⛽ Density at 15 °C and vapour pressure of liquefied petroleum gases LPG calculated from the molar composition (gas chromatography according to ISO 7941) and component factors; PN-EN ISO 8973

Petrochemistry PN-EN ISO 8973

Overview

WHAT THE STANDARD COVERS. ISO 8973:1997 (clause 1): a simplified method for calculating the density and vapour pressure of liquefied petroleum gases LPG from compositional data and from the density and vapour pressure factors of the individual components given in the standard. The method is intended for product quality specifications and not for the measurement of quantities on transfer (settlements) — here the standard refers to ISO 6578. Definitions (clause 3): LPG — a hydrocarbon gas that can be stored or handled in the liquid phase under moderate pressure and at ambient temperature, consisting mainly of C3 and C4 alkanes or alkenes, usually with less than 5 % by volume liquid of components with a higher number of carbon atoms, with a gauge vapour pressure not exceeding about 1600 kPa at 40 °C; density factor — the density of the component in the liquid phase under its own vapour pressure at 15 °C, in kg/m³; vapour pressure — in kPa absolute (gauge pressure + ambient pressure); vapour pressure factor — the absolute vapour pressure of the component at 37.8, 40, 50 or 70 °C, in kPa. The "Scope" field of the PKN card of PN-EN ISO 8973:2000, in full (our translation): "A simplified method is described for calculating the density and vapour pressure of liquefied petroleum gases (LPG) based on additive data of the density and vapour pressure factors for the individual LPG components. A list of these factors is given in Annex A. The method is intended for use in product quality requirements, but it is not intended for use in quantity measurements in commercial trade (see ISO 6578)" (without a full stop on the card).

PROCEDURE AND CALCULATIONS (ISO 8973:1997, clauses 4–9 and Annex A — full text of the standard read). Principle: the molar composition of LPG is determined by gas chromatography according to ISO 7941, and from it the liquid density and vapour pressure are calculated using the factors of the standard. Density: the mass fraction of each component wi = xi·Mi / Σ(xi·Mi) (xi — mole fraction, Mi — relative molecular mass), and the density of LPG at 15 °C ρ = 1 / Σ(wi/ρi), where ρi — density factor of the component. Vapour pressure: partial vapour pressure of the component pvp,i = xi·pv,i (pv,i — vapour pressure factor at 37.8, 40, 50 or 70 °C), absolute vapour pressure of LPG pv = Σ pvp,i, and the gauge vapour pressure pve = pv − atmospheric pressure (101.325 kPa). Result: density to the nearest 0.1 kg/m³, vapour pressure to 1 kPa. Precision depends on the precision of the compositional analysis by chromatography and on the accuracy of the factors. Report: reference to the standard, type and identification of the product, result, deviations from the procedure, date. Factors from Table A.1 (relative molecular mass; density factor at 15 °C, kg/m³; vapour pressure factors at 37.8 / 40 / 50 / 70 °C, kPa), selected rows: propane — 44.097 2; 507.30; 1317 / 1352 / 1672 / 2634; butane — 58.123 0; 584.06; 355 / 377 / 486 / 831; isobutane (2-methylpropane) — 58.123 0; 562.98; 507 / 531 / 659 / 1115; propene — 42.081 4; 521.33; 1570 / 1661 / 2026 / 3141; ethane — 30.069 4; density factor 375.76 in the 1997 edition, and after amendment Amd 1:2020 — 359.78; 5269 / 5611 / 6282 / 9119. The table also covers ethene, butenes, butadienes, pentanes and 1-pentene; the standard stresses that the factors are empirical values to be used only in these calculations.

AMENDMENT ISO 8973:1997/Amd 1:2020 (full text, 1 page). Reference to ISO 7941 undated (instead of ISO 7941:1988); ISO 6578:1991 moved to the bibliography (with a footnote that it has been withdrawn); in 6.3.2 the missing word "pressure" added in the sentence on vapour pressure; in Table A.1 the ethane density factor 375.76 replaced by 359.78.

STATUS (state of the PKN cards on 10.10.2026). The card of PN-EN ISO 8973:2000 (Polish version) carries no withdrawal label: title "Skroplone gazy węglowodorowe -- Oznaczanie gęstości i prężności par metodą obliczeniową" (Liquefied petroleum gases — Determination of density and vapour pressure by a calculation method; our translation), publication 03-07-2000, 9 pages, Sector of Chemistry, KT 223 Technical Gases, "Introduces: EN ISO 8973:1999 [IDT], ISO 8973:1997 [IDT]", ICS 75.160.30, in the field "Additional elements" — PN-EN ISO 8973:2000/A1:2020-10E. The card of the amendment PN-EN ISO 8973:2000/A1:2020-10 (English version): publication 15-10-2020, 12 pages, "Introduces: EN ISO 8973:1999/A1:2020 [IDT], ISO 8973:1997/Amd 1:2020 [IDT]", "Scope" field: "The first sentence in 6.3.2 and the density factor for ethane in Table A.1 have been changed." (our translation). PKN search engine, query "ISO 8973" (10.10.2026): only these two cards. The Polish text of the standard therefore dates from 2000 and has Table A.1 from the 1997 edition, while the amendment with the new ethane factor is available in English only. In the iTeh catalogue (read on 10.10.2026) ISO 8973:1997 has the status "Published", and the last periodic review ended with confirmation of the standard on 29.08.2026; EN ISO 8973:1999 — "Published".

