📋 Table of Contents

Density is one of the most fundamental physical properties of any polymer — it reveals crystallinity, predicts mechanical performance, confirms material grade, and enables material identification. Yet density testing is often overlooked as "routine" when it is, in fact, a precise and information-rich measurement. This guide explains the Archimedes method from first principles, covers every standard, and shows exactly how to perform the test correctly.

The Archimedes immersion method is the most widely used technique for determining polymer density. It requires no expensive specialised optics or chromatography equipment — just a high-precision digital balance, distilled water, and the correct accessories. The Digital Density Apparatus from International Equipments bundles everything needed into a single, ready-to-use system.

Why Density Matters in Polymer Testing

Density is not just a physical constant — it is a window into a polymer's microstructure and predicted performance. Here is what polymer density tells you:

🔬

Crystallinity indicator

In semi-crystalline polymers like PE and PP, higher density = higher crystallinity = better stiffness, strength, and barrier properties.

🏷️

Grade verification

Density confirms you have the right polymer grade. HDPE (0.941+), LDPE (0.910–0.940), and LLDPE (0.915–0.940) differ only in density.

🌡️

Relates to MFI/MW

For PE, density and molecular weight together define the grade. IS 4984 specifies both MFI and density to fully characterise pipe-grade HDPE.

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MVR → MFI conversion

Melt density at test temperature is required to convert MVR (cm³/10 min) to MFI (g/10 min) — see our MFI vs MVR blog.

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Material identification

Unknown plastic samples can often be identified by density — polyolefins float in water, PVC and ABS sink. Density narrows material ID.

📦

Process quality control

Changes in density of finished products signal processing problems: improper cooling rates, contamination, or wrong compound used.

What is Density and Specific Gravity?

Density (ρ) is the mass per unit volume of a material, expressed in grams per cubic centimetre (g/cm³) or kilograms per cubic metre (kg/m³).

ρ (g/cm³) = mass (g) / volume (cm³)

1 g/cm³ = 1,000 kg/m³

Specific Gravity (SG) — also called Relative Density — is the ratio of a material's density to the density of water at 4°C (1.000 g/cm³). Since water at 4°C has density = 1.000 g/cm³ exactly, the numerical value of SG equals density in g/cm³ — but SG is dimensionless (no units).

💡 Practical note: In polymer testing, we usually measure at 23°C using water at 23°C (density = 0.9975 g/cm³, not exactly 1.000). The difference is small (< 0.3%) and negligible for most QC purposes. For high-precision work, apply the temperature correction factor. ISO 1183-1 and ASTM D792 both tabulate water density vs temperature.

The Archimedes Principle — The Science Behind the Test

The Archimedes method is named after Archimedes of Syracuse (c. 287–212 BCE), who discovered the hydrostatic principle that bears his name: a body fully immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid it displaces.

"A body immersed in a fluid is buoyed up by a force equal to the weight of the fluid displaced."

W_air

Weight of specimen in air (g)

W_liquid

Apparent weight of specimen in water (g)

W_air − W_liquid

Buoyant force = weight of water displaced (g)

From the buoyant force, we can calculate the volume of water displaced — which equals the volume of the specimen. Combining this with the specimen's mass gives its density:

ρspecimen = Wair × ρliquid / (Wair − Wliquid)

ρ_specimen

Density of plastic (g/cm³)

W_air

Weight in air (g)

W_liquid

Weight in water (g)

ρ_liquid (water at 23°C) = 0.9975 g/cm³ — typically approximated as 1.000 g/cm³ for routine QC

Physical intuition — why it works

When you suspend a specimen in water, it appears lighter than it does in air. The difference in apparent weight (W_air − W_liquid) is the buoyant force — which equals the weight of water displaced by the specimen. Since we know the density of water, we can calculate the volume of water displaced, and therefore the volume of the specimen. With volume known, density = mass / volume. Elegantly simple, yet extraordinarily precise when performed correctly.

