--- title: Online vs Laboratory vs Portable Gas Analysis url: https://www.sarveshanalytics.com/learn/online-vs-laboratory-vs-portable-gas-analysis.html description: When to use an online process gas analyser, a laboratory method or a portable instrument: speed, cost, accuracy, what each can and cannot do. updated: 2026-11-02 source: html --- Learn · Foundations # Online, laboratory or portable gas analysis: which one when? They answer different questions. An online analyser tells you now and always; a laboratory tells you precisely and later; a portable instrument tells you here and today. Beginner 3 min read Lesson 2 of 9[Foundations](https://www.sarveshanalytics.com/learn/track-foundations.html) Helpful first: [What is online gas analysis? A plain introduction](https://www.sarveshanalytics.com/learn/what-is-online-gas-analysis.html) Quick answer Use an online analyser when the answer is needed continuously to control or protect a process. Use laboratory analysis when you need the highest accuracy or many components occasionally. Use portable instruments for spot checks, audits and commissioning. Most plants use all three, with the lab and portable tools checking the online analyser. Updated 2 November 2026 · Figures from Sarvesh datasheets **After this lesson you can** - Compare online, laboratory and portable analysis on speed, accuracy and cost - Choose the method that fits a given question - Explain how laboratory and portable results are used to check an online analyser ## Online: continuous and in the loop An online analyser is installed permanently and reports every second or so. Its strength is time: it sees the upset at once, can feed a controller and can raise an alarm. Its limits are that it covers a fixed set of gases and a fixed range, needs a sample system, and must be calibrated and maintained. ## Laboratory: precise and flexible A laboratory instrument such as a gas chromatograph can separate and measure many components with high accuracy, but the sample is taken in a cylinder or bag, carried to the lab and measured later. The result may arrive hours or days after the event, and the act of sampling can itself change the gas. Laboratory analysis is how plants verify certificates, investigate problems and measure components that no online analyser covers. ## Portable: a quick, local check Portable analysers are carried to the point of use. They suit commissioning checks, audits, troubleshooting and plants that cannot justify a fixed installation. They are usually less accurate than a laboratory and are used by a person, so they give snapshots rather than a trend. ## Side by side | | Online | Laboratory | Portable | |---|---|---|---| | Result time | Seconds to minutes, continuous | Hours to days | Seconds to minutes, on demand | | Best at | Control, protection, trend | Accuracy, many components | Spot checks, audits | | Needs | Sample system, calibration | Sampling container, skilled analyst | A person on site | | Weakness | Fixed gases and range | Late, sampling can alter the gas | Snapshot only | **Rule of thumb** If a wrong number for an hour would cost money or safety, measure it online. If you only need to know once a month, a portable instrument or a lab sample is cheaper. ## How they work together A well-run plant uses the laboratory and portable instruments to check the online analyser. A periodic comparison against a laboratory result, or against a portable reference, reveals drift and confirms the calibration. When the two disagree, the sample, the calibration gas and the method are examined before anyone blames the analyser. See [How often should you calibrate a gas analyser](https://www.sarveshanalytics.com/learn/how-often-to-calibrate-a-gas-analyser.html). **In short** - Online analysis is continuous and suits control and protection. - Laboratory analysis is precise and broad but late. - Portable analysis gives quick spot checks. - Use lab and portable results to verify the online analyser. ## Check your understanding Answer in your head or on paper first, then open the answer. Recalling an answer teaches more than rereading. **A tank blanket must be kept below a set oxygen level at all times. Which method?** Online, because the answer is needed continuously and must trip an alarm. **Why can a laboratory result differ from an online reading of the same gas?** The lab sample may have changed in the container or in transit, and the online reading may be affected by drift or by its sample system. **What do you do when online and portable readings disagree?** Check the sample point and line, the calibration gas and the calibration of both instruments before changing anything. ## Key terms - [Analyser (analyzer)](https://www.sarveshanalytics.com/learn/glossary.html#t-analyser-analyzer) - [Calibration](https://www.sarveshanalytics.com/learn/glossary.html#t-calibration) - [Span gas](https://www.sarveshanalytics.com/learn/glossary.html#t-span-gas) - [Drift](https://www.sarveshanalytics.com/learn/glossary.html#t-drift) ## Frequently asked questions **Can portable analysers replace online analysers?** Not for control or protection. They give a snapshot, not a continuous reading, and need a person present. **Why use a laboratory if I have an online analyser?** To verify the online instrument, to measure gases it does not cover, and to certify product to a standard. **Is online analysis more accurate than laboratory analysis?** Usually not. The laboratory can be more accurate; online analysis is faster and continuous. **Which is cheapest?** A portable instrument has the lowest outlay; online has the lowest cost per reading when you need continuous data. ## Related products [ ### ACCURRA-S Online analyser for trace, percent and purity O2, CO, CO2, CH4 and dew point; can measure two gases from one sample. View ACCURRA-S ](https://www.sarveshanalytics.com/safe-gas-analyser.html) ## Where this applies - [Gas purity certification and cylinder filling](https://www.sarveshanalytics.com/applications/gas-purity-certification-and-cylinder-filling.html)