SARVESH ANALYTICSANALYSIS SIMPLIFIED
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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.
Explained in: Analyser outputs: 4-20 mA, relays and Modbus explained, Wiring, grounding and electrical noise for gas analysers

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
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.
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.
Explained in: Connecting a gas analyser to PLC, DCS and SCADA
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
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, Online vs laboratory vs portable gas analysis
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.
Explained in: Where to mount a gas analyser: panel, shelter or field
Asphyxiant
A gas, such as nitrogen or argon, that harms by displacing oxygen rather than by being toxic: Oxygen enrichment and oxygen deficiency hazards.
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
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
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.
Explained in: Records, audit trails and qualification for regulated plants
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, Total cost of ownership of a gas analyser
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.
Explained in: Calibration gases: choosing, handling and certificates
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
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, Measuring hydrogen: purity, mixtures and safety limits, Steel plant gases: coke oven, blast furnace, converter
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.
Explained in: Inert gas blanketing in pharma and chemical plants
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.
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, Filters for gas analysers: dust, mist, membrane, Probes and blowback for hot, dusty and tarry gas
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.
Explained in: Sample pressure, flow and fast bypass loops

C

Calibration
Adjusting an analyser against gas of known concentration so its reading is correct.
Explained in: Why ambient pressure affects oxygen purity readings, What is online gas analysis? A plain introduction, Online vs laboratory vs portable gas analysis
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.
Explained in: How gas analyser calibration works: zero and span, How often should you calibrate a gas analyser?, Auto-Calibration and validation checks for gas analysers
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.
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, Biogas, landfill gas and digesters explained
CEMS
Continuous emission monitoring system: equipment that measures stack gases and reports them at regulatory conditions: Emission monitoring (CEMS) basics: what a system does.
Explained in: Emission monitoring (CEMS) basics: what a system does
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.
Explained in: Moisture measurement methods compared for dry gas
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.
Coalescing filter
A filter that merges fine liquid droplets into larger ones that drain away.
Explained in: Filters for gas analysers: dust, mist, membrane
Coke oven gas
A by-product gas of coke making, rich in hydrogen and methane: Steel plant gases: coke oven, blast furnace, converter.
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.
Controlled atmosphere (CA)
Storage with oxygen and CO2 held at set levels to slow ripening: Controlled-Atmosphere fruit storage explained.
Explained in: Controlled-Atmosphere fruit storage explained
Converter gas
A CO-rich by-product gas of oxygen steelmaking, recovered when its composition allows: Steel plant gases: coke oven, blast furnace, converter.
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, How an air separation plant works and what is measured

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
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, Trace oxygen measurement and air ingress: ppm done right, Choosing the sample point for a gas analyser
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
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?, ppm, percent and ppb explained for gas analysis, Dry basis vs wet basis in gas measurement
Digester
A sealed vessel in which organic matter is broken down without air to produce biogas: Biogas, landfill gas and digesters explained.
Drain
A route for condensate to leave a sample system, which must be continuous and safe: Condensate, moisture and sample cooling for gas analysis.
Explained in: Site utilities checklist for gas analysers
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, Glove boxes and Controlled-Atmosphere chambers explained

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, Gas analysis technologies compared: which sensor for which gas, Cross-Sensitivity and interference in gas analysers
Electrolyser
A device that splits water into hydrogen and oxygen using electricity: Hydrogen electrolysers and gas analysis: what to measure.
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
Endothermic gas
A furnace atmosphere made by partly burning natural gas over a catalyst, mostly CO, hydrogen and nitrogen: Heat treatment furnace atmospheres explained.
Explained in: Heat treatment furnace atmospheres explained
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.
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.
Explained in: Combustion, excess air and flue gas oxygen explained
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, What is online gas analysis? A plain introduction

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)?, How to read a gas analyser datasheet, Analyser outputs: 4-20 mA, relays and Modbus explained
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, Filters for gas analysers: dust, mist, membrane, Maintenance schedule and spares for gas analysers
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, Protection concepts: flameproof, intrinsic safety, purging, Inert gas blanketing in pharma and chemical plants
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.
Explained in: Sampling cryogenic and liquefied gases: vaporisers
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

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
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
Gas group IIC
The most demanding hazardous-area gas group, covering hydrogen and acetylene.
Explained in: Hazardous area gas analysers explained, Measuring hydrogen: purity, mixtures and safety limits, Flammability, explosive limits and gas safety basics
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.
Explained in: Wiring, grounding and electrical noise for gas analysers

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.
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, Condensate, moisture and sample cooling for gas analysis, Probes and blowback for hot, dusty and tarry gas
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.
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
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

I

IECEx
An international scheme for certifying equipment for explosive atmospheres.
Explained in: Hazardous area classification: zones, gas groups, T-Classes
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, Extractive vs In-Situ gas analysis, What is online gas analysis? A plain introduction
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)?, Flammability, explosive limits and gas safety basics, Inerting and purging: how they work and how to prove them
Interference
An error in a reading caused by another gas or condition in the sample.
Explained in: Cross-Sensitivity and interference in gas analysers
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.
Explained in: Protection concepts: flameproof, intrinsic safety, purging

