⛽ Density by hydrometer

Petrochemistry PN-EN ISO 3675

Overview

STATUS (as of 02.11.2026). PN-EN ISO 3675:2004 — Polish version, published 14.04.2004, implements EN ISO 3675:1998 and ISO 3675:1998 [IDT]; it replaced PN-EN ISO 3675:2002 (English version). The PKN catalogue card has no withdrawal note — the standard is current. A new edition is in preparation: draft ISO/DIS 3675:2026 (in CEN prEN ISO 3675) is a technical revision; according to its foreword, an appendix on the principles of hydrometer construction has been added because ISO 649-1 (density hydrometers for general purposes), referred to in the standard, was withdrawn in 2024.

SCOPE (clause 1). Laboratory determination, using a glass hydrometer, of the density at 15 °C of crude petroleum, liquid petroleum products and mixtures of petroleum and non-petroleum products normally handled as liquids, with a Reid vapour pressure (RVP) of 100 kPa or less. The method is suitable for mobile transparent liquids; viscous liquids are tested at temperatures above ambient using a liquid bath, and opaque liquids by reading the scale where the top of the meniscus meets the stem of the hydrometer and applying the correction from Table 1. A hydrometer is calibrated to read correctly at a specified temperature, so readings at another temperature are only hydrometer readings and not the density at that temperature. The standard warns that for volatile or waxy crude oils containing free or suspended water and sediments the accuracy can be lower than inferred from the precision data, owing to the loss of light components during sample mixing; the density at 15 °C can be converted with tables to API gravity or relative density.

DEFINITIONS (clause 3). Density — mass per unit volume expressed in kilograms per cubic metre or grams per millilitre at 15 °C and 101.325 kPa. The standard also defines the cloud point, the wax appearance temperature (WAT) and the pour point — terms needed for preparing waxy samples.

WHY IT IS TESTED. According to the corresponding American standard ASTM D1298 (5.1, 5.5), an accurate density is needed to convert measured volumes to volumes or masses at the reference temperatures of 15 °C or 60 °F during custody transfer, and in automotive, aviation and marine fuels it is a quality indicator affecting storage, handling and combustion.

HOW THE TEST PROCEEDS (clauses 4–7). The sample is brought to a specified temperature and a test portion is transferred to a cylinder at approximately the same temperature; the hydrometer, whose temperature has also been regulated, is lowered into the sample and allowed to settle. After temperature equilibrium has been reached, the hydrometer scale and the temperature of the test portion are read, and the reading is reduced to 15 °C with the measurement tables; if necessary, the cylinder with the sample is placed in a constant-temperature bath (clause 4). The cylinder, of glass, plastics or metal, has an inside diameter at least 25 mm greater than the outside diameter of the hydrometer, and its height ensures at least 25 mm clearance between the bottom of the floating hydrometer and the bottom of the cylinder. Glass hydrometers conforming to ISO 649-1, graduated in units of density, cover the range 600–1,100 kg/m³ at 15 °C (a single hydrometer spans 20 kg/m³ with a scale interval of 0.2 kg/m³ or 50 kg/m³ with scale intervals of 0.5 and 1.0 kg/m³); they have a scale interval of 0.2, 0.5 or 1.0 kg/m³, a maximum scale error of ±0.2, ±0.3 or ±0.6 kg/m³ and a meniscus correction of +0.3, +0.7 or +1.4 kg/m³ (Table 1). The bath (if needed) maintains the test temperature to within ±0.25 °C; the thermometers have a range from −1 °C to 38 °C with a graduation interval of 0.1 °C (error ±0.1 °C) or from −20 °C to 102 °C with a graduation interval of 0.2 °C (error ±0.15 °C), and resistance thermometers are permitted if the uncertainty of the whole system is not greater (clause 5). Samples are taken according to ISO 3170 or ISO 3171; when volatile liquids are sampled automatically without a variable-volume receiver, light components may be lost (clause 6). The test portion must be as representative as possible of the bulk sample, so the sample may need to be mixed while maintaining its integrity (7.1).

READING — ACCORDING TO THE 1993 EDITION (ISO 3675:1993; the publicly available text of the 1998 edition ends at 7.1). The hydrometer is lowered gently without wetting the stem above the immersion level; the liquid is stirred with the thermometer and the temperature recorded. The hydrometer is depressed about two scale divisions and released so that it floats freely away from the walls. In a transparent liquid, the reading is the point at which the principal, horizontal surface of the liquid cuts the scale: the eye is placed slightly below the liquid level and raised until the surface, first seen as a distorted ellipse, becomes a straight line. In an opaque liquid, the top of the meniscus is read and a correction applied. Immediately after the reading, the temperature is measured again; if it differs from the previous value by more than 0.5 °C, the readings are repeated. The corrected reading is converted with the ISO 91-1 tables (Table 53A for crude oils, 53B for products) to the density at 15 °C.