ISO 1183 and ASTM D792 — The Governing Standards

Two major standards govern density testing of solid plastics:

StandardMethodTemperatureSpecimenNotes
ISO 1183-1 Method AImmersion in liquid23°C ± 2°C waterSolid, non-porous specimensInternational — used globally, referenced by EN 12201
ISO 1183-1 Method BLiquid pycnometer23°C ± 0.1°CPellets, granules, powdersHigh precision for small/irregular specimens
ISO 1183-1 Method CDensity gradient column23°C ± 0.1°CAll solid specimensHighest precision — lab-intensive
ASTM D792 Method AImmersion in water23°C ± 2°CSolid specimens ≥ 1 gUSA standard — numerically equivalent to ISO 1183-1A
ASTM D792 Method BImmersion in liquid23°C ± 2°CFor specimens <1 g or brittleUses isopropanol or ethanol liquid
IS 7328Equivalent to ISO 118323°C ± 2°CSolid PE specimensIndian standard — referenced by IS 4984
📋 For IS 4984 HDPE pipe testing: IS 7328 (aligned with ISO 1183-1 Method A) is the specified standard. For export to European markets (EN 12201), ISO 1183-1 Method A is required. For North American markets (ASTM D3350), ASTM D792 Method A is used. All three give essentially the same result.

The Density Formula — Both Cases Explained

Case 1: Specimens that sink (density > 1.0 g/cm³)

For plastics denser than water — PVC, ABS, PC, POM, Nylon, PTFE — the specimen sinks unaided. This is the simpler case:

ρ = Wair × ρwater / (Wair − Wliquid)

Where ρ_water at 23°C = 0.9975 g/cm³

Worked example — PVC specimen:

W_air = 5.243 g  ·  W_liquid = 1.195 g  ·  ρ_water = 0.9975 g/cm³
ρ = 5.243 × 0.9975 / (5.243 − 1.195) = 5.230 / 4.048 = 1.292 g/cm³

Case 2: Specimens that float (density < 1.0 g/cm³)

For plastics lighter than water — all polyethylenes (HDPE, LDPE, LLDPE) and polypropylene (PP) — the specimen must be forced below the water surface using a sinker (a small dense metal weight attached by a thread). The formula is slightly different:

ρ = Wair × ρwater / (Wair + Wsinker,water − Wsinker+specimen,water)

W_air

Specimen weight in air (g)

W_sinker,water

Sinker weight in water — alone (g)

W_sinker+specimen,water

Sinker + specimen together in water (g)

ρ_water

0.9975 g/cm³ at 23°C

Worked example — HDPE pipe sample:

W_air (HDPE specimen) = 4.821 g
W_sinker,water (sinker alone in water) = 12.345 g
W_sinker+specimen,water (sinker + HDPE in water) = 12.191 g
Denominator = 4.821 + 12.345 − 12.191 = 4.975
ρ = 4.821 × 0.9975 / 4.975 = 4.809 / 4.975 = 0.9667 g/cm³
→ This result indicates PE 100 grade HDPE. ✓ Passes IS 4984 (minimum 0.941 g/cm³)

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Handling Floating Specimens (PE and PP)

Since all polyethylenes and polypropylene float in water, the sinker procedure is the standard for these materials. The key is accurate measurement of the sinker's weight in water — this is the "tare" for the submerged weighing. The Digital Density Apparatus accessory set includes a sinker precisely suited for this purpose.

⚠️ Critical step — remove air bubbles. After submerging the HDPE specimen (with sinker attached), air bubbles trapped on the specimen surface add apparent buoyancy and give a falsely low density reading. Gently brush the specimen surface with a soft brush or add a drop of wetting agent (e.g. 0.1% Triton X-100) to the water to reduce surface tension. Wait for bubbles to clear before recording the weight.

Step-by-Step Procedure (ISO 1183-1 Method A)

1

Prepare and condition the specimen

Cut a representative specimen of 1–5 g from the plastic material. Specimens must be free of voids, surface cracks, and contamination. For pipe, cut a section from the pipe wall mid-thickness. Condition at 23°C ± 2°C for at least 2 hours before testing.

2

Weigh the specimen in air (W_air)

Place the specimen directly on the balance pan (no suspension). Record the weight to 1 mg (0.001 g) precision. Ensure no air currents disturb the reading — shield the balance from drafts. Record as W_air.

3

Set up the immersion system

Fill the beaker with distilled or deionised water. Measure and record the water temperature — it must be 23°C ± 2°C. Set up the suspension stand so that the beaker sits above the balance pan without any mechanical contact (otherwise the beaker weight is transmitted to the balance and corrupts the reading).

