--- title: Gas Analysis Glossary | Sarvesh Analytics url: https://www.sarveshanalytics.com/learn/glossary.html description: 170 plain definitions of gas analysis terms: dew point, NDIR, TCD, T90, span gas, extractive, flameproof, LOC, SIL, PSA, CBG and more. updated: 2026-10-21 source: html --- Learn ยท Glossary # Gas analysis glossary 170 plain definitions of the terms used on analyser data sheets, in sample handling and in hazardous-area work, with a link to the lesson that explains each one. ## 4 **4-20 mA** An analogue signal standard in which the reading is carried as a current, 4 mA at the bottom of the range and 20 mA at the top. See [Analyser outputs: 4-20 mA, relays and Modbus explained](https://www.sarveshanalytics.com/learn/analyser-outputs-4-20-ma-relays-and-modbus.html). Explained in: [Analyser outputs: 4-20 mA, relays and Modbus explained](https://www.sarveshanalytics.com/learn/analyser-outputs-4-20-ma-relays-and-modbus.html), [Wiring, grounding and electrical noise for gas analysers](https://www.sarveshanalytics.com/learn/wiring-grounding-and-electrical-noise.html) ## A **Absolute pressure** Pressure measured from a perfect vacuum. Gauge pressure plus atmospheric pressure. Gas laws and partial pressure use absolute pressure. Explained in: [How gases behave: pressure, temperature and partial pressure](https://www.sarveshanalytics.com/learn/how-gases-behave-pressure-temperature-and-partial-pressure.html) **Accuracy** How close a reading is to the true value. See [Accuracy, precision, repeatability and drift in gas analysers](https://www.sarveshanalytics.com/learn/accuracy-precision-repeatability-and-drift.html). Explained in: [Accuracy, precision, repeatability and drift in gas analysers](https://www.sarveshanalytics.com/learn/accuracy-precision-repeatability-and-drift.html), [How to read a gas analyser datasheet](https://www.sarveshanalytics.com/learn/reading-a-gas-analyser-datasheet.html), [What accuracy do you really need from a gas analyser?](https://www.sarveshanalytics.com/learn/what-accuracy-do-you-really-need.html) **Adsorption** The sticking of gas molecules to a solid surface. Used in dryers and PSA beds, and a cause of slow moisture readings as water adsorbs on tube walls. **Air ingress** Air leaking into a process or a sample line. At ppm oxygen levels even a very small leak is significant: [Trace oxygen measurement and air ingress: ppm done right](https://www.sarveshanalytics.com/learn/trace-oxygen-measurement-and-air-ingress.html). **Alarm set point** The value at which an alarm activates. Normally paired with a deadband to avoid chatter: [Connecting a gas analyser to PLC, DCS and SCADA](https://www.sarveshanalytics.com/learn/connecting-an-analyser-to-plc-dcs-and-scada.html). Explained in: [Connecting a gas analyser to PLC, DCS and SCADA](https://www.sarveshanalytics.com/learn/connecting-an-analyser-to-plc-dcs-and-scada.html) **Ambient temperature** The temperature of the air around an instrument, which has limits in the datasheet. Explained in: [Where to mount a gas analyser: panel, shelter or field](https://www.sarveshanalytics.com/learn/where-to-mount-a-gas-analyser.html) **Analyser (analyzer)** An instrument that measures the concentration of a gas or other property of a process sample and outputs a signal such as 4-20 mA. Explained in: [What is online gas analysis? A plain introduction](https://www.sarveshanalytics.com/learn/what-is-online-gas-analysis.html), [Online vs laboratory vs portable gas analysis](https://www.sarveshanalytics.com/learn/online-vs-laboratory-vs-portable-gas-analysis.html) **Analyser house (shelter)** A small building or cabin that holds analysers close to the process, protected from the weather and often heated and ventilated: [Where to mount a gas analyser: panel, shelter or field](https://www.sarveshanalytics.com/learn/where-to-mount-a-gas-analyser.html). Explained in: [Where to mount a gas analyser: panel, shelter or field](https://www.sarveshanalytics.com/learn/where-to-mount-a-gas-analyser.html) **Asphyxiant** A gas, such as nitrogen or argon, that harms by displacing oxygen rather than by being toxic: [Oxygen enrichment and oxygen deficiency hazards](https://www.sarveshanalytics.com/learn/oxygen-enrichment-and-oxygen-deficiency-hazards.html). **ASU (air separation unit)** A plant that separates air into oxygen, nitrogen and argon, usually by cryogenic distillation. Explained in: [How an air separation plant works and what is measured](https://www.sarveshanalytics.com/learn/how-an-air-separation-plant-works.html) **ATEX** European directives covering equipment for explosive atmospheres. Marking states group, category and temperature class. Other schemes, such as IECEx, are similar in intent. Explained in: [Hazardous area classification: zones, gas groups, T-Classes](https://www.sarveshanalytics.com/learn/hazardous-area-classification-zones-gas-groups-temperature-classes.html) **Audit trail** A record of actions, such as calibrations and setting changes, with date and who did them, used by quality systems: [Records, audit trails and qualification for regulated plants](https://www.sarveshanalytics.com/learn/records-audit-trails-and-qualification-for-regulated-plants.html). Explained in: [Records, audit trails and qualification for regulated plants](https://www.sarveshanalytics.com/learn/records-audit-trails-and-qualification-for-regulated-plants.html) **Auto-calibration** A function that switches zero and span gas to the analyser and adjusts it automatically, started by a timer, keypad or Modbus command. Optional on most ACCURRA-S models. Explained in: [Auto-Calibration and validation checks for gas analysers](https://www.sarveshanalytics.com/learn/auto-calibration-and-validation-checks.html), [Total cost of ownership of a gas analyser](https://www.sarveshanalytics.com/learn/total-cost-of-ownership-of-a-gas-analyser.html) **Autoignition temperature** The temperature at which a flammable mixture ignites without a spark. Equipment surface temperature class must stay below it. ## B **Back pressure** Pressure at the vent or outlet of a sample line, which changes the sample pressure at the sensor. **Balance gas** The main gas in a calibration mixture, such as nitrogen, that carries the small amounts of target gas: [Calibration gases: choosing, handling and certificates](https://www.sarveshanalytics.com/learn/calibration-gases-choosing-and-handling.html). Explained in: [Calibration gases: choosing, handling and certificates](https://www.sarveshanalytics.com/learn/calibration-gases-choosing-and-handling.html) **Barometric pressure** The pressure of the atmosphere, which varies with weather and altitude; about 1013.25 hPa at sea level in a standard atmosphere. Explained in: [How gases behave: pressure, temperature and partial pressure](https://www.sarveshanalytics.com/learn/how-gases-behave-pressure-temperature-and-partial-pressure.html) **Binary mixture** A gas mixture of two components. Thermal conductivity analysers measure a binary mixture directly. Explained in: [How thermal conductivity gas analysers (TCD) work](https://www.sarveshanalytics.com/learn/how-thermal-conductivity-gas-analysers-work.html), [Measuring hydrogen: purity, mixtures and safety limits](https://www.sarveshanalytics.com/learn/measuring-hydrogen-purity-and-hydrogen-in-gas-mixtures.html), [Steel plant gases: coke oven, blast furnace, converter](https://www.sarveshanalytics.com/learn/steel-plant-gases-explained.html) **Biomethane** Biogas upgraded to a high methane content, usable like natural gas. **Blanketing** Maintaining a slight