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.
Explained in: Combustion, excess air and flue gas oxygen explained
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.
Explained in: Measuring hydrogen: purity, mixtures and safety limits, Flammability, explosive limits and gas safety basics, Hydrogen electrolysers and gas analysis: what to measure
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)?, Inerting and purging: how they work and how to prove them, Inert gas blanketing in pharma and chemical plants
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
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

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.
Explained in: Filters for gas analysers: dust, mist, membrane
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, Analyser outputs: 4-20 mA, relays and Modbus explained, Connecting a gas analyser to PLC, DCS and SCADA
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
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
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?, Gas analysis technologies compared: which sensor for which gas, Cross-Sensitivity and interference in gas analysers
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.
Explained in: ppm to mg/Nm3: volume vs mass concentration

O

Oxygen deficiency
An atmosphere with less oxygen than normal, commonly below about 19.5 %, which endangers life: Oxygen enrichment and oxygen deficiency hazards.
Explained in: Oxygen enrichment and oxygen deficiency hazards, Controlled-Atmosphere fruit storage explained
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

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, Gas analysis technologies compared: which sensor for which gas
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.
Explained in: How gases behave: pressure, temperature and partial pressure
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, Dry basis vs wet basis in gas measurement, Condensate, moisture and sample cooling for gas analysis
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
PLC
Programmable logic controller: a small industrial computer that runs control logic.
Explained in: Connecting a gas analyser to PLC, DCS and SCADA
Poisoning (sensor)
Loss of sensor performance caused by a contaminant, such as sulphur, chlorine or silicone: Sensor life, poisoning and replacement in gas analysers.
Explained in: Sensor life, poisoning and replacement in gas analysers
ppb
Parts per billion by volume: one part in a billion. 1000 ppb is 1 ppm: ppm, percent and ppb explained for gas analysis.
Explained in: ppm, percent and ppb explained for gas analysis
Precision
How closely repeated readings of the same gas agree with each other.
Explained in: Accuracy, precision, repeatability and drift in gas analysers
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).
Explained in: Compressed air quality and dew point (ISO 8573-1)
Pressure regulator
A device that reduces and holds a steady downstream pressure.
Explained in: Sample pressure, flow and fast bypass loops
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, Choosing the sample point for a gas analyser, Probes and blowback for hot, dusty and tarry gas
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
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
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
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, Choosing an oxygen analyser: trace, percent or purity, How an air separation plant works and what is measured

R

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, Emission monitoring (CEMS) basics: what a system does
Reference gas
A second gas supplied to some analysers for comparison. ACCURRA-TCD needs none.
Explained in: How a zirconia oxygen analyser works, How thermal conductivity gas analysers (TCD) work
Repeatability
The agreement between repeated readings of the same gas under the same conditions.
Explained in: Accuracy, precision, repeatability and drift in gas analysers
Resolution
The smallest change an instrument can display or output.
Explained in: Accuracy, precision, repeatability and drift in gas analysers
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
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, How to read a gas analyser datasheet
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, Controlled-Atmosphere fruit storage explained
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

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.
Explained in: What SIL means for gas analysers, and what it does not
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, How to design a gas sample handling system, What is online gas analysis? A plain introduction
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.
Explained in: Response time, T90 and sample lag in gas analysis, Sample lines: materials, length, diameter and lag
Sample point (tapping)
The connection on the process from which gas is taken for analysis: Choosing the sample point for a gas analyser.
Explained in: Choosing the sample point for a gas analyser
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
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, What samples an analyser cannot take: know the limits
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?, Response time, T90 and sample lag in gas analysis, Moisture measurement methods compared for dry gas
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
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, What SIL means for gas analysers, and what it does not, Hydrogen electrolysers and gas analysis: what to measure
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, How gas analyser calibration works: zero and span
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?, Online vs laboratory vs portable gas analysis, Sample system troubleshooting: symptoms to causes
Stoichiometric
Exactly the proportions of fuel and air needed for complete combustion.
Explained in: Combustion, excess air and flue gas oxygen explained
Stratification
Layering of a gas stream so that composition or temperature differs across a duct: Choosing the sample point for a gas analyser.
Explained in: Choosing the sample point for a gas analyser
Syngas
Synthesis gas, a mixture mainly of carbon monoxide and hydrogen made by gasification.
Explained in: Steel plant gases: coke oven, blast furnace, converter

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, How thermal conductivity gas analysers (TCD) work, Measuring hydrogen: purity, mixtures and safety limits
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
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
Traceability
The unbroken chain of comparisons linking a measurement to a national or international standard.
Explained in: Calibration gases: choosing, handling and certificates, How often should you calibrate a gas analyser?, Records, audit trails and qualification for regulated plants

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
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.
Explained in: Records, audit trails and qualification for regulated plants
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, How an air separation plant works and what is measured
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, Site utilities checklist for gas analysers, Inerting and purging: how they work and how to prove them
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

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.
Explained in: Dry basis vs wet basis in gas measurement

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, How gas analyser calibration works: zero and span
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?, Sample system troubleshooting: symptoms to causes, Commissioning an online gas analyser: A Step-by-Step guide
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, Electrochemical vs paramagnetic oxygen sensors, Gas analysis technologies compared: which sensor for which gas
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, Where to mount a gas analyser: panel, shelter or field, Hazardous area classification: zones, gas groups, T-Classes
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