⛽ Density by hydrometer (hydrometry)

Petrochemistry PN-EN ISO 3675

Overview

Density is one of the most basic physical parameters of petroleum products, commonly used in commerce, logistics, quality control, and product characterization. Density at 15°C is the standard reference temperature for petroleum products in Europe (in USA, 60°F = 15.56°C is used).

The hydrometer method (PN-EN ISO 3675) is a classic, simple and widely used procedure for density measurement using a glass hydrometer. A hydrometer is a thin glass tube with a weight in the lower part and a scale in the upper part, which is immersed in the test liquid. The immersion depth of the hydrometer depends on the liquid density — the higher the density, the less immersion.

Measurement is performed in a graduated cylinder after bringing the sample to a specified temperature (preferably 15°C). Reading the hydrometer scale requires meniscus correction (for transparent liquids, reading at the lower meniscus point, for opaque liquids — at the upper point). If measurement was performed at a temperature other than 15°C, the result is converted to density at 15°C using conversion tables (ASTM D1250 / IP 200).

Density is a key parameter in fuel specifications: gasoline 720–775 kg/m³, diesel fuel 820–845 kg/m³ (per PN-EN 590), crude oil 780–1000 kg/m³. It also serves to convert volume to mass and vice versa in commercial and logistics operations — every fuel transport requires density measurement.

Method principle

The test sample is brought to a temperature close to 15°C and poured into a measuring cylinder of appropriate diameter. The hydrometer, also brought to approximate sample temperature, is carefully lowered into the cylinder with sample and allowed to float freely. After the hydrometer position stabilizes (approx. 1–2 min), the density value is read on the graduated scale at the point where the meniscus contacts the hydrometer stem. Simultaneously, sample temperature is measured with a thermometer accurate to 0.5°C. The read density value is corrected to 15°C temperature using ASTM tables or correction factors.

Applications

• Quality control of liquid fuels (gasoline, diesel, LPG) per PN-EN 228 and PN-EN 590
• Crude oil and petroleum products trading — volume to mass conversion
• Quality monitoring in fuel terminals and refineries
• Classification and identification of petroleum products
• Crude oil testing — API gravity determination
• Quality control of lubricating, base, and industrial oils
• Proficiency testing (PT) in petroleum laboratories
• Rapid fuel control at gas stations and warehouses

Key parameters

Standard

Standard number

PN-EN ISO 3675:1998

Title (PL)

Ropa naftowa i ciekle przetwory naftowe — Laboratoryjne oznaczanie gestosci — Metoda areometryczna

Title (EN)

Crude petroleum and liquid petroleum products — Laboratory determination of density — Hydrometer method

Step-by-step procedure

1. Sample preparation

   Bring sample to temperature close to 15°C. For viscous samples, higher temperature is allowed (record!). Gently mix, avoiding aeration.

   ⏱ Time: 15 min • 🌡 Temperature: 15°C

2. Hydrometer selection

   Select hydrometer with range matching expected sample density. Hydrometer should be clean and dry.

   ⏱ Time: 2 min

3. Cylinder filling

   Pour sample into clean measuring cylinder. Fill to level allowing free hydrometer immersion (approx. 3/4 volume).

   ⏱ Time: 2 min

4. Hydrometer introduction

   Carefully lower hydrometer into cylinder with sample. Do not release suddenly — hydrometer should immerse smoothly without hitting bottom.

   ⏱ Time: 1 min

5. Stabilization

   Wait until hydrometer stabilizes its position and stops swaying (1–2 minutes). Hydrometer should not touch cylinder walls.

   ⏱ Time: 2 min

6. Density reading

   For transparent liquids: read scale at lower meniscus point (sight line at meniscus level). For opaque liquids: reading at upper meniscus point.

   ⏱ Time: 1 min

7. Temperature measurement

   Immediately after density reading, measure sample temperature in cylinder. Record with 0.5°C accuracy.

   ⏱ Time: 1 min

8. Temperature correction

   Convert density to 15°C temperature using ASTM D1250 tables or correction factors for given product type.

   ⏱ Time: 5 min

9. Measurement repetition

   Perform second determination. Difference between two results should not exceed repeatability (0.5 kg/m³).

   ⏱ Time: 5 min

10. Hydrometer cleaning

    Wash hydrometer with white spirit and acetone. Dry and store in protective box. Do not touch graduated stem with fingers.

    ⏱ Time: 5 min

11. Result reporting

    Report density in kg/m³ with 0.1 kg/m³ accuracy, referenced to 15°C. Include measurement temperature and applied correction.

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Petroleum products — flammable (gasoline, naphtha), work away from ignition sources
• Hydrocarbon vapors — toxic by inhalation, work under fume hood
• Glass hydrometers — fragile, risk of cuts if broken
• Avoid fuel contact with skin — use nitrile gloves
• Safety glasses and laboratory coat required

Frequently asked questions