positive pressure of inert gas above a liquid or powder so air cannot enter: [Inert gas blanketing in pharma and chemical plants](https://www.sarveshanalytics.com/learn/inert-gas-blanketing-in-pharma-and-chemical-plants.html). Explained in: [Inert gas blanketing in pharma and chemical plants](https://www.sarveshanalytics.com/learn/inert-gas-blanketing-in-pharma-and-chemical-plants.html) **Blast furnace gas** A lean by-product fuel gas of iron making, mostly nitrogen, CO and CO2: [Steel plant gases: coke oven, blast furnace, converter](https://www.sarveshanalytics.com/learn/steel-plant-gases-explained.html). **Blowback** A burst of clean air or gas sent back through a probe filter to clear dust, triggered on a timer or when sample flow fails. Explained in: [How a zirconia oxygen analyser works](https://www.sarveshanalytics.com/learn/how-a-zirconia-oxygen-analyser-works.html), [Filters for gas analysers: dust, mist, membrane](https://www.sarveshanalytics.com/learn/filters-and-particulate-protection.html), [Probes and blowback for hot, dusty and tarry gas](https://www.sarveshanalytics.com/learn/hot-dusty-gas-probes-and-blowback.html) **Bypass loop (fast loop)** A line that circulates a large flow past the tapping, from which the analyser draws a small branch, to reduce lag: [Sample pressure, flow and fast bypass loops](https://www.sarveshanalytics.com/learn/sample-pressure-flow-and-bypass-loops.html). Explained in: [Sample pressure, flow and fast bypass loops](https://www.sarveshanalytics.com/learn/sample-pressure-flow-and-bypass-loops.html) ## C **Calibration** Adjusting an analyser against gas of known concentration so its reading is correct. Explained in: [Why ambient pressure affects oxygen purity readings](https://www.sarveshanalytics.com/learn/why-ambient-pressure-affects-oxygen-purity-readings.html), [What is online gas analysis? A plain introduction](https://www.sarveshanalytics.com/learn/what-is-online-gas-analysis.html), [Online vs laboratory vs portable gas analysis](https://www.sarveshanalytics.com/learn/online-vs-laboratory-vs-portable-gas-analysis.html) **Calibration check** Applying a known gas and recording the deviation without adjusting the analyser. Compare calibration, which adjusts: [How gas analyser calibration works: zero and span](https://www.sarveshanalytics.com/learn/how-gas-analyser-calibration-works.html). Explained in: [How gas analyser calibration works: zero and span](https://www.sarveshanalytics.com/learn/how-gas-analyser-calibration-works.html), [How often should you calibrate a gas analyser?](https://www.sarveshanalytics.com/learn/how-often-to-calibrate-a-gas-analyser.html), [Auto-Calibration and validation checks for gas analysers](https://www.sarveshanalytics.com/learn/auto-calibration-and-validation-checks.html) **Calibration gas** A certified gas mixture of known concentration used to calibrate or check an analyser: [Calibration gases: choosing, handling and certificates](https://www.sarveshanalytics.com/learn/calibration-gases-choosing-and-handling.html). Explained in: [Site utilities checklist for gas analysers](https://www.sarveshanalytics.com/learn/site-utilities-checklist-for-analysers.html), [Calibration gases: choosing, handling and certificates](https://www.sarveshanalytics.com/learn/calibration-gases-choosing-and-handling.html), [What accuracy do you really need from a gas analyser?](https://www.sarveshanalytics.com/learn/what-accuracy-do-you-really-need.html) **Carbon molecular sieve** An adsorbent used in nitrogen PSA generators that adsorbs oxygen faster than nitrogen. **Carbon potential** The carbon content steel would reach at equilibrium with a furnace atmosphere; the control variable in carburising: [Heat treatment furnace atmospheres explained](https://www.sarveshanalytics.com/learn/heat-treatment-furnace-atmospheres-explained.html). **Catchpot** A vessel that removes liquid condensate from a sample gas before it reaches downstream components. **CBG (compressed biogas)** Biogas upgraded to a high methane content and compressed for use as fuel. Explained in: [Measuring methane and biogas composition online](https://www.sarveshanalytics.com/learn/measuring-methane-and-biogas.html), [Biogas, landfill gas and digesters explained](https://www.sarveshanalytics.com/learn/biogas-landfill-gas-and-digesters-explained.html) **CEMS** Continuous emission monitoring system: equipment that measures stack gases and reports them at regulatory conditions: [Emission monitoring (CEMS) basics: what a system does](https://www.sarveshanalytics.com/learn/emission-monitoring-cems-basics.html). Explained in: [Emission monitoring (CEMS) basics: what a system does](https://www.sarveshanalytics.com/learn/emission-monitoring-cems-basics.html) **Certificate of analysis** The document that comes with a calibration gas cylinder, stating the concentration, its uncertainty, the date and the traceability. **Chilled mirror hygrometer** An instrument that cools a mirror until dew forms and detects it optically. A reference method for dew point: [Moisture measurement methods compared for dry gas](https://www.sarveshanalytics.com/learn/moisture-measurement-methods-compared.html). Explained in: [Moisture measurement methods compared for dry gas](https://www.sarveshanalytics.com/learn/moisture-measurement-methods-compared.html) **Clinker** The hot, partly fused product of a cement kiln that is ground to make cement: [Cement kiln gas analysis explained: O2, CO and safety](https://www.sarveshanalytics.com/learn/cement-kiln-gas-analysis-explained.html). **Coalescing filter** A filter that merges fine liquid droplets into larger ones that drain away. Explained in: [Filters for gas analysers: dust, mist, membrane](https://www.sarveshanalytics.com/learn/filters-and-particulate-protection.html) **Coke oven gas** A by-product gas of coke making, rich in hydrogen and methane: [Steel plant gases: coke oven, blast furnace, converter](https://www.sarveshanalytics.com/learn/steel-plant-gases-explained.html). **Combustibles** Unburnt gases such as CO and hydrogen in flue gas. They can burn at a hot zirconia cell and make the oxygen reading too low. **Condensate** Liquid that forms when a gas is cooled below its dew point. Explained in: [How to design a gas sample handling system](https://www.sarveshanalytics.com/learn/how-to-design-a-gas-sample-handling-system.html), [Measuring carbon monoxide and carbon dioxide online](https://www.sarveshanalytics.com/learn/measuring-carbon-monoxide-and-carbon-dioxide.html), [Measuring methane and biogas composition online](https://www.sarveshanalytics.com/learn/measuring-methane-and-biogas.html) **Controlled atmosphere (CA)** Storage with oxygen and CO2 held at set levels to slow ripening: [Controlled-Atmosphere fruit storage explained](https://www.sarveshanalytics.com/learn/controlled-atmosphere-fruit-storage-explained.html). Explained in: [Controlled-Atmosphere fruit storage explained](https://www.sarveshanalytics.com/learn/controlled-atmosphere-fruit-storage-explained.html) **Converter gas** A CO-rich by-product gas of oxygen steelmaking, recovered when its composition allows: [Steel plant gases: coke oven, blast furnace, converter](https://www.sarveshanalytics.com/learn/steel-plant-gases-explained.html). **Cross-sensitivity** A sensor's response to gases other than its target: [Cross-Sensitivity and interference in gas analysers](https://www.sarveshanalytics.com/learn/cross-sensitivity-and-interference-in-gas-analysers.html). Explained in: [What is an NDIR gas analyser?](https://www.sarveshanalytics.com/learn/what-is-an-ndir-gas-analyser.html), [How