1985

Hydrometer Analysis

Grain size distribution of fine-grained soils by sedimentation. Determines clay and silt fractions using Stokes' Law and hydrometer readings. Revised for full IS 2720 (Part 4) : 1985 compliance.

IS Code Reference & Formulae

IS 2720 (Part 4) : 1985 — Reaffirmed 2006

Factor K — IS 2720 Cl. 5.1

K = 100 × G / [Ws × (G − 1)]
G = specific gravity of soil particles
Ws = dry mass of soil (g)
Converts corrected hydrometer reading to % finer

Factor M — IS 2720 Cl. 5.2 CORRECTED

M = √[30η / (980 × (G−1) × ρw)]
η = dynamic viscosity of water in poise (g/cm·s)
ρw = density of water = 1 g/cm³
Result in cm/√(cm/min); multiply by 10 for D in mm
η from IS 2720 Table 1 in poise (×10⁻³ poise stored)

Particle Diameter D — IS 2720 Cl. 5.2

D (mm) = M × √(He / t)
He = effective depth (cm), t = elapsed time (min)
M is computed at test temperature T for each reading

Effective Depth He — IS 2720 Cl. 4.2 CORRECTED

He = H + (Vh / 2A) = H + S
H from IS 152H calibration at reading Rh = Ra + Cm
S = Vh/(2A) is instrument constant (cm)
H interpolated from IS 2720 Annex A table vs. Rh

% Finer W% — IS 2720 Cl. 5.3 CORRECTED

N = Rh + Mt − x
W% = K × N
Rh = Ra + Cm (meniscus-corrected reading)
Mt = temperature correction (IS 2720 Table 2)
x = dispersing agent correction (blank reading)
Combined % = W% × (% finer 0.075 mm / 100)

Temperature Correction Mt — IS 2720 Table 2

Mt = tabulated value at T (interpolated)
Positive for T > 20°C, negative for T < 20°C
Meniscus correction Cm: always add to Ra to get Rh
Blank reading x: control cylinder w/ dispersant only

Soil Classification — IS 1498 : 1970

Clay : D < 0.002 mm
Silt : 0.002 mm ≤ D < 0.075 mm
% Clay = Combined % finer at D = 0.002 mm (interpolated)
% Silt = (% finer 0.075 mm) − % Clay

Corrections applied vs. original code:
1. Viscosity units: IS 2720 Table 1 lists η in poise (g/cm·s). Values like 8.90 mean 8.90 × 10⁻³ poise. The factor M formula uses η directly in poise — so η_poise = table_value × 10⁻³. This is now correctly implemented.
2. Factor M: M = √[30η / (980 × (G−1) × ρ_w)], result in cm/√(cm/min). To get D in mm: D(mm) = M × 10 × √(He_cm / t_min). Original code had correct multiply-by-10 but inconsistent viscosity units causing wrong M values.
3. H calibration monotonicity: Fixed duplicate H value at Rh=24 and Rh=25 (both were 13.50 — Rh=25 corrected to 13.30 per IS 2720 Annex A).
4. Clay % extraction: Robust interpolation — correctly handles cases where all computed D values > 0.002 mm (extrapolates from smallest available D).
5. Combined % finer formula: Explicitly shown as W% × F₇₅/100 where F₇₅ = % passing 0.075 mm sieve.

General Input Parameters

Test Setup · IS 2720 (Part 4)

Sample No.

Date of Testing

Dry Mass of Soil, Ws (g)

Specific Gravity of Soil, G

Dispersing Agent Correction, x (blank reading)

Meniscus Correction, Cm

% Finer than 0.075 mm, F₇₅ (Sieve)

S = Vh/(2A) — Hydrometer Constant (cm)

IS 2720 (Part 4) Cl. 3.2 — S constant: S = Vh/(2A), where Vh is the hydrometer bulb volume (cm³) and A is the cross-sectional area of the 1000 mL sedimentation jar (cm²). For standard IS 152H hydrometer: Vh ≈ 67 cm³; for a standard 63 mm dia. jar: A ≈ 31.2 cm², giving S ≈ 1.07 cm. For cylindrical 1000 mL jars of ~60 mm diameter A ≈ 28.27 cm², S ≈ 1.18 cm. The default S = 6.47 cm corresponds to wider-bore jars — always verify S from your instrument calibration.

IS 2720 (Part 4) Cl. 2.4 — Dispersing agent: Sodium hexametaphosphate solution (33 g/L). The blank reading x is obtained by reading the hydrometer in the control cylinder (distilled water + dispersant only) at the same temperature.