4

For sinking specimens — weigh in water directly

Attach a fine wire or thread to the specimen. Suspend the specimen from the balance hook (using the suspension accessory) so it is fully immersed in the water. The specimen must not touch the beaker walls or bottom. Remove air bubbles. Record the weight as W_liquid.

5

For floating specimens (PE/PP) — use sinker procedure

First: Attach the sinker to the suspension wire and weigh it in water alone. Record as W_sinker,water. Then: Attach the specimen to the sinker using thread. Submerge both in water. Remove air bubbles from specimen surface. Record weight as W_sinker+specimen,water.

6

Calculate density

For sinking specimens: ρ = W_air × 0.9975 / (W_air − W_liquid). For floating specimens: ρ = W_air × 0.9975 / (W_air + W_sinker,water − W_sinker+specimen,water). Report density to 3 decimal places (e.g. 0.956 g/cm³).

7

Run duplicate and average

Measure at least 3 specimens from the same sample. Calculate the mean density. Results should agree within ±0.003 g/cm³ for replicate specimens — if variability is higher, check for specimen inhomogeneity (voids, contamination, or variable crystallinity across the sample).

8

Compare against specification and report

Compare the mean density against the material specification (e.g. IS 4984: ≥ 0.941 g/cm³ for HDPE pipe grade). Prepare the test report: density result, water temperature, test standard (ISO 1183-1 Method A), instrument, date, operator, specimen source.

Density Reference Values for Common Polymers

PolymerDensity (g/cm³)Water behaviourCommon Applications
HDPE (pipe grade PE 80)0.941 – 0.960Sinks in saltwater; floats in freshIS 4984 min: 0.941 g/cm³
HDPE (pipe grade PE 100)0.950 – 0.965Higher crystallinity gradeIS 4984 min: 0.941 g/cm³
LDPE0.910 – 0.940Floats in waterPackaging film, bags
LLDPE0.915 – 0.940Floats in waterStretch film, flexible packaging
Polypropylene (PP homo)0.900 – 0.910Floats — lowest density common plasticFibres, injection, pipe
PVC (rigid / UPVC)1.30 – 1.58Sinks readily in waterPipe, profiles, fittings
ABS1.01 – 1.08Marginally sinks in waterInjection moulding, consumer goods
Polycarbonate (PC)1.18 – 1.22Sinks in waterEngineering, optical
Nylon 6 (PA 6)1.12 – 1.15Sinks in waterFibres, engineering plastics
POM (Acetal / Delrin)1.39 – 1.43Sinks readilyPrecision gears, valves
PTFE (Teflon)2.10 – 2.20Very dense — sinks quicklyChemical linings, seals
Natural Rubber (NR)0.91 – 0.93Floats in waterGaskets, tyres, gloves
SBR (Styrene-butadiene)0.93 – 0.96Floats to marginally sinksTyres, hoses, belts
EPDM rubber0.86 – 0.90Floats clearly in waterSeals, roofing, automotive
💡 Quick identification test: Plastics with density < 1.0 g/cm³ (PE, PP, most rubbers) float in plain water. Plastics with density > 1.0 g/cm³ (PVC, ABS, PC, Nylon, POM) sink. This simple float/sink test in a beaker of water is a fast preliminary material identification technique — accurate density measurement using the Archimedes apparatus gives the precise value.

IS 4984 Density Requirements for HDPE Pipes

IS 4984:2016 includes density testing as a mandatory requirement for HDPE pressure pipes:

Requirement Value
Minimum density (IS 4984 — all pipe grades)0.941 g/cm³ at 23°C
Test standardIS 7328 / ISO 1183-1 Method A / ASTM D792
Test temperature23°C ± 2°C
Immersion liquidDistilled or deionised water
Specimen requirementCut from pipe wall — free of voids and surface defects
FrequencyEvery batch of incoming compound; each production run of finished pipe
ReportingDensity in g/cm³ to 3 decimal places, water temperature recorded

What density tells you about HDPE pipe quality

⚠️
< 0.941 g/cm³
Below IS 4984 minimum. Insufficient crystallinity. Reject batch — inadequate pressure resistance and ESCR.
✅
0.941 – 0.960 g/cm³
PE 80 grade range. Meets IS 4984. Good for standard water supply applications.
🏆
0.950 – 0.965 g/cm³
PE 100 grade range. Excellent crystallinity. Higher stiffness, better ESCR, preferred for demanding applications.

