--- title: Sampling Cryogenic and Liquefied Gases: Vaporisers url: https://www.sarveshanalytics.com/learn/sampling-cryogenic-and-liquefied-gases.html description: Why a liquid sample must be fully vaporised before analysis, how a sample vaporiser works, and the practical points for liquid oxygen, nitrogen, argon and LPG draws. updated: 2026-11-03 source: html --- Learn · Sampling and conditioning # Sampling cryogenic and liquefied gases: why vaporise first Analysers measure gas. A liquid draw from a tank or column has to become gas without changing composition on the way. Intermediate 3 min read Lesson 8 of 10[Sampling and conditioning](https://www.sarveshanalytics.com/learn/track-sampling.html) Helpful first: [Sample pressure, flow and fast bypass loops](https://www.sarveshanalytics.com/learn/sample-pressure-flow-and-bypass-loops.html) Quick answer A liquid sample must be vaporised completely before it reaches an analyser. If it boils off slowly, the lighter component leaves first and the sample no longer has the liquid's composition. A sample vaporiser heats the draw so that all of it becomes gas at once. For cryogenic oxygen, nitrogen and argon, use a vaporiser and keep the line short, dry and warm. Updated 3 November 2026 · Figures from Sarvesh datasheets **After this lesson you can** - Explain fractionation when a liquid boils off partially - Describe what a sample vaporiser does - Plan a cryogenic sample line with a vaporiser and bypass ## The problem: boiling off in stages A liquid mixture boils off its most volatile component first. If a liquid sample warms gradually in a line, the first vapour is rich in the light component and the residue is rich in the heavy one. An analyser fed this sample sees a composition that moves with time and does not equal the liquid. This is called fractionation. ## What a vaporiser does A sample vaporiser heats the liquid so quickly and completely that every component turns to gas together. The gas then has the composition of the liquid. SV-200 is the cryogenic sample vaporiser in the Sarvesh range, with a bypass line for quick flushing; SV-400 serves wet samples and others. They are designed so that the sample is vaporised without loss: [sample-vaporisers](https://www.sarveshanalytics.com/sample-vaporisers.html). ## Building the sample path 1. Take the draw from the liquid line through a short insulated line to the vaporiser. 2. Vaporise at the vaporiser, then regulate pressure and flow of the gas. 3. Use the bypass to flush the vaporiser quickly at start-up and when changing sample. 4. Keep downstream tube warm and dry so the gas does not recondense or pick up moisture. 5. Vent safely: oxygen-enriched vent gas is a hazard: [Oxygen enrichment and oxygen deficiency hazards](https://www.sarveshanalytics.com/learn/oxygen-enrichment-and-oxygen-deficiency-hazards.html). ## Cryogenic safety - Liquid oxygen and oxidising liquids need materials cleaned for oxygen service; hydrocarbon grease and oil are not allowed. - Trapped liquid in a closed line can build dangerous pressure as it warms; include relief where the design requires. - Cold lines and parts cause burns. **Rule of thumb** Treat any liquid draw as a safety design item as well as an analysis item. Ask an engineer to review the arrangement. A vaporiser turns the whole liquid draw to gas at once, so the composition reaching the analyser is the composition of the liquid. **In short** - Liquid samples fractionate if they boil off slowly. - A vaporiser turns the whole draw to gas at once. - A bypass line flushes the vaporiser quickly. - Cryogenic and oxygen service raise safety requirements. ## Check your understanding Answer in your head or on paper first, then open the answer. Recalling an answer teaches more than rereading. **What is fractionation?** The lighter component of a liquid mixture boiling off first, so the vapour no longer has the liquid's composition. **What is the bypass on a cryogenic vaporiser for?** To flush the vaporiser and line quickly at start-up or when changing sample. **Why must oxygen service use cleaned components?** Oil and grease can ignite in oxygen-enriched conditions. ## Key terms - [Vaporiser](https://www.sarveshanalytics.com/learn/glossary.html#t-vaporiser) - [Cryogenic](https://www.sarveshanalytics.com/learn/glossary.html#t-cryogenic) - [Fractionation](https://www.sarveshanalytics.com/learn/glossary.html#t-fractionation) ## Frequently asked questions **Why can't I analyse liquid oxygen directly?** Analysers measure gas. The liquid must be vaporised, and completely, to keep its composition. **Which vaporiser does Sarvesh offer for cryogenic samples?** SV-200, with a bypass line; SV-400 is also available. **Can I analyse LPG with ACCURRA-S?** Flammable samples are outside the standard ACCURRA-S sample specification; ask an engineer. **How do I flush a vaporiser?** Use the bypass line, per the product instructions. ## Related products [ ### SV-200 / SV-400 vaporisers Sample vaporisers for cryogenic and wet samples with no loss of sample. View SV-200 / SV-400 vaporisers ](https://www.sarveshanalytics.com/sample-vaporisers.html) ## Where this applies - [Oxygen purity in air separation plants](https://www.sarveshanalytics.com/applications/oxygen-purity-air-separation-plant.html) - [Sample vaporisers SV-200 and SV-400](https://www.sarveshanalytics.com/sample-vaporisers.html)