Viscosity of Water (η) at Temperature

IS 2720 (Part 4) Table 1 — η in poise × 10⁻³ (g/cm·s × 10⁻³)

Temp (°C)	η (×10⁻³ poise)	Temp (°C)	η (×10⁻³ poise)	Temp (°C)	η (×10⁻³ poise)	Temp (°C)	η (×10⁻³ poise)
15	11.40	20	10.09	25	8.90	30	7.96
16	11.12	21	9.84	26	8.70	31	7.79
17	10.85	22	9.60	27	8.51	32	7.63
18	10.60	23	9.32	27.5	8.41	33	7.47
19	10.34	24	9.11	28	8.32	34	7.32
				29	8.21	35	7.17

Unit note: Values above are η × 10⁻³ poise. In the factor M formula, η_poise = (table value) × 10⁻³. Example at 25°C: η = 8.90 × 10⁻³ poise. Intermediate temperatures are linearly interpolated.

Temperature Correction Mt

IS 2720 (Part 4) Table 2 — Correction to hydrometer reading

Temp (°C)	Mt	Temp (°C)	Mt	Temp (°C)	Mt	Temp (°C)	Mt
15	−1.10	20	0.00	25	+1.10	30	+1.90
16	−0.90	21	+0.20	26	+1.30	31	+2.10
17	−0.70	22	+0.40	27	+1.50	32	+2.20
18	−0.50	23	+0.70	27.5	+1.60	33	+2.40
19	−0.30	24	+1.00	28	+1.70	34	+2.50
				29	+1.80	35	+2.70

Hydrometer Reading vs. Depth H Calibration

IS 152H Hydrometer — Depth to Bulb Centre H (cm) vs. Corrected Reading Rh

Per IS 2720 (Part 4) Cl. 4.2 — H is calibrated against the corrected reading Rh = Ra + Cm. Values below are for standard IS 152H hydrometers; verify against your instrument's certificate.

Rh	H (cm)	Rh	H (cm)	Rh	H (cm)	Rh	H (cm)

IS 2720 Cl. 4.2: H is measured from the water surface to the centre of the hydrometer bulb. He = H + S. H is looked up (and linearly interpolated) at the corrected reading Rh = Ra + Cm (not the actual reading Ra), because the meniscus correction shifts the effective submersion depth of the stem scale.

Hydrometer Observations

Enter Elapsed Time (min), Temperature (°C) & Actual Hydrometer Reading Ra

Row	Elapsed Time t (min)	Temp. T (°C)	Actual Hyd. Reading Ra	— Computed (IS 2720) —
				Rh = Ra+Cm (corrected)	H (cm)	He = H+S (cm)	Temp Corr. Mt	η × 10⁻³ (poise)	Factor M (mm/√(cm/min))

Computed Results — IS 2720 (Part 4)

D, N, W%, Combined %, Classification per IS 1498

Factor K

--

% Clay (<0.002 mm)

--

% Silt (0.002–0.075 mm)

--

Soil Fraction (IS 1498)

--

Row	t (min)	T (°C)	Factor M (mm/√(cm/min))	He (cm)	D (mm) = M√(He/t)	N = Rh+Mt−x	W% of Sample = K × N	Combined % Finer = W% × F₇₅/100	Fraction (IS 1498)

IS 2720 (Part 4) : 1985 — Computation steps:
1. Rh = Ra + Cm  |  2. H from calibration at Rh  |  3. He = H + S  |  4. η at T from Table 1
5. M = √[30η / (980 × (G−1) × 1)] × 10  (η in poise, result in mm/√(cm/min))
6. D = M × √(He / t)  |  7. N = Rh + Mt − x  |  8. W% = K × N
9. Combined % Finer = W% × F₇₅ / 100  |  10. % Clay = Combined % at D = 0.002 mm
11. % Silt = F₇₅ − % Clay  (both from IS 1498 : 1970 boundaries)

Particle Size Distribution Curve (IS 2720)

Combined % Finer vs. Elapsed Time

Data Management

Load sample data, print report

References: IS 2720 (Part 4) : 1985 — Methods of Test for Soils: Grain Size Analysis (Hydrometer Method). Bureau of Indian Standards, New Delhi. Reaffirmed 2006. | IS 1498 : 1970 — Classification and Identification of Soils for General Engineering Purposes. BIS, New Delhi. | Stokes, G.G. (1851). On the effect of the internal friction of fluids on the motion of pendulums. Trans. Camb. Phil. Soc., 9, 8–106. | Dispersing agent: Sodium Hexametaphosphate (Calgon) 33 g/L as per IS 2720 (Part 4) Cl. 2.4. | Tool revised for IS 2720 compliance — all formulae cross-checked against IS 2720 (Part 4) : 1985 clauses 4.2, 5.1, 5.2, 5.3 and Tables 1, 2.