thermal conductivity gas analysers (TCD) work](https://www.sarveshanalytics.com/learn/how-thermal-conductivity-gas-analysers-work.html), [Cross-Sensitivity and interference in gas analysers](https://www.sarveshanalytics.com/learn/cross-sensitivity-and-interference-in-gas-analysers.html) **Cryogenic** Relating to very low temperatures, such as liquid oxygen, nitrogen and argon, which boil at about minus 183, minus 196 and minus 186 C. Explained in: [Sampling cryogenic and liquefied gases: vaporisers](https://www.sarveshanalytics.com/learn/sampling-cryogenic-and-liquefied-gases.html), [How an air separation plant works and what is measured](https://www.sarveshanalytics.com/learn/how-an-air-separation-plant-works.html) ## D **DCS** Distributed control system: a plant control system made of networked controllers and operator stations. Explained in: [Connecting a gas analyser to PLC, DCS and SCADA](https://www.sarveshanalytics.com/learn/connecting-an-analyser-to-plc-dcs-and-scada.html) **Dead leg** A branch of tubing with no flow. It holds old gas and adds lag and contamination. Explained in: [Response time, T90 and sample lag in gas analysis](https://www.sarveshanalytics.com/learn/response-time-t90-and-sample-lag.html), [Trace oxygen measurement and air ingress: ppm done right](https://www.sarveshanalytics.com/learn/trace-oxygen-measurement-and-air-ingress.html), [Choosing the sample point for a gas analyser](https://www.sarveshanalytics.com/learn/choosing-the-sample-point.html) **Dead volume** Volume in a sample system that is not swept by the flow, such as dead legs and oversized fittings. **Detection limit** The smallest concentration an analyser can distinguish from zero. Explained in: [Accuracy, precision, repeatability and drift in gas analysers](https://www.sarveshanalytics.com/learn/accuracy-precision-repeatability-and-drift.html) **Dew point** The temperature at which water vapour in a gas begins to condense. Explained in: [What is dew point and how is it measured?](https://www.sarveshanalytics.com/learn/what-is-dew-point-and-how-is-it-measured.html), [ppm, percent and ppb explained for gas analysis](https://www.sarveshanalytics.com/learn/ppm-percent-and-ppb-units-explained.html), [Dry basis vs wet basis in gas measurement](https://www.sarveshanalytics.com/learn/dry-basis-vs-wet-basis-gas-measurement.html) **Digester** A sealed vessel in which organic matter is broken down without air to produce biogas: [Biogas, landfill gas and digesters explained](https://www.sarveshanalytics.com/learn/biogas-landfill-gas-and-digesters-explained.html). **Drain** A route for condensate to leave a sample system, which must be continuous and safe: [Condensate, moisture and sample cooling for gas analysis](https://www.sarveshanalytics.com/learn/condensate-moisture-and-sample-cooling.html). Explained in: [Site utilities checklist for gas analysers](https://www.sarveshanalytics.com/learn/site-utilities-checklist-for-analysers.html) **Drift** A slow change in an analyser's reading over time with the same input gas. Explained in: [Online vs laboratory vs portable gas analysis](https://www.sarveshanalytics.com/learn/online-vs-laboratory-vs-portable-gas-analysis.html), [Accuracy, precision, repeatability and drift in gas analysers](https://www.sarveshanalytics.com/learn/accuracy-precision-repeatability-and-drift.html), [How often should you calibrate a gas analyser?](https://www.sarveshanalytics.com/learn/how-often-to-calibrate-a-gas-analyser.html) **Dry basis** A gas value stated as if all water vapour were removed. Contrast wet basis: [Dry basis vs wet basis in gas measurement](https://www.sarveshanalytics.com/learn/dry-basis-vs-wet-basis-gas-measurement.html). Explained in: [Dry basis vs wet basis in gas measurement](https://www.sarveshanalytics.com/learn/dry-basis-vs-wet-basis-gas-measurement.html), [Combustion, excess air and flue gas oxygen explained](https://www.sarveshanalytics.com/learn/combustion-excess-air-and-flue-gas-oxygen.html), [Emission monitoring (CEMS) basics: what a system does](https://www.sarveshanalytics.com/learn/emission-monitoring-cems-basics.html) **Dual-channel analyser** An analyser that measures two gases from one sample in one enclosure, such as ACCURRA-S oxygen with CO2 or oxygen with dew point. Explained in: [Measuring methane and biogas composition online](https://www.sarveshanalytics.com/learn/measuring-methane-and-biogas.html), [Glove boxes and Controlled-Atmosphere chambers explained](https://www.sarveshanalytics.com/learn/glove-boxes-and-controlled-atmosphere-chambers.html) ## E **Electrochemical cell** A sensor that generates a current from a chemical reaction with the target gas, used for trace oxygen and CO. The cell is consumed over time. Explained in: [Electrochemical vs paramagnetic oxygen sensors](https://www.sarveshanalytics.com/learn/electrochemical-vs-paramagnetic-oxygen-sensors.html), [Gas analysis technologies compared: which sensor for which gas](https://www.sarveshanalytics.com/learn/gas-analysis-technologies-compared.html), [Cross-Sensitivity and interference in gas analysers](https://www.sarveshanalytics.com/learn/cross-sensitivity-and-interference-in-gas-analysers.html) **Electrolyser** A device that splits water into hydrogen and oxygen using electricity: [Hydrogen electrolysers and gas analysis: what to measure](https://www.sarveshanalytics.com/learn/hydrogen-electrolysers-and-gas-analysis.html). **Enclosure rating (IP)** The ingress protection rating that states how well an enclosure keeps out dust and water, for example IP65. Explained in: [Where to mount a gas analyser: panel, shelter or field](https://www.sarveshanalytics.com/learn/where-to-mount-a-gas-analyser.html) **Endothermic gas** A furnace atmosphere made by partly burning natural gas over a catalyst, mostly CO, hydrogen and nitrogen: [Heat treatment furnace atmospheres explained](https://www.sarveshanalytics.com/learn/heat-treatment-furnace-atmospheres-explained.html). Explained in: [Heat treatment furnace atmospheres explained](https://www.sarveshanalytics.com/learn/heat-treatment-furnace-atmospheres-explained.html) **ESP (electrostatic precipitator)** A dust collector that charges particles with high voltage. It can ignite gas if CO and unburnt fuel are present: [Cement kiln gas analysis explained: O2, CO and safety](https://www.sarveshanalytics.com/learn/cement-kiln-gas-analysis-explained.html). **Excess air** Air supplied to a burner beyond the amount needed for exact combustion, estimated from flue gas oxygen: [Excess air from flue gas oxygen calculator](https://www.sarveshanalytics.com/calculators/excess-air-calculator.html). Explained in: [Combustion, excess air and flue gas oxygen explained](https://www.sarveshanalytics.com/learn/combustion-excess-air-and-flue-gas-oxygen.html) **Explosive atmosphere** A mixture of air with flammable gas, vapour or dust, within its explosive limits, that can burn or explode if ignited. **Extractive analysis** Measuring a gas by drawing a sample from the process to an analyser, usually through a conditioning system. Explained in: [Extractive vs In-Situ gas analysis](https://www.sarveshanalytics.com/learn/extractive-vs-in-situ-gas-analysis.html), [What is online gas analysis? A plain introduction](https://www.sarveshanalytics.com/learn/what-is-online-gas-analysis.html) ## F **Failsafe relay** A relay that moves to its alarm state if power or the instrument fails. Explained in: [What is limiting oxygen concentration (LOC)?](https://www.sarveshanalytics.com/learn/what-is-limiting-oxygen-concentration.html), [How