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.

The separate other-gas converter for neon, argon, krypton and xenon 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 helium quantity and price converter instead uses reference densities from its documented real-gas model.

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.

In the helium converter, metric gas litres and cubic metres use a selected reference of 0 °C, 15 °C or 20 °C at 101.325 kPa absolute. The 0 °C output is labelled Nm³. SCF always uses 60 °F and 101.325 kPa absolute, independently of the metric selection. MCF means 1 000 SCF and MMCF means 1 000 000 SCF. These are this tool's explicit conventions, not a claim that every contract uses them.

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.

Gas Calc uses a density grid generated from CoolProp 8.0.0 HEOS for helium, with bilinear interpolation between pressure and temperature nodes. Its vessel calculation accepts −40 to +60 °C and pressures equivalent to 0 to 450 bar gauge, with atmospheric pressure from 0.5 to 1.1 bar absolute. It does not extrapolate. This is the calculator's domain, not an approved operating envelope for any container.

More than 900 000 numerical checks against direct calculations from the pinned model found a maximum relative interpolation difference of approximately 0.0052%. That figure measures numerical interpolation, not the accuracy of the equation of state, your pressure gauge or water-capacity record. The model is not NIST-certified; NIST's publication is a related technical reference. The calculation excludes mixtures and enriched isotope products.

Reproducible worked examples

  • Liquid: 100 liquid L is approximately 12.474 kg at the stated Air Liquide reference state.
  • Reference gas: 1 m³ helium at 0 °C and 101.325 kPa absolute corresponds to approximately 0.178481 kg in the CoolProp reference calculation. This is a model 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.
  • Vessel inventory: 50 L water capacity at 200.0 bar gauge, 20.0 °C and 1.01325 bar atmospheric pressure contains approximately 1.5064 kg in the model. At zero gauge pressure a vessel still contains gas. Neither result is an allowable filling quantity or a measure of withdrawable gas.

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

Unit prices without changing the amount

Enter an amount in any helium quantity field and a price in any price field. The columns update independently. Each price is normalised to price per kilogram, then converted to the other units at the stated reference conditions. Quantity multiplied by its matching unit price gives the same total. For example, 1 MMCF at USD 200 per MCF gives USD 200 000, because 1 MMCF equals 1 000 MCF.

These are equivalents of a price you supply, not SG Trading quotations or market prices. A liquid-equivalent price is not a liquefaction quotation: losses, packaging, freight, duties and taxes are excluded. Selecting a currency changes the label only, with no exchange-rate conversion. Prices remain in the calculator and are not transferred to the RFQ.

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 market price. Agree the required measurement basis and supporting documents with the parties to the transaction.

Technical references: CIAAW atomic weights, Air Liquide helium properties, CoolProp helium documentation and NIST helium thermodynamic model. Helium calculation method updated 13 October 2026.