The Digital Density Apparatus — Specifications

The Digital Density Apparatus from International Equipments is a purpose-built system for ISO 1183-1 / ASTM D792 density testing. It includes the high-precision digital balance and all required accessories as a complete, ready-to-use kit:

Specification Detail
Balance capacity220 grams — covers virtually all plastic specimen sizes
Balance resolution1 milligram (0.001 g) — ASTM D792 minimum requirement
Accuracy±0.001 g — traceable calibration
Accessories includedBeaker (for water) · Stand (suspension frame) · Sinker (for floating specimens) · Thread (for suspension)
StandardsISO 1183-1 Method A · ASTM D792 Method A · IS 7328
Applicable materialsAll solid, non-porous plastics, rubber, and polymer compounds
FinishElectrostatic powder coat — corrosion resistant
Power supply230 V, 50 Hz, single phase
CertificationCE & ISO certified
📋 Why the accessory set matters: The sinker accessory is specifically sized and weighted for PE and PP specimens in the standard density range. The stand accessory ensures the beaker sits over the balance without mechanical contact. The thread is non-hygroscopic to prevent moisture absorption during suspension. Together, these accessories eliminate the major setup errors that affect homemade Archimedes density rigs.

Common Sources of Error in Density Testing

🫧 Air bubbles on specimen surface

Bubbles trapped on the surface or in surface pores add apparent buoyancy to the specimen, making it appear lighter in water than it is. Always brush the surface gently after immersion.

Effect: Falsely LOW density reading

🌡️ Water temperature out of range

Water density varies with temperature — 0.9970 g/cm³ at 25°C vs 0.9997 g/cm³ at 10°C. A 10°C temperature error without correction causes ~0.003 g/cm³ error in density.

Effect: Systematic density error

🔬 Specimen voids or porosity

Internal bubbles or pores reduce the true volume without affecting mass — giving falsely high density readings. Use only dense, void-free specimens. Examine cross-section before testing.

Effect: Falsely HIGH density reading

🏗️ Beaker touching the suspension stand or balance

Any mechanical connection between the beaker/stand and the balance transmits beaker weight to the balance. The suspension frame must be completely isolated from the balance. Check after every setup.

Effect: Large random errors in W_liquid

💧 Hygroscopic specimen absorbing water

Nylon, POM, and other moisture-sensitive plastics absorb water during immersion. Weigh quickly (< 30 s immersion) to avoid moisture pickup affecting results.

Effect: Falsely HIGH apparent weight in water

🧵 Sinker weight not accurately determined

For floating specimens, the sinker weight in water (W_sinker,water) must be measured fresh at each session — sinker corrosion or residual polymer coating changes its weight. Re-measure if in doubt.

Effect: Systematic error in floating specimen density

Practical Applications of Density Testing

Beyond IS 4984 HDPE pipe testing, the Digital Density Apparatus is used across a wide range of polymer and rubber testing applications:

HDPE / LDPE / PP grade verification

Confirm polymer grade on incoming raw material. Distinguish HDPE from LDPE by density alone.

Rubber compound quality control

Density confirms correct filler loading (carbon black, silica) and compound formulation consistency.

PVC compound verification

PVC density increases with plasticiser reduction. Density tracks plasticiser content changes batch-to-batch.

MFI to MVR conversion

Melt density at test temperature converts MVR (cm³/10 min) to MFI (g/10 min) — see MFI vs MVR blog.

Carbon fibre / glass fibre compound

Filler volume fraction and fibre density affect composite density — tracked for quality verification.

Material identification (unknown samples)

Density narrows material identity. Combine with MFI and other tests for positive ID.

Blow moulded container QC

Wall density of HDPE bottles tracks crystallinity and predicts ESCR and stacking performance.

Film and sheet quality control

LDPE / LLDPE film density reflects polymerisation conditions and predicts seal strength and optics.

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Key Takeaways

✉ Equip your density testing lab today. Contact International Equipments for a quotation on the Digital Density Apparatus — complete with accessories, CE and ISO certification, and 12-month warranty. Request a free quote →

Frequently Asked Questions

Common questions about plastic density testing, the Archimedes method, ISO 1183, and HDPE pipe density requirements.