to read a gas analyser datasheet](https://www.sarveshanalytics.com/learn/reading-a-gas-analyser-datasheet.html), [Analyser outputs: 4-20 mA, relays and Modbus explained](https://www.sarveshanalytics.com/learn/analyser-outputs-4-20-ma-relays-and-modbus.html) **Fine filter** A final-stage filter before the analyser; in ACCURRA-S a glass micro fibre filter is built in. Explained in: [How to design a gas sample handling system](https://www.sarveshanalytics.com/learn/how-to-design-a-gas-sample-handling-system.html), [Filters for gas analysers: dust, mist, membrane](https://www.sarveshanalytics.com/learn/filters-and-particulate-protection.html), [Maintenance schedule and spares for gas analysers](https://www.sarveshanalytics.com/learn/maintenance-schedule-and-spares-for-gas-analysers.html) **Flame arrestor** A device that stops a flame travelling along a pipe by absorbing its heat in narrow passages. **Flameproof enclosure (Ex d)** An enclosure that withstands an internal explosion and prevents it igniting the surrounding atmosphere. Explained in: [Hazardous area gas analysers explained](https://www.sarveshanalytics.com/learn/hazardous-area-gas-analysers-explained.html), [Protection concepts: flameproof, intrinsic safety, purging](https://www.sarveshanalytics.com/learn/protection-concepts-flameproof-intrinsic-safety-and-purging.html), [Inert gas blanketing in pharma and chemical plants](https://www.sarveshanalytics.com/learn/inert-gas-blanketing-in-pharma-and-chemical-plants.html) **Flue gas** The gas leaving a furnace, boiler or kiln after combustion. **Fractionation** The separation of a liquid mixture by boiling, the lighter component leaving first. It is why liquid samples must be vaporised completely: [Sampling cryogenic and liquefied gases: vaporisers](https://www.sarveshanalytics.com/learn/sampling-cryogenic-and-liquefied-gases.html). Explained in: [Sampling cryogenic and liquefied gases: vaporisers](https://www.sarveshanalytics.com/learn/sampling-cryogenic-and-liquefied-gases.html) **Frost point** The temperature at which moisture deposits as ice. Dew points below 0 C in dry gas are usually frost points. Explained in: [Moisture measurement methods compared for dry gas](https://www.sarveshanalytics.com/learn/moisture-measurement-methods-compared.html) ## G **Galvanic isolation** Electrical separation between circuits so no direct current path exists, used to break ground loops and protect electronics. Explained in: [Wiring, grounding and electrical noise for gas analysers](https://www.sarveshanalytics.com/learn/wiring-grounding-and-electrical-noise.html) **Gas chromatograph** An instrument that separates the components of a mixture in a column and measures each. Used for multi-component analysis; not in the Sarvesh range. Explained in: [Gas analysis technologies compared: which sensor for which gas](https://www.sarveshanalytics.com/learn/gas-analysis-technologies-compared.html) **Gas group IIC** The most demanding hazardous-area gas group, covering hydrogen and acetylene. Explained in: [Hazardous area gas analysers explained](https://www.sarveshanalytics.com/learn/hazardous-area-gas-analysers-explained.html), [Measuring hydrogen: purity, mixtures and safety limits](https://www.sarveshanalytics.com/learn/measuring-hydrogen-purity-and-hydrogen-in-gas-mixtures.html), [Flammability, explosive limits and gas safety basics](https://www.sarveshanalytics.com/learn/flammability-explosive-limits-and-gas-safety-basics.html) **Gauge pressure** Pressure measured relative to the atmosphere. Add atmospheric pressure to get absolute pressure. **Ground loop** A circuit formed by two ground connections at different potentials, which drives current through a signal path and adds noise: [Wiring, grounding and electrical noise for gas analysers](https://www.sarveshanalytics.com/learn/wiring-grounding-and-electrical-noise.html). Explained in: [Wiring, grounding and electrical noise for gas analysers](https://www.sarveshanalytics.com/learn/wiring-grounding-and-electrical-noise.html) ## H **H2S** Hydrogen sulphide: a toxic, corrosive, flammable gas found in biogas and some process streams. It is outside the standard ACCURRA-S sample limits. **Hazardous area** A place where an explosive atmosphere may occur. Classified into zones: [Hazardous area classification: zones, gas groups, T-Classes](https://www.sarveshanalytics.com/learn/hazardous-area-classification-zones-gas-groups-temperature-classes.html). **Heat tracing** Electrical or steam heating of a line to keep it above the dew point of the gas inside. Explained in: [Sample lines: materials, length, diameter and lag](https://www.sarveshanalytics.com/learn/sample-lines-materials-length-and-lag.html), [Condensate, moisture and sample cooling for gas analysis](https://www.sarveshanalytics.com/learn/condensate-moisture-and-sample-cooling.html), [Probes and blowback for hot, dusty and tarry gas](https://www.sarveshanalytics.com/learn/hot-dusty-gas-probes-and-blowback.html) **Hot-wet extraction** Keeping the sample hot and wet from the probe to a heated analyser so that soluble gases are not lost: [Emission monitoring (CEMS) basics: what a system does](https://www.sarveshanalytics.com/learn/emission-monitoring-cems-basics.html). **HRSG** Heat recovery steam generator, which makes steam from the hot exhaust of a gas turbine. **Hygroscopic** Readily absorbing moisture from the surroundings. Explained in: [Moisture measurement methods compared for dry gas](https://www.sarveshanalytics.com/learn/moisture-measurement-methods-compared.html) **Hysteresis** A deadband that makes an alarm switch on at one value and off at a different value, to prevent chatter. Explained in: [Connecting a gas analyser to PLC, DCS and SCADA](https://www.sarveshanalytics.com/learn/connecting-an-analyser-to-plc-dcs-and-scada.html) ## I **IECEx** An international scheme for certifying equipment for explosive atmospheres. Explained in: [Hazardous area classification: zones, gas groups, T-Classes](https://www.sarveshanalytics.com/learn/hazardous-area-classification-zones-gas-groups-temperature-classes.html) **In-situ analysis** Measuring the gas where it flows, with the sensor in the duct or pipe and no sample line. Explained in: [How a zirconia oxygen analyser works](https://www.sarveshanalytics.com/learn/how-a-zirconia-oxygen-analyser-works.html), [Extractive vs In-Situ gas analysis](https://www.sarveshanalytics.com/learn/extractive-vs-in-situ-gas-analysis.html), [What is online gas analysis? A plain introduction](https://www.sarveshanalytics.com/learn/what-is-online-gas-analysis.html) **Inerting** Replacing air in a vessel with an inert gas such as nitrogen to keep oxygen below the level that supports combustion. Explained in: [What is limiting oxygen concentration (LOC)?](https://www.sarveshanalytics.com/learn/what-is-limiting-oxygen-concentration.html), [Flammability, explosive limits and gas safety basics](https://www.sarveshanalytics.com/learn/flammability-explosive-limits-and-gas-safety-basics.html), [Inerting and purging: how they work and how to prove them](https://www.sarveshanalytics.com/learn/inerting-and-purging-how-they-work.html) **Interference** An error in a reading caused by another gas or condition in the sample. Explained in: [Cross-Sensitivity and interference in gas analysers](https://www.sarveshanalytics.com/learn/cross-sensitivity-and-interference-in-gas-analysers.html) **Interlock** A control or safety logic that stops or prevents an action when a condition, such as high oxygen, is met. **Intrinsic safety (Ex i)** A protection