HOW MANY LABORATORIES AND IN WHAT FORM (copy of the accreditation scope database of 10.10.2026 and current PCA documents read on 10.10.2026). The number 8973 appears in 30 records at 7 laboratories: AB 1275 (10 records), AB 079 (6), AB 009, AB 103, AB 484, AB 1278 (3 each) and AB 474 (2). In the current PCA documents the standard is present at six laboratories, without suspended entries; on 10.10.2026 the PCA page of laboratory AB 474 had no scope document attached. Forms: "PN-EN ISO 8973:2000+A1:2020-10" (most), "PN-EN ISO 8973" without a year (AB 1278 and one record of AB 009). The "Laboratories" tab ("PN-EN ISO 8973") shows all 7 laboratories.

WHAT THE LABORATORIES TEST (copy and PCA documents). Subject: gaseous fuels — LPG, liquefied petroleum gases, at AB 009 also bio-components (bio-propane). Quantities "by calculation": density at 15 °C (AB 079, AB 103, AB 474, AB 1275, AB 1278); vapour pressure estimated at 37.8, 40, 50 and 70 °C, and at some laboratories also at −10, −5, 0, 10 and 20 °C (AB 079, AB 103, AB 474, AB 484, AB 1275); gauge vapour pressure at 20 °C (AB 1275); the temperature at which the estimated gauge vapour pressure is not less than 150 kPa (AB 009, AB 079, AB 103, AB 484, AB 1275, AB 1278) or 200 kPa (AB 009, AB 1275). For the quantities at low temperatures and for the threshold temperature the laboratories also cite PN-EN 589 (requirements for automotive LPG), Annex C — all seven, in different editions (2019-04, +A1:2022-07, 2024-08 or without a year).

WHERE A SEEMINGLY CORRECT RESULT IS EASY TO GET. First — the old ethane factor: a calculation with the factor 375.76 instead of 359.78 kg/m³ (Amd 1:2020; whoever calculates from the Polish text of PN-EN ISO 8973:2000 alone has the old factor) gives an overstated density for mixtures containing ethane (a larger ρi reduces the term wi/ρi of the sum in the denominator); in the calculation spreadsheet the table of factors has to be checked, not only the formulae. Second — absolute versus gauge pressure: the factors give ABSOLUTE vapour pressure; the gauge pressure is the result minus 101.325 kPa, and product requirements are sometimes given as gauge pressure. Third — temperatures outside the table: the text of ISO 8973 gives vapour pressure factors only for 37.8, 40, 50 and 70 °C; the values at −10…20 °C and the threshold temperatures of 150/200 kPa in the laboratories' scopes are calculated together with PN-EN 589 Annex C — we have not read the content of that annex here. Fourth — this is a calculation, not a measurement: the accuracy depends on the chromatographic analysis and on the factors, and the method is not suitable for quantity settlements (ISO 6578). Fifth — the density at 15 °C is the density of the liquid under its own vapour pressure, not the density of the gas.

WHAT WE DO NOT GIVE. Standard and amendment read in full; we do not give the chromatographic procedure of ISO 7941, the requirements of PN-EN 589 or the content of its Annex C (threshold temperatures of vapour pressure), or all rows of Table A.1 (some rows in the sample are illegible under the watermark).

Method principle

The molar composition of LPG is determined by gas chromatography (ISO 7941); the liquid density at 15 °C is calculated as ρ = 1/Σ(wi/ρi) from the mass fractions wi = xiMi/Σ(xiMi) and the density factors of the components, and the vapour pressure as the sum of the partial pressures pv = Σ xi·pv,i from the factors at 37.8, 40, 50 or 70 °C; gauge pressure = pv − 101.325 kPa (ISO 8973:1997, clauses 4–6; factors in Table A.1, ethane density factor according to Amd 1:2020).

Applications

• Liquefied petroleum gases LPG (C3–C4 alkanes and alkenes, gauge vapour pressure up to approx. 1600 kPa at 40 °C) — product quality specifications (ISO 8973:1997, clauses 1 and 3)
• Not for the measurement of quantities on transfer and settlements (reference to ISO 6578)
• In the accreditation scopes: 7 laboratories — density at 15 °C, vapour pressure at several temperatures and threshold vapour pressure temperature (with PN-EN 589 Annex C)

Key parameters

Standard

Standard number

PN-EN ISO 8973:2000 + PN-EN ISO 8973:2000/A1:2020-10 (norma w wersji polskiej, zmiana w wersji angielskiej; wprowadzają EN ISO 8973:1999 [IDT], ISO 8973:1997 [IDT], EN ISO 8973:1999/A1:2020 [IDT] oraz ISO 8973:1997/Amd 1:2020 [IDT])

Title (PL)

Skroplone gazy węglowodorowe — Oznaczanie gęstości i prężności par metodą obliczeniową

Title (EN)

Liquefied petroleum gases — Calculation method for density and vapour pressure

Step-by-step procedure

1. Sampling and compositional analysis

   Molar composition of LPG by gas chromatography according to ISO 7941 (sampling of LPG — according to a separate standard, e.g. ISO 4257).

2. Density

   Mass fractions wi = xiMi/Σ(xiMi); ρ(15 °C) = 1/Σ(wi/ρi) from the factors of Table A.1 (ethane: 359.78 after Amd 1:2020).

3. Vapour pressure

   pv = Σ xi·pv,i at the selected temperature (37.8, 40, 50 or 70 °C); gauge pressure pve = pv − 101.325 kPa.

4. Result and report

   Density to 0.1 kg/m³, vapour pressure to 1 kPa; in the report reference to the standard, identification of the product, result, deviations, date. PN-EN ISO 8973:2000 with amendment A1:2020-10 current (state of the PKN cards on 10.10.2026).

Required equipment and apparatus

Health and safety (OHS)

• Sampling and analysis of LPG — extremely flammable gas under pressure; risk of frostbite on contact with the liquid; ventilation and no ignition sources
• NOTE ON SUBSTANCE: the result is a calculation from the composition, not a measurement of density or vapour pressure

Frequently asked questions