What is the Archimedes method for measuring density of plastics?+
The Archimedes method determines density of solid plastics by measuring two weights: weight in air (W_air) and apparent weight while fully immersed in distilled water (W_liquid). Density is calculated as: ρ = W_air × ρ_water / (W_air − W_liquid). For floating specimens (PE, PP), a sinker is attached and a modified formula is used. The method is standardised in ISO 1183-1 Method A and ASTM D792 Method A.
What is the density of HDPE, LDPE, PP, PVC and other common plastics?+
Typical densities at 23°C: HDPE (PE 100) — 0.950–0.965 g/cm³; HDPE (PE 80) — 0.941–0.960 g/cm³; LDPE — 0.910–0.940 g/cm³; LLDPE — 0.915–0.940 g/cm³; PP — 0.900–0.910 g/cm³; PVC (rigid) — 1.30–1.58 g/cm³; ABS — 1.01–1.08 g/cm³; PC — 1.18–1.22 g/cm³; Nylon 6 — 1.12–1.15 g/cm³; POM — 1.39–1.43 g/cm³; PTFE — 2.10–2.20 g/cm³; Natural Rubber — 0.91–0.93 g/cm³.
What are ISO 1183 and ASTM D792?+
ISO 1183 (Plastics — Methods for Determining the Density of Non-Cellular Plastics) and ASTM D792 (Density and Specific Gravity of Plastics by Displacement) are the primary standards for plastic density measurement. ISO 1183-1 Method A and ASTM D792 Method A both use the Archimedes immersion method in distilled water at 23°C ± 2°C. IS 7328 is the Indian equivalent, referenced by IS 4984 for HDPE pipe testing.
Why is density important for HDPE pipe quality control?+
Density in HDPE pipe material is directly related to crystallinity — which governs stiffness, tensile strength, chemical resistance, and long-term creep resistance. IS 4984:2016 requires minimum density of 0.941 g/cm³ for all HDPE pressure pipe grades. Lower density indicates insufficient crystallinity — the pipe will have reduced stiffness and poorer long-term performance. PE 100 (higher density) consistently outperforms PE 80.
How do you handle floating specimens (PE and PP) in density testing?+
Since PE and PP have density < 1.0 g/cm³, they float in water. A sinker (small dense metal weight) is attached to the specimen by thread. Measure: (1) W_air — specimen weight in air; (2) W_sinker,water — sinker weight in water alone; (3) W_sinker+specimen,water — sinker + specimen in water. Density = W_air × ρ_water / (W_air + W_sinker,water − W_sinker+specimen,water). The sinker accessory is included with the Digital Density Apparatus.
What is specific gravity and how does it differ from density?+
Specific gravity (SG) is the ratio of material density to water density at 4°C (1.000 g/cm³). Since water at 4°C has density = 1.000 g/cm³ exactly, SG equals density numerically in g/cm³ — but SG is dimensionless. When measuring at 23°C (water density = 0.9975 g/cm³), density in g/cm³ equals SG × 0.9975. For routine QC, the difference (0.25%) is negligible and the two terms are used interchangeably.
What factors affect the accuracy of Archimedes density measurement?+
(1) Air bubbles on specimen — give falsely LOW density; brush surface gently after immersion. (2) Water temperature variation — causes systematic error; maintain 23°C ± 2°C. (3) Specimen voids or porosity — give falsely HIGH density; use only void-free specimens. (4) Beaker contacting the stand or balance — causes large errors; ensure full mechanical isolation. (5) Hygroscopic specimens (Nylon, POM) absorbing water — weigh quickly (< 30 s immersion). (6) Sinker weight not measured accurately — re-measure sinker in water at each test session.
What equipment is needed for the Archimedes density test?+
The Digital Density Apparatus from International Equipments provides the complete system: high-precision digital balance (220 g capacity, 1 mg resolution), beaker for water, suspension stand frame, sinker for floating specimens, and thread. The stand ensures the beaker sits above the balance without mechanical contact. All accessories are purpose-designed for ISO 1183-1 Method A accuracy. CE and ISO certified.
What is the density specification for HDPE pipes per IS 4984?+
IS 4984:2016 requires HDPE pipe material density ≥ 0.941 g/cm³ at 23°C, measured per IS 7328 / ISO 1183-1 Method A. Testing is required on incoming compound and finished pipe. PE 100 pipe typically has density 0.950–0.965 g/cm³. A result below 0.941 indicates insufficient crystallinity — batch must be rejected.