concept that limits electrical energy so that no spark or heat can ignite a flammable atmosphere: [Protection concepts: flameproof, intrinsic safety, purging](https://www.sarveshanalytics.com/learn/protection-concepts-flameproof-intrinsic-safety-and-purging.html). Explained in: [Protection concepts: flameproof, intrinsic safety, purging](https://www.sarveshanalytics.com/learn/protection-concepts-flameproof-intrinsic-safety-and-purging.html) ## L **Lambda** The ratio of actual air to the air needed for exact combustion. Lambda 1 is stoichiometric: [Excess air from flue gas oxygen calculator](https://www.sarveshanalytics.com/calculators/excess-air-calculator.html). Explained in: [Combustion, excess air and flue gas oxygen explained](https://www.sarveshanalytics.com/learn/combustion-excess-air-and-flue-gas-oxygen.html) **Landfill gas** A methane-rich gas produced by waste in landfills, drawn from wells and used or flared. **Leak test** A test, with pressure and a detection method, that shows whether a sample system or enclosure leaks. **LEL** Lower explosive limit: the lowest concentration of a flammable gas in air that can burn or explode. About 4 % for hydrogen and 5 % for methane: [Flammability, explosive limits and gas safety basics](https://www.sarveshanalytics.com/learn/flammability-explosive-limits-and-gas-safety-basics.html). Explained in: [Measuring hydrogen: purity, mixtures and safety limits](https://www.sarveshanalytics.com/learn/measuring-hydrogen-purity-and-hydrogen-in-gas-mixtures.html), [Flammability, explosive limits and gas safety basics](https://www.sarveshanalytics.com/learn/flammability-explosive-limits-and-gas-safety-basics.html), [Hydrogen electrolysers and gas analysis: what to measure](https://www.sarveshanalytics.com/learn/hydrogen-electrolysers-and-gas-analysis.html) **Limiting oxygen concentration (LOC)** The oxygen level below which a fuel and inert gas mixture cannot sustain a flame. Explained in: [What is limiting oxygen concentration (LOC)?](https://www.sarveshanalytics.com/learn/what-is-limiting-oxygen-concentration.html), [Inerting and purging: how they work and how to prove them](https://www.sarveshanalytics.com/learn/inerting-and-purging-how-they-work.html), [Inert gas blanketing in pharma and chemical plants](https://www.sarveshanalytics.com/learn/inert-gas-blanketing-in-pharma-and-chemical-plants.html) **Linearity** How closely the response of an analyser follows a straight line over its range. Explained in: [Accuracy, precision, repeatability and drift in gas analysers](https://www.sarveshanalytics.com/learn/accuracy-precision-repeatability-and-drift.html) **Live zero** The use of 4 mA, not 0 mA, for the bottom of a 4-20 mA range, so a broken wire can be recognised. Explained in: [Analyser outputs: 4-20 mA, relays and Modbus explained](https://www.sarveshanalytics.com/learn/analyser-outputs-4-20-ma-relays-and-modbus.html) ## M **Membrane filter** A thin porous membrane, often PTFE, that removes fine particles and aerosols and repels water: [Filters for gas analysers: dust, mist, membrane](https://www.sarveshanalytics.com/learn/filters-and-particulate-protection.html). Explained in: [Filters for gas analysers: dust, mist, membrane](https://www.sarveshanalytics.com/learn/filters-and-particulate-protection.html) **Modbus RTU** A serial communication protocol, here over RS 485, used to read measurements, status and history from an analyser. Explained in: [How to read a gas analyser datasheet](https://www.sarveshanalytics.com/learn/reading-a-gas-analyser-datasheet.html), [Analyser outputs: 4-20 mA, relays and Modbus explained](https://www.sarveshanalytics.com/learn/analyser-outputs-4-20-ma-relays-and-modbus.html), [Connecting a gas analyser to PLC, DCS and SCADA](https://www.sarveshanalytics.com/learn/connecting-an-analyser-to-plc-dcs-and-scada.html) **Molar volume** The volume of one mole of gas: 22.414 litres at 0 C and 1013.25 hPa for an ideal gas. Explained in: [ppm to mg/Nm3: volume vs mass concentration](https://www.sarveshanalytics.com/learn/volume-mass-and-mg-per-nm3-concentration-units.html) **Mole fraction** The share of molecules in a mixture that are a given gas. Percent and ppm by volume are mole fractions. Explained in: [How gases behave: pressure, temperature and partial pressure](https://www.sarveshanalytics.com/learn/how-gases-behave-pressure-temperature-and-partial-pressure.html) **Molecular sieve** A porous solid that adsorbs molecules by size, used to dry gas and remove CO2 and in PSA beds. ## N **NDIR** Non-dispersive infrared: a method that measures gases such as CO, CO2 and methane by their absorption of infrared light. Explained in: [What is an NDIR gas analyser?](https://www.sarveshanalytics.com/learn/what-is-an-ndir-gas-analyser.html), [Gas analysis technologies compared: which sensor for which gas](https://www.sarveshanalytics.com/learn/gas-analysis-technologies-compared.html), [Cross-Sensitivity and interference in gas analysers](https://www.sarveshanalytics.com/learn/cross-sensitivity-and-interference-in-gas-analysers.html) **Needle valve** A fine adjustment valve used to set sample flow. **Normal cubic metre (Nm3)** A cubic metre of gas at normal conditions, usually 0 C and 1013.25 hPa: [ppm to mg/Nm3: volume vs mass concentration](https://www.sarveshanalytics.com/learn/volume-mass-and-mg-per-nm3-concentration-units.html). Explained in: [ppm to mg/Nm3: volume vs mass concentration](https://www.sarveshanalytics.com/learn/volume-mass-and-mg-per-nm3-concentration-units.html) ## O **Oxygen deficiency** An atmosphere with less oxygen than normal, commonly below about 19.5 %, which endangers life: [Oxygen enrichment and oxygen deficiency hazards](https://www.sarveshanalytics.com/learn/oxygen-enrichment-and-oxygen-deficiency-hazards.html). Explained in: [Oxygen enrichment and oxygen deficiency hazards](https://www.sarveshanalytics.com/learn/oxygen-enrichment-and-oxygen-deficiency-hazards.html), [Controlled-Atmosphere fruit storage explained](https://www.sarveshanalytics.com/learn/controlled-atmosphere-fruit-storage-explained.html) **Oxygen enrichment** An atmosphere with more oxygen than normal, commonly above about 23.5 %, in which materials ignite easily. Explained in: [Oxygen enrichment and oxygen deficiency hazards](https://www.sarveshanalytics.com/learn/oxygen-enrichment-and-oxygen-deficiency-hazards.html) ## P **Paramagnetic sensor** An oxygen sensor that uses oxygen's attraction to a magnetic field. It has no consumed element. Explained in: [Electrochemical vs paramagnetic oxygen sensors](https://www.sarveshanalytics.com/learn/electrochemical-vs-paramagnetic-oxygen-sensors.html), [Gas analysis technologies compared: which sensor for which gas](https://www.sarveshanalytics.com/learn/gas-analysis-technologies-compared.html) **Partial pressure** The pressure a gas in a mixture would exert alone: its mole fraction times the total pressure: [How gases behave: pressure, temperature and partial pressure](https://www.sarveshanalytics.com/learn/how-gases-behave-pressure-temperature-and-partial-pressure.html). Explained in: [How gases behave: pressure, temperature and partial pressure](https://www.sarveshanalytics.com/learn/how-gases-behave-pressure-temperature-and-partial-pressure.html) **Peltier cooler** A solid-state cooler with no refrigerant or compressor, used to cool and dry sample gas. Explained in: [How to design a gas sample handling system](https://www.sarveshanalytics.com/learn/how-to-design-a-gas-sample-handling-system.html), [Dry basis vs wet basis in gas