--- title: Gas Analysis Foundations: Units, Behaviour, Accuracy url: https://www.sarveshanalytics.com/learn/track-foundations.html description: Start here: what online gas analysis is, how gases behave, ppm and percent, dry and wet basis, accuracy, drift, T90 and how to read a datasheet. updated: 2026-11-03 source: html --- Learn ยท Track 1 of 8 # Foundations What a number on an analyser really means: units, gas behaviour, accuracy and speed. 9 lessons About 29 min ## What this track covers Everything else in gas analysis rests on a small set of ideas: how a concentration is stated, how pressure and temperature change what a sensor sees, and what words like accuracy, drift and T90 promise. This track builds those ideas in nine short lessons. If you already work with analysers you can skim it. If you are new, read it in order; later tracks assume it. ## Lessons, in order 1. [ **What is online gas analysis? A plain introduction** Online gas analysis measures the composition of a process gas continuously, as it flows, and sends the result to a display, an alarm or a control system. Beginner*3 min*](https://www.sarveshanalytics.com/learn/what-is-online-gas-analysis.html) 2. [ **Online vs laboratory vs portable gas analysis** Use an online analyser when the answer is needed continuously to control or protect a process. Beginner*3 min*](https://www.sarveshanalytics.com/learn/online-vs-laboratory-vs-portable-gas-analysis.html) 3. [ **How gases behave: pressure, temperature and partial pressure** In a gas mixture each component contributes a partial pressure equal to its fraction times the total pressure. Beginner*3 min*](https://www.sarveshanalytics.com/learn/how-gases-behave-pressure-temperature-and-partial-pressure.html) 4. [ **ppm, percent and ppb explained for gas analysis** Percent, ppm and ppb are volume fractions: 1 % equals 10,000 ppm, and 1 ppm equals 1000 ppb. Beginner*3 min*](https://www.sarveshanalytics.com/learn/ppm-percent-and-ppb-units-explained.html) 5. [ **ppm to mg/Nm3: volume vs mass concentration** Mass concentration in mg per cubic metre equals ppm times molar mass divided by molar volume, and the molar volume depends on temperature and pressure. Intermediate*3 min*](https://www.sarveshanalytics.com/learn/volume-mass-and-mg-per-nm3-concentration-units.html) 6. [ **Dry basis vs wet basis in gas measurement** A wet-basis reading counts water vapour as part of the gas; a dry-basis reading removes it. Intermediate*3 min*](https://www.sarveshanalytics.com/learn/dry-basis-vs-wet-basis-gas-measurement.html) 7. [ **Accuracy, precision, repeatability and drift in gas analysers** Accuracy is how close a reading is to the true value; precision or repeatability is how closely repeated readings agree; drift is how the reading moves over time. Intermediate*3 min*](https://www.sarveshanalytics.com/learn/accuracy-precision-repeatability-and-drift.html) 8. [ **Response time, T90 and sample lag in gas analysis** T90 is the time an analyser takes to reach 90 % of a step change in concentration. Intermediate*3 min*](https://www.sarveshanalytics.com/learn/response-time-t90-and-sample-lag.html) 9. [ **How to read a gas analyser datasheet** Read a datasheet in four groups: what it measures (gas, range, minimum range), how well (accuracy, repeatability, drift, response), what it needs (sample conditions, power, utilities, ambient limits) and what it gives (outputs, approvals, options). Intermediate*5 min*](https://www.sarveshanalytics.com/learn/reading-a-gas-analyser-datasheet.html) Next track: [Measurement principles](https://www.sarveshanalytics.com/learn/track-measurement-principles.html)