⚖️ Density by pycnometer

Petrochemistry PN-EN ISO 3838

Overview

STATUS (as of 06.11.2026). According to the PKN catalogue card (read on 06.11.2026) PN-EN ISO 3838:2008 is a Polish version published on 02.07.2008 (22 pages), implementing EN ISO 3838:2004 and ISO 3838:2004 [IDT] and replacing PN-EN ISO 3838:2005 (English version), with no withdrawal note; the card does not list Amendment 1:2023. ISO 3838:2004 (second edition, 30.04.2004) superseded the 1983 edition as a technical revision; in December 2023 ISO issued amendment ISO 3838:2004/Amd 1:2023 (prepared with CEN/TC 19) — we did not have its content. The description is based on the preview of ISO 3838:2004 (Clauses 1–7.1: scope, references, definitions, principle, apparatus, preparation and conditioning of the pyknometer); we did not have Clauses 7.2–12 (calibration, procedures, calculations, precision).

SCOPE. The standard specifies methods for determining the density or relative density of crude petroleum and petroleum products handled as liquids. The capillary-stoppered pyknometer method is also for use with solids and may be used for coal tar products (road tars, creosote, tar pitches) and their mixtures with petroleum products; it is not suitable for highly volatile liquids — with a Reid vapour pressure above 50 kPa (ISO 3007) or an initial boiling point below 40 °C. The graduated bicapillary pyknometer method is recommended for the accurate determination of the density of all except the more viscous products and is useful when only small amounts of sample are available; it is restricted to liquids with a Reid vapour pressure of 130 kPa or less and a kinematic viscosity below 50 mm²/s at the test temperature. For highly volatile liquids the standard specifies special precautions.

PYKNOMETERS. The capillary-stoppered pyknometer has one of three forms: with a ground glass cap ("warden") — recommended for all products except viscous and solid ones and always for volatile ones, because it reduces expansion and evaporation losses and allows testing below laboratory temperature; the Gay-Lussac type — for non-volatile liquids except highly viscous ones; the wide-mouth (Hubbard) form — for very viscous liquids and solids. Pyknometers without a warden cannot be used at test temperatures so low that sample is lost through the capillary during weighing. The graduated bicapillary pyknometer (Lipkin type) has a capacity of 1 ml to 10 ml and a mass of at most 30 g. The pyknometers have to conform to ISO 3507.

DEFINITIONS. Density is the mass of the substance divided by its volume; when it is reported, the unit and the temperature have to be stated. Relative density is the ratio of the density of the substance at temperature t₁ to the density of water at temperature t₂, both temperatures being stated explicitly. For conversions to reference temperatures the standard refers to the tables of ISO 91-1 (15 °C and 60 °F) and ISO 91-2 (20 °C) and to the reference conditions of ISO 5024.

WHAT WE DO NOT GIVE HERE. Without Clauses 7.2–12 we do not give the calibration and measurement procedure, temperature equilibration times, formulae with corrections for glass expansion and air buoyancy or precision. We have removed figures and assessments without a source: an accuracy of "± 0.1 kg/m³", "referee method", a bath tolerance of "± 0.1 °C" (the standard: 0.05 °C), the densities of water and toluene, the formula for API gravity, step times, brands and prices, and confused pyknometer types (the Gay-Lussac type is a capillary-stoppered pyknometer, not a bicapillary one).

Method principle

Capillary-stoppered pyknometer: the masses of equal volumes of sample and water are compared. Equal volumes are ensured by filling the pyknometer so that it overflows through the capillary while standing in a bath at the test temperature until equilibrium is reached. Graduated bicapillary pyknometer: the graduated arms are calibrated with water in terms of the apparent mass in air of water, and a graph is prepared; the dry pyknometer is filled with sample, after temperature equilibrium in the bath the liquid levels are read and the pyknometer is weighed, and the apparent mass of an equal volume of water is read from the graph. In both methods the calculation of density or relative density includes corrections for the thermal expansion of glass and for air buoyancy.

Applications

• Density and relative density of crude petroleum and petroleum products handled as liquids
• Solids and semi-solids — capillary-stoppered pyknometer
• Coal tar products (road tars, creosote, tar pitches) and their mixtures with petroleum products — capillary-stoppered pyknometer
• Accurate determinations with small amounts of sample — bicapillary pyknometer (liquids with a viscosity below 50 mm²/s)
• Density for conversions in other standards, e.g. in the determination of benzene in petrol according to EN 12177

Key parameters

Standard

Standard number

PN-EN ISO 3838:2008

Title (PL)

Ropa naftowa i ciekłe lub stałe przetwory naftowe — Oznaczanie gęstości lub gęstości względnej — Metody z użyciem piknometru z korkiem kapilarnym i piknometru dwukapilarnego z podziałką

Title (EN)

Crude petroleum and liquid or solid petroleum products — Determination of density or relative density — Capillary-stoppered pyknometer and graduated bicapillary pyknometer methods

Step-by-step procedure

1. Preparation of the pyknometer

   Clean the pyknometer and stopper with surfactant cleaning fluid, rinse with distilled water, then with a water-soluble volatile solvent (e.g. acetone) and dry, using a current of filtered air if necessary. Clean in this way before every calibration and whenever liquid fails to drain cleanly from the walls; between determinations light petroleum spirit 40/60 °C followed by vacuum drying is normally sufficient.

2. Conditioning and weighing the empty pyknometer

   After drying, allow the pyknometer to reach room temperature, dissipate any static charge and weigh to the nearest 0.1 mg; after dissipating the charge by breathing on it, make sure the mass has returned to a constant value. For the greatest accuracy make all weighings within a 5 °C temperature range.

3. Calibration with water

   Calibrate the pyknometer with water according to Clauses 7.2–7.5 of the standard (capillary-stoppered pyknometer — mass of water at the test temperature; bicapillary — graph of the apparent mass of water against the readings on the arms).

4. Measurement of the sample

   Fill the pyknometer with sample and bring it to equilibrium in the bath at the test temperature according to Clause 8 (liquids, solids and semi-solids) or Clause 9 (bicapillary pyknometer); for highly volatile liquids apply the precautions given in the standard.

5. Calculation

   Calculate the density or relative density according to Clause 10, with corrections for the thermal expansion of glass and air buoyancy; report the result with unit and temperature (relative density — with temperatures t₁ and t₂).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Chromosulfuric acid cleaning solution (permitted by the standard only when surfactant does not clean adequately): contains chromium(VI) compounds — toxic, carcinogenic, highly corrosive and hazardous in contact with organic materials; eye protection and protective clothing, never pipette by mouth, after use neutralize carefully and dispose of as toxic waste (chromium is highly dangerous to the environment)
• Acetone (harmonised CLP classification): highly flammable (H225), causes serious eye irritation (H319), may cause drowsiness or dizziness (H336)
• Highly volatile liquids: the standard specifies special precautions for them; the capillary-stoppered pyknometer is not used at a Reid vapour pressure above 50 kPa

Frequently asked questions