measurement](https://www.sarveshanalytics.com/learn/dry-basis-vs-wet-basis-gas-measurement.html), [Condensate, moisture and sample cooling for gas analysis](https://www.sarveshanalytics.com/learn/condensate-moisture-and-sample-cooling.html) **Permeation** The slow passage of gas through the wall of a material such as plastic tubing, a cause of oxygen and moisture error at trace levels. Explained in: [Sample lines: materials, length, diameter and lag](https://www.sarveshanalytics.com/learn/sample-lines-materials-length-and-lag.html) **PLC** Programmable logic controller: a small industrial computer that runs control logic. Explained in: [Connecting a gas analyser to PLC, DCS and SCADA](https://www.sarveshanalytics.com/learn/connecting-an-analyser-to-plc-dcs-and-scada.html) **Poisoning (sensor)** Loss of sensor performance caused by a contaminant, such as sulphur, chlorine or silicone: [Sensor life, poisoning and replacement in gas analysers](https://www.sarveshanalytics.com/learn/sensor-life-poisoning-and-replacement.html). Explained in: [Sensor life, poisoning and replacement in gas analysers](https://www.sarveshanalytics.com/learn/sensor-life-poisoning-and-replacement.html) **ppb** Parts per billion by volume: one part in a billion. 1000 ppb is 1 ppm: [ppm, percent and ppb explained for gas analysis](https://www.sarveshanalytics.com/learn/ppm-percent-and-ppb-units-explained.html). Explained in: [ppm, percent and ppb explained for gas analysis](https://www.sarveshanalytics.com/learn/ppm-percent-and-ppb-units-explained.html) **ppm** Parts per million by volume. 10,000 ppm is 1 %. Explained in: [What is dew point and how is it measured?](https://www.sarveshanalytics.com/learn/what-is-dew-point-and-how-is-it-measured.html), [How gases behave: pressure, temperature and partial pressure](https://www.sarveshanalytics.com/learn/how-gases-behave-pressure-temperature-and-partial-pressure.html), [ppm, percent and ppb explained for gas analysis](https://www.sarveshanalytics.com/learn/ppm-percent-and-ppb-units-explained.html) **Precision** How closely repeated readings of the same gas agree with each other. Explained in: [Accuracy, precision, repeatability and drift in gas analysers](https://www.sarveshanalytics.com/learn/accuracy-precision-repeatability-and-drift.html) **Preheater** A tower of cyclones that heats raw meal with kiln gas before the kiln in a cement plant. **Pressure dew point** The dew point of a gas at its line pressure, usually much higher than the dew point after expansion to atmosphere: [Compressed air quality and dew point (ISO 8573-1)](https://www.sarveshanalytics.com/learn/compressed-air-quality-and-dew-point.html). Explained in: [Compressed air quality and dew point (ISO 8573-1)](https://www.sarveshanalytics.com/learn/compressed-air-quality-and-dew-point.html) **Pressure regulator** A device that reduces and holds a steady downstream pressure. Explained in: [Sample pressure, flow and fast bypass loops](https://www.sarveshanalytics.com/learn/sample-pressure-flow-and-bypass-loops.html) **Probe** The first part of a sampling system, inserted in the process to draw a sample through a filter. Explained in: [Extractive vs In-Situ gas analysis](https://www.sarveshanalytics.com/learn/extractive-vs-in-situ-gas-analysis.html), [Choosing the sample point for a gas analyser](https://www.sarveshanalytics.com/learn/choosing-the-sample-point.html), [Probes and blowback for hot, dusty and tarry gas](https://www.sarveshanalytics.com/learn/hot-dusty-gas-probes-and-blowback.html) **Proof test** A periodic test that reveals hidden failures in a safety function, such as applying test gas to an analyser. Explained in: [What SIL means for gas analysers, and what it does not](https://www.sarveshanalytics.com/learn/what-sil-means-for-gas-analysers.html) **PSA / VPSA** Pressure swing adsorption and vacuum pressure swing adsorption, processes that make nitrogen or oxygen on site using adsorbent beds. Explained in: [PSA, VPSA and membrane gas generators explained](https://www.sarveshanalytics.com/learn/nitrogen-and-oxygen-generators-psa-vpsa-and-membrane.html) **Purge** To replace the gas in a vessel or line with another gas, usually inert, before use or opening: [Inerting and purging: how they work and how to prove them](https://www.sarveshanalytics.com/learn/inerting-and-purging-how-they-work.html). Explained in: [Trace oxygen measurement and air ingress: ppm done right](https://www.sarveshanalytics.com/learn/trace-oxygen-measurement-and-air-ingress.html), [Commissioning an online gas analyser: A Step-by-Step guide](https://www.sarveshanalytics.com/learn/commissioning-an-online-gas-analyser.html), [Inerting and purging: how they work and how to prove them](https://www.sarveshanalytics.com/learn/inerting-and-purging-how-they-work.html) **Purging (Ex p)** A protection concept in which an enclosure is kept at a slight positive pressure of clean air or inert gas to keep hazardous gas out. Explained in: [Protection concepts: flameproof, intrinsic safety, purging](https://www.sarveshanalytics.com/learn/protection-concepts-flameproof-intrinsic-safety-and-purging.html) **Purity analyser** An analyser that measures a high concentration of a gas, near 100 %, such as oxygen purity from 95 to 100 %. Explained in: [Why ambient pressure affects oxygen purity readings](https://www.sarveshanalytics.com/learn/why-ambient-pressure-affects-oxygen-purity-readings.html), [Choosing an oxygen analyser: trace, percent or purity](https://www.sarveshanalytics.com/learn/choosing-an-oxygen-analyser-trace-percent-or-purity.html), [How an air separation plant works and what is measured](https://www.sarveshanalytics.com/learn/how-an-air-separation-plant-works.html) ## R **Range** The span of concentration an analyser measures, for example 0 to 1000 ppm. Explained in: [Why ambient pressure affects oxygen purity readings](https://www.sarveshanalytics.com/learn/why-ambient-pressure-affects-oxygen-purity-readings.html), [Electrochemical vs paramagnetic oxygen sensors](https://www.sarveshanalytics.com/learn/electrochemical-vs-paramagnetic-oxygen-sensors.html), [Choosing an oxygen analyser: trace, percent or purity](https://www.sarveshanalytics.com/learn/choosing-an-oxygen-analyser-trace-percent-or-purity.html) **Reference conditions** The temperature and pressure, dry or wet basis and reference oxygen to which a concentration is stated. Explained in: [ppm to mg/Nm3: volume vs mass concentration](https://www.sarveshanalytics.com/learn/volume-mass-and-mg-per-nm3-concentration-units.html), [Emission monitoring (CEMS) basics: what a system does](https://www.sarveshanalytics.com/learn/emission-monitoring-cems-basics.html) **Reference gas** A second gas supplied to some analysers for comparison. ACCURRA-TCD needs none. Explained in: [How a zirconia oxygen analyser works](https://www.sarveshanalytics.com/learn/how-a-zirconia-oxygen-analyser-works.html), [How thermal conductivity gas analysers (TCD) work](https://www.sarveshanalytics.com/learn/how-thermal-conductivity-gas-analysers-work.html) **Repeatability** The agreement between repeated readings of the same gas under the same conditions. Explained in: [Accuracy, precision, repeatability and drift in gas analysers](https://www.sarveshanalytics.com/learn/accuracy-precision-repeatability-and-drift.html) **Resolution** The smallest change an instrument can display or output. Explained in: [Accuracy, precision, repeatability and drift in gas analysers](https://www.sarveshanalytics.com/learn/accuracy-precision-repeatability-and-drift.html) **Respiratory quotient** The ratio of CO2 produced to oxygen consumed by