An enquiry for “50 litres of helium” can describe three very different things: liquid helium, the internal capacity of a cylinder, or the equivalent volume of gaseous helium at specified conditions. A supplier cannot compare these quantities without knowing which meaning the buyer intends.

Use the SG Trading Gas Calc for transparent quantity estimates. This guide explains the inputs and assumptions to retain when sending a result with an RFQ.

What does a litre describe?

A litre is a volume, not a mass. In a liquid-helium enquiry it may describe the volume of liquid. On a cylinder record it commonly describes internal water capacity: the space inside the vessel. In a gas-volume statement it must be tied to temperature and absolute pressure.

Write the meaning alongside the number: “100 L liquid helium at the stated liquid condition”, “50 L cylinder water capacity”, or “1 m³ gas equivalent at 0 °C and 101.325 kPa absolute”. Keeping the qualifier removes an avoidable ambiguity before pricing or packaging is discussed.

Why mass and volume need different information

Mass identifies how much material is present without choosing a gas-volume reference temperature. To convert between mass and volume, the calculation needs density or a suitable thermodynamic relationship. The gas identity matters: one reference cubic metre of helium does not have the same mass as one reference cubic metre of xenon.

Our ordinary-gas converter uses the ideal-gas relationship, with compressibility factor Z = 1: amount of substance equals mass divided by molar mass, and volume equals that amount multiplied by R and temperature, divided by absolute pressure. Temperature in this equation is in kelvin.

The molar masses follow CIAAW's atomic-weight data. The argon preset assumes atmospheric composition, for which 39.948 g/mol is used. CIAAW explains why argon's atomic weight varies with origin. An enriched isotope or a mixture requires its own composition data; it must not be calculated using an ordinary-gas preset.

Normal and standard volumes: state the definition

The letters Nm³, Sm³ and SCF are incomplete purchasing information unless the temperature, absolute pressure and calculation convention are agreed. Do not silently substitute a familiar definition for the one used in a contract or a supplier's document.

Gas Calc offers four explicit reference presets: 0 °C, 15 °C and 20 °C at 101.325 kPa absolute, plus 60 °F at 1 atm absolute. The 0 °C preset is labelled normal in this tool. The 60 °F preset is one specified convention, not a claim that every SCF contract uses it. Each output states its actual conditions.

Choosing a different reference temperature changes the equivalent volume, not the mass. Cubic feet and cubic metres are geometry conversions once the reference state is identical. Changing both the unit and reference state is a separate thermodynamic conversion.

Liquid helium at a defined state

Air Liquide's helium property table gives a liquid density of 124.74 kg/m³ at the boiling point of −268.93 °C and 1.013 bar. This is 0.12474 kg per liquid litre. It is the defined state used by the liquid mode, not a density promised for every pressurised storage condition.

Accordingly, 100 liquid L × 0.12474 kg/L gives approximately 12.474 kg. Conversely, dividing 12.474 kg by that density gives 100 liquid L. Actual tank quantities, vapour space, losses and delivery measurements remain separate questions. A tank's geometric volume is not its deliverable liquid volume.

Water capacity is not gas content

A bundle of 12 cylinders, each with 50 L internal water capacity, has 600 L total water capacity. That arithmetic says nothing by itself about the mass of compressed gas inside. Gas identity, pressure, temperature and an appropriate density model are also required.

For a common-state estimate, all cylinders must contain the same gas at the same pressure and equilibrium temperature. If individual cylinders differ, one average pressure is not automatically a valid substitute for separate inventories. Refer to the actual package specification and measurement records.

Gauge pressure versus absolute pressure

Gauge pressure is measured relative to the surrounding atmosphere. Absolute pressure is measured relative to vacuum. Converting between them requires the atmospheric pressure used in the calculation; converting bar to psi alone does not change that basis.

For example, 0 bar gauge corresponds to 1.01325 bar absolute when the atmospheric pressure is 101325 Pa. The direct converter displays this default and allows a different atmospheric pressure to be entered. An unqualified “200 bar” should therefore be clarified as gauge or absolute before an inventory calculation.

Real-gas behaviour and equilibrium temperature

A reference-volume ideal-gas estimate must not be extended to arbitrary cylinder pressure. A real-gas equation of state describes how density depends on pressure and temperature beyond that approximation. NIST's helium equation of state is a Helmholtz-energy formulation, a different method from a simple ideal-gas or Van der Waals estimate.

A compressed-helium calculation also needs a representative equilibrium gas temperature. A reading taken while a recently filled cylinder is changing temperature may not represent the condition assumed by the calculation. This guide is not a filling or operating procedure.

The table-based compressed-helium mode is held for method verification. Compatibility with a legacy calculator alone is not independent evidence of physical accuracy. No cylinder inventory example is presented here as an approved commercial quantity.

Reproducible worked examples

  • Liquid: 100 liquid L is approximately 12.474 kg at the stated Air Liquide reference state.
  • Reference gas: 1 m³ ordinary helium at 0 °C and 101.325 kPa absolute corresponds to approximately 0.178576 kg using Z = 1. This is a calculated ideal-gas value, not a measured delivery density.
  • Package geometry: 12 × 50 L gives 600 L water capacity, not 600 L of liquid helium and not 600 normal cubic metres of gas.

Keep the complete calculation summary, not just a rounded headline result. Extra displayed digits do not create measurement accuracy.

What to include in the RFQ

State the gas and isotopic composition, gaseous or liquid supply, quantity and unit, reference conditions where applicable, packaging and tare ownership. Add the destination, delivery basis, timing and quality requirements. For liquid helium, use the liquid helium RFQ checklist for tank and return context.

A calculation transferred to Request a quote is a draft estimate for review. It does not set the working pressure, choose a package, guarantee availability or submit the form for you.

Limits and sources

These estimates are not filling instructions, certificates of quantity or custody-transfer measurements. They do not predict boil-off, remaining withdrawable gas, safe fill capacity, impurity levels or price. Agree the required measurement basis and supporting documents with the parties to the transaction.

Technical references: CIAAW atomic weights, Air Liquide helium properties and NIST helium thermodynamic model. Sources checked 22 September 2026.