LEGAL REQUIREMENTS. The Regulation of the Minister of Climate and Environment of 26.06.2024 on test methods for the quality of liquid fuels (Journal of Laws 2024, item 1058, Annex Part I point 4 and Part II point 3) allows the density at 15 °C of petrols and diesel fuel to be determined with an oscillating density meter (PN-EN ISO 12185) or with a hydrometer immersed in the sample in a cylinder: the scale reading is taken, the sample temperature noted and the result referred to 15 °C read from conversion tables — according to PN-EN ISO 3675. Requirements of the regulation on quality requirements for liquid fuels (Journal of Laws 2024, item 1018): petrols — from 720.0 kg/m³ to 775.0 kg/m³ (Annexes 1 and 2 item 4); diesel fuel — from 820.0 kg/m³ in the summer period (815.0 kg/m³ in the transition and winter periods) to 845.0 kg/m³, and with improved low-temperature properties from 800.0 kg/m³ to 840.0 kg/m³ (Annex 3 item 3).

WHAT WE DO NOT GIVE HERE. The publicly available text of ISO 3675:1998 ends at 7.1. We do not give the further sample preparation (including volatile and waxy samples), the reading procedure and corrections (clauses 8–11), the rounding of the result or the repeatability and reproducibility values according to that edition; the reading procedure above comes from the 1993 edition.

Method principle

The sample is brought to a specified temperature and the test portion is transferred to a hydrometer cylinder at approximately the same temperature. The appropriate hydrometer, whose temperature has also been regulated, is lowered into the test portion and allowed to settle.

After temperature equilibrium has been reached, the hydrometer scale and the temperature of the test portion are read, and the reading is reduced to 15 °C with the measurement tables (ISO 91-1). To avoid excessive temperature variation during the test, the cylinder with the sample can be placed in a constant-temperature bath (clause 4).

Applications

• Crude petroleum, liquid petroleum products and their mixtures with non-petroleum products with a Reid vapour pressure of 100 kPa or less (clause 1)
• Motor petrols — density at 15 °C 720.0–775.0 kg/m³ (Journal of Laws 2024, item 1018, Annexes 1 and 2 item 4)
• Diesel fuel — density at 15 °C from 820.0 kg/m³ in summer (815.0 kg/m³ in the transition and winter periods) to 845.0 kg/m³; diesel fuel with improved low-temperature properties 800.0–840.0 kg/m³ (Journal of Laws 2024, item 1018, Annex 3 item 3)
• Method permitted alongside the oscillating density meter (PN-EN ISO 12185) for testing the quality of petrols and diesel fuel (Journal of Laws 2024, item 1058, Annex Part I point 4, Part II point 3)
• Viscous liquids — at temperatures above ambient with a bath; opaque liquids — with a meniscus correction (clause 1)
• Conversion of measured volumes to volumes or masses at the reference temperature during custody transfer (ASTM D1298, 5.1)

Key parameters

Standard

Standard number

PN-EN ISO 3675:2004

Title (PL)

Ropa naftowa i ciekłe przetwory naftowe — Laboratoryjne oznaczanie gęstości — Metoda z areometrem

Title (EN)

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

Step-by-step procedure

1. Sampling and mixing

   According to ISO 3170 or ISO 3171; the test portion must be as representative as possible of the bulk sample, and mixing must not impair the integrity of the sample — light components are protected with volatile liquids (clause 6, 7.1).

2. Test temperature

   The sample is brought to a specified temperature, and the cylinder, hydrometer and thermometer to approximately the same temperature (clause 4); viscous liquids are tested at temperatures above ambient with a bath (clause 1, 5.3). Detailed conditions — the rest of clause 7 and clause 8, outside the available text.

3. Filling the cylinder and immersing the hydrometer

   1993 edition: the test portion is transferred without splashing and air bubbles, the cylinder is placed vertically in a location free from air currents; the hydrometer is lowered gently without wetting the stem above the immersion level, depressed about two scale divisions and released so that it floats freely away from the walls (7.2–7.5).

4. Scale reading

   1993 edition: in a transparent liquid — the point at which the principal surface of the liquid cuts the scale (eye slightly below the liquid level until the surface becomes a straight line); in an opaque liquid — the top of the meniscus with a correction (7.6–7.7); meniscus corrections of the 1998 edition — Table 1. With a plastics cylinder, the static charge, which can prevent the hydrometer from floating freely, is dissipated before the reading by wiping the cylinder with a moist cloth (note 9).

5. Temperature

   1993 edition: the temperature is measured before and immediately after the reading; if the difference exceeds 0.5 °C, the readings are repeated (7.4, 7.8).

6. Conversion and result

   The hydrometer reading and temperature are converted with the ISO 91-1 measurement tables to the density at 15 °C (clause 4); 1993 edition: Table 53A for crude oils, 53B for products (8.2). Rounding of the result and precision according to the 1998 edition — outside the available text.

Required equipment and apparatus

Health and safety (OHS)

• Crude petroleum and volatile petroleum products are flammable — precautions according to the safety data sheets

Frequently asked questions