respiring fruit or cells. Used in some storage and fermentation controls. Explained in: [Controlled-Atmosphere fruit storage explained](https://www.sarveshanalytics.com/learn/controlled-atmosphere-fruit-storage-explained.html) **Response time (T90)** The time for a reading to reach 90 % of a step change in concentration. Explained in: [Response time, T90 and sample lag in gas analysis](https://www.sarveshanalytics.com/learn/response-time-t90-and-sample-lag.html), [How to read a gas analyser datasheet](https://www.sarveshanalytics.com/learn/reading-a-gas-analyser-datasheet.html) **Rotameter** A flow meter with a float in a tapered tube, used to see and set sample flow. Explained in: [Sample pressure, flow and fast bypass loops](https://www.sarveshanalytics.com/learn/sample-pressure-flow-and-bypass-loops.html), [Controlled-Atmosphere fruit storage explained](https://www.sarveshanalytics.com/learn/controlled-atmosphere-fruit-storage-explained.html) **RS-485** A two-wire serial communication standard used by Modbus RTU, allowing many devices on one cable over long distances. Explained in: [Analyser outputs: 4-20 mA, relays and Modbus explained](https://www.sarveshanalytics.com/learn/analyser-outputs-4-20-ma-relays-and-modbus.html) ## S **Safe area** A place classified as free of explosive atmospheres, where ordinary equipment may be used. **Safety instrumented function** A function that detects a dangerous condition and brings the process to a safe state, with a defined safety integrity level: [What SIL means for gas analysers, and what it does not](https://www.sarveshanalytics.com/learn/what-sil-means-for-gas-analysers.html). Explained in: [What SIL means for gas analysers, and what it does not](https://www.sarveshanalytics.com/learn/what-sil-means-for-gas-analysers.html) **Sample conditioning** Treating a sample, by filtering, cooling, drying and regulating, so that it suits the analyser. **Sample handling system** The probe, lines, cooler, pump and filters that deliver a conditioned sample to an analyser. Explained in: [Extractive vs In-Situ gas analysis](https://www.sarveshanalytics.com/learn/extractive-vs-in-situ-gas-analysis.html), [How to design a gas sample handling system](https://www.sarveshanalytics.com/learn/how-to-design-a-gas-sample-handling-system.html), [What is online gas analysis? A plain introduction](https://www.sarveshanalytics.com/learn/what-is-online-gas-analysis.html) **Sample lag** The time for a sample to travel from the tapping to the analyser, equal to line volume divided by flow: [Gas sample line lag time calculator](https://www.sarveshanalytics.com/calculators/sample-lag-time-calculator.html). Explained in: [Response time, T90 and sample lag in gas analysis](https://www.sarveshanalytics.com/learn/response-time-t90-and-sample-lag.html), [Sample lines: materials, length, diameter and lag](https://www.sarveshanalytics.com/learn/sample-lines-materials-length-and-lag.html) **Sample point (tapping)** The connection on the process from which gas is taken for analysis: [Choosing the sample point for a gas analyser](https://www.sarveshanalytics.com/learn/choosing-the-sample-point.html). Explained in: [Choosing the sample point for a gas analyser](https://www.sarveshanalytics.com/learn/choosing-the-sample-point.html) **SCADA** Supervisory control and data acquisition: software that collects plant data, shows it to operators and stores it. Explained in: [Connecting a gas analyser to PLC, DCS and SCADA](https://www.sarveshanalytics.com/learn/connecting-an-analyser-to-plc-dcs-and-scada.html) **Scrubber** A filter or chemical bed that removes an interfering or harmful gas from a sample before the sensor. Explained in: [Cross-Sensitivity and interference in gas analysers](https://www.sarveshanalytics.com/learn/cross-sensitivity-and-interference-in-gas-analysers.html), [What samples an analyser cannot take: know the limits](https://www.sarveshanalytics.com/learn/what-samples-an-analyser-cannot-take.html) **Selectivity** The ability of a sensor to respond only to its target gas. **Sensor bypass valve** A valve that isolates trace oxygen or dew point sensors during purging or upsets so they recover quickly. Explained in: [What is dew point and how is it measured?](https://www.sarveshanalytics.com/learn/what-is-dew-point-and-how-is-it-measured.html), [Response time, T90 and sample lag in gas analysis](https://www.sarveshanalytics.com/learn/response-time-t90-and-sample-lag.html), [Moisture measurement methods compared for dry gas](https://www.sarveshanalytics.com/learn/moisture-measurement-methods-compared.html) **Sensor life** The time over which a sensor performs within specification, which depends on the type and the sample: [Sensor life, poisoning and replacement in gas analysers](https://www.sarveshanalytics.com/learn/sensor-life-poisoning-and-replacement.html). Explained in: [Maintenance schedule and spares for gas analysers](https://www.sarveshanalytics.com/learn/maintenance-schedule-and-spares-for-gas-analysers.html), [Sensor life, poisoning and replacement in gas analysers](https://www.sarveshanalytics.com/learn/sensor-life-poisoning-and-replacement.html), [Total cost of ownership of a gas analyser](https://www.sarveshanalytics.com/learn/total-cost-of-ownership-of-a-gas-analyser.html) **Shielded cable** A cable with a conductive screen that protects the signal from electrical noise. Explained in: [Wiring, grounding and electrical noise for gas analysers](https://www.sarveshanalytics.com/learn/wiring-grounding-and-electrical-noise.html) **SIL** Safety integrity level, a measure of the reliability of a safety function under functional safety standards. Sarvesh makes no SIL claim for its analysers. Explained in: [Hazardous area gas analysers explained](https://www.sarveshanalytics.com/learn/hazardous-area-gas-analysers-explained.html), [What SIL means for gas analysers, and what it does not](https://www.sarveshanalytics.com/learn/what-sil-means-for-gas-analysers.html), [Hydrogen electrolysers and gas analysis: what to measure](https://www.sarveshanalytics.com/learn/hydrogen-electrolysers-and-gas-analysis.html) **Siloxane** A silicon-containing organic compound found in landfill gas and some biogas that can damage sensors and engines. **Span** The difference between the top and bottom of an analyser's range. A span error scales readings; compare zero error. Explained in: [Accuracy, precision, repeatability and drift in gas analysers](https://www.sarveshanalytics.com/learn/accuracy-precision-repeatability-and-drift.html), [How gas analyser calibration works: zero and span](https://www.sarveshanalytics.com/learn/how-gas-analyser-calibration-works.html) **Span drift** A change over time in the scale of an analyser's response. **Span gas** Gas of known concentration near the top of the range, used to set the analyser's gain during calibration. Explained in: [What is an NDIR gas analyser?](https://www.sarveshanalytics.com/learn/what-is-an-ndir-gas-analyser.html), [Online vs laboratory vs portable gas analysis](https://www.sarveshanalytics.com/learn/online-vs-laboratory-vs-portable-gas-analysis.html), [Sample system troubleshooting: symptoms to causes](https://www.sarveshanalytics.com/learn/sample-system-troubleshooting.html) **Stoichiometric** Exactly the proportions of fuel and air needed for complete combustion. Explained in: [Combustion, excess air and flue gas oxygen explained](https://www.sarveshanalytics.com/learn/combustion-excess-air-and-flue-gas-oxygen.html) **Stratification** Layering of a gas stream so that composition or temperature differs