⛽ Density by hydrometer (hydrometry)
Laboratory determination of density of crude petroleum and liquid petroleum products by hydrometer method at 15°C.
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
| Parameter | Value |
|---|---|
| Reference temperature | 15°C (Europe) / 60°F (USA) |
| Gasoline density | 720–775 kg/m³ (per PN-EN 228) |
| Diesel fuel density | 820–845 kg/m³ (per PN-EN 590) |
| Hydrometer accuracy | 0.5 kg/m³ (class 0) / 1.0 kg/m³ (class 1) |
| Repeatability | 0.5 kg/m³ |
| Reproducibility | 1.0 kg/m³ |
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.
2. Hydrometer selection
Select hydrometer with range matching expected sample density. Hydrometer should be clean and dry.
3. Cylinder filling
Pour sample into clean measuring cylinder. Fill to level allowing free hydrometer immersion (approx. 3/4 volume).
4. Hydrometer introduction
Carefully lower hydrometer into cylinder with sample. Do not release suddenly — hydrometer should immerse smoothly without hitting bottom.
5. Stabilization
Wait until hydrometer stabilizes its position and stops swaying (1–2 minutes). Hydrometer should not touch cylinder walls.
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.
7. Temperature measurement
Immediately after density reading, measure sample temperature in cylinder. Record with 0.5°C accuracy.
8. Temperature correction
Convert density to 15°C temperature using ASTM D1250 tables or correction factors for given product type.
9. Measurement repetition
Perform second determination. Difference between two results should not exceed repeatability (0.5 kg/m³).
10. Hydrometer cleaning
Wash hydrometer with white spirit and acetone. Dry and store in protective box. Do not touch graduated stem with fingers.
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
| Equipment | Example | Indicative price |
|---|---|---|
| Glass hydrometers (set) | Ludwig Schneider, Alla France — ranges 600–1100 kg/m³, class 0 or 1 | 150–500 PLN / pc |
| Measuring cylinder (hydrometry) | Glass cylinder d = 50 mm, h = 400 mm, borosilicate glass or PP | 50–200 PLN |
| Laboratory thermometer | Liquid-in-glass or Pt100 thermometer, range -20 to +50°C, accuracy 0.5°C | 100–800 PLN |
| Thermostatic bath | Julabo CORIO, Huber, Lauda — for thermostating samples at 15°C | 5,000–15,000 PLN |
| Automatic densitometer (option) | Anton Paar DMA 4100 M, DMA 4500 M (oscillation method) | 50,000–120,000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| White spirit | 8032-32-4 | For cleaning hydrometers and cylinders after measurement |
| Acetone | 67-64-1 | For final rinsing and quick drying of hydrometers |
| Distilled water (density CRM) | 7732-18-5 | Density standard (998.203 kg/m³ at 20°C) for hydrometer verification |
| Reference density liquid | — | Certified density standard (e.g., Paragon Scientific) for calibration |
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