across a duct: [Choosing the sample point for a gas analyser](https://www.sarveshanalytics.com/learn/choosing-the-sample-point.html). Explained in: [Choosing the sample point for a gas analyser](https://www.sarveshanalytics.com/learn/choosing-the-sample-point.html) **Syngas** Synthesis gas, a mixture mainly of carbon monoxide and hydrogen made by gasification. Explained in: [Steel plant gases: coke oven, blast furnace, converter](https://www.sarveshanalytics.com/learn/steel-plant-gases-explained.html) ## T **TCD (thermal conductivity detector)** A sensor that measures a gas by how well it carries heat away from a heated element. Explained in: [Gas analysis technologies compared: which sensor for which gas](https://www.sarveshanalytics.com/learn/gas-analysis-technologies-compared.html), [How thermal conductivity gas analysers (TCD) work](https://www.sarveshanalytics.com/learn/how-thermal-conductivity-gas-analysers-work.html), [Measuring hydrogen: purity, mixtures and safety limits](https://www.sarveshanalytics.com/learn/measuring-hydrogen-purity-and-hydrogen-in-gas-mixtures.html) **TDLAS** Tunable diode laser absorption spectroscopy: measuring a gas by laser light absorbed on one narrow line. Not in the Sarvesh range. Explained in: [Gas analysis technologies compared: which sensor for which gas](https://www.sarveshanalytics.com/learn/gas-analysis-technologies-compared.html) **Temperature class** A marking, T1 to T6, giving the maximum surface temperature of hazardous-area equipment, from 450 C to 85 C. Explained in: [Hazardous area classification: zones, gas groups, T-Classes](https://www.sarveshanalytics.com/learn/hazardous-area-classification-zones-gas-groups-temperature-classes.html) **Trace measurement** Measurement at low concentration, typically ppm. Explained in: [What is dew point and how is it measured?](https://www.sarveshanalytics.com/learn/what-is-dew-point-and-how-is-it-measured.html), [Choosing an oxygen analyser: trace, percent or purity](https://www.sarveshanalytics.com/learn/choosing-an-oxygen-analyser-trace-percent-or-purity.html), [ppm, percent and ppb explained for gas analysis](https://www.sarveshanalytics.com/learn/ppm-percent-and-ppb-units-explained.html) **Traceability** The unbroken chain of comparisons linking a measurement to a national or international standard. Explained in: [Calibration gases: choosing, handling and certificates](https://www.sarveshanalytics.com/learn/calibration-gases-choosing-and-handling.html), [How often should you calibrate a gas analyser?](https://www.sarveshanalytics.com/learn/how-often-to-calibrate-a-gas-analyser.html), [Records, audit trails and qualification for regulated plants](https://www.sarveshanalytics.com/learn/records-audit-trails-and-qualification-for-regulated-plants.html) ## U **UEL** Upper explosive limit: the highest concentration of a flammable gas in air that can still burn. About 75 % for hydrogen. Explained in: [Flammability, explosive limits and gas safety basics](https://www.sarveshanalytics.com/learn/flammability-explosive-limits-and-gas-safety-basics.html) **ULO (ultra-low oxygen)** A controlled-atmosphere regime that holds oxygen at around 1 to 2 % or lower. **Uncertainty** A stated range within which the true value is expected to lie, given on calibration certificates. ## V **Validation** Checking, with a known gas, that an analyser reads correctly, without necessarily adjusting it: [Auto-Calibration and validation checks for gas analysers](https://www.sarveshanalytics.com/learn/auto-calibration-and-validation-checks.html). Explained in: [Records, audit trails and qualification for regulated plants](https://www.sarveshanalytics.com/learn/records-audit-trails-and-qualification-for-regulated-plants.html) **Vaporiser** A heated device that turns a liquid or cold sample into gas of the same composition. Explained in: [Sampling cryogenic and liquefied gases: vaporisers](https://www.sarveshanalytics.com/learn/sampling-cryogenic-and-liquefied-gases.html), [How an air separation plant works and what is measured](https://www.sarveshanalytics.com/learn/how-an-air-separation-plant-works.html) **Vent** The outlet where a sample leaves an analyser, which must be safe and have steady low back pressure. Explained in: [Sample pressure, flow and fast bypass loops](https://www.sarveshanalytics.com/learn/sample-pressure-flow-and-bypass-loops.html), [Site utilities checklist for gas analysers](https://www.sarveshanalytics.com/learn/site-utilities-checklist-for-analysers.html), [Inerting and purging: how they work and how to prove them](https://www.sarveshanalytics.com/learn/inerting-and-purging-how-they-work.html) **Volume fraction** The share of a mixture's volume taken by a component. For gases, equal to mole fraction. Explained in: [ppm, percent and ppb explained for gas analysis](https://www.sarveshanalytics.com/learn/ppm-percent-and-ppb-units-explained.html) ## W **Warm-up time** The time after power-up for a sensor or heater to stabilise before readings are reliable. **Wet basis** A gas value stated with water vapour counted as part of the gas: [Dry basis vs wet basis in gas measurement](https://www.sarveshanalytics.com/learn/dry-basis-vs-wet-basis-gas-measurement.html). Explained in: [Dry basis vs wet basis in gas measurement](https://www.sarveshanalytics.com/learn/dry-basis-vs-wet-basis-gas-measurement.html) ## Z **Zeolite** A porous mineral used as an adsorbent, for example in oxygen PSA generators, where it adsorbs nitrogen. **Zero** The analyser reading with no target gas present. A zero error shifts every reading equally. Explained in: [Accuracy, precision, repeatability and drift in gas analysers](https://www.sarveshanalytics.com/learn/accuracy-precision-repeatability-and-drift.html), [How gas analyser calibration works: zero and span](https://www.sarveshanalytics.com/learn/how-gas-analyser-calibration-works.html) **Zero drift** A change over time in the analyser's reading at zero gas. **Zero gas** Gas free of the target component, used to set the analyser's zero during calibration. Explained in: [What is an NDIR gas analyser?](https://www.sarveshanalytics.com/learn/what-is-an-ndir-gas-analyser.html), [Sample system troubleshooting: symptoms to causes](https://www.sarveshanalytics.com/learn/sample-system-troubleshooting.html), [Commissioning an online gas analyser: A Step-by-Step guide](https://www.sarveshanalytics.com/learn/commissioning-an-online-gas-analyser.html) **Zirconia sensor** A heated ceramic sensor that measures oxygen from the voltage generated between two sides of a zirconia element. Explained in: [How a zirconia oxygen analyser works](https://www.sarveshanalytics.com/learn/how-a-zirconia-oxygen-analyser-works.html), [Electrochemical vs paramagnetic oxygen sensors](https://www.sarveshanalytics.com/learn/electrochemical-vs-paramagnetic-oxygen-sensors.html), [Gas analysis technologies compared: which sensor for which gas](https://www.sarveshanalytics.com/learn/gas-analysis-technologies-compared.html) **Zone 1 / Zone 2** Hazardous-area zones where flammable gas may occur occasionally (Zone 1) or is not likely in normal operation (Zone 2). Explained in: [Hazardous area gas analysers explained](https://www.sarveshanalytics.com/learn/hazardous-area-gas-analysers-explained.html), [Where to mount a gas analyser: panel, shelter or field](https://www.sarveshanalytics.com/learn/where-to-mount-a-gas-analyser.html), [Hazardous area classification: zones, gas groups, T-Classes](https://www.sarveshanalytics.com/learn/hazardous-area-classification-zones-gas-groups-temperature-classes.html)