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The practical guide to online gas analysis and sampling.
75 lessons, 7 calculators and a 170-term glossary, written by the engineers who build the analysers. Start with a learning path, browse by track, or search for the question you have.
Start here
Pick the path that matches your job.
Each path is a short, ordered set of lessons with the reason for every step.
10 lessons · 32 min
New to gas analysis
Engineers, buyers and students meeting process gas analysis for the first time
Start this path12 lessons · 41 min
Specify an analyser
Process, instrumentation and procurement engineers writing or reviewing a specification
Start this path9 lessons · 28 min
Keep analysers running
Instrument technicians, maintenance planners and plant operators
Start this path8 lessons · 25 min
Safety and hazardous areas
Safety engineers, project engineers and anyone installing analysers where gas can burn or explode
Start this path10 lessons · 33 min
Design a sample system
Instrument and process engineers designing or fixing a sample line
Start this pathBrowse by track
Eight topics, from first principles to the purchase order.
From what a number on an analyser means, to the sample system that feeds it, to how it is certified, run and bought.
Track 1 · 9 lessons
Foundations
What a number on an analyser really means: units, gas behaviour, accuracy and speed.
Read: 0 of 9 lessons read
Open the trackTrack 2 · 15 lessons
Measurement principles
How each technology works, where it fails, and how each common gas is measured.
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Open the trackTrack 3 · 10 lessons
Sampling and conditioning
Getting a representative, clean, dry sample to the analyser, which decides most field problems.
Read: 0 of 10 lessons read
Open the trackTrack 4 · 6 lessons
Installation and integration
Mounting, signals, wiring, control-system integration and commissioning.
Read: 0 of 6 lessons read
Open the trackTrack 5 · 8 lessons
Calibration and maintenance
Keeping the reading true: calibration, validation, troubleshooting, spares and records.
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Open the trackTrack 6 · 8 lessons
Safety and hazardous areas
Flammability, inerting, hazardous areas, protection concepts, SIL and oxygen hazards.
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Open the trackTrack 7 · 13 lessons
Process primers
How real processes use gas analysis, from boilers to biogas to glove boxes.
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Open the trackTrack 8 · 6 lessons
Choosing and buying
Specifying, accuracy needs, total cost, comparing quotes and supplier questions.
Read: 0 of 6 lessons read
Open the trackCalculators
Do the arithmetic without the spreadsheet.
Each one states its formula and its assumptions.
ppm to mg/m3 and percent gas converter
Percent, ppm and ppb are volume fractions: 1 % equals 10,000 ppm and 1 ppm equals 1000 ppb.
OpenDew point to ppm calculator for gases
Water content in ppm by volume equals the saturation vapour pressure at the dew point divided by the total pressure, times a million.
OpenExcess air from flue gas oxygen calculator
With complete combustion, excess air in percent equals O2 divided by (20.95 minus O2), times 100, using dry flue gas oxygen.
OpenEmission correction to reference oxygen
Emission limits are stated at a reference oxygen level so that diluting the flue gas with extra air cannot hide pollution.
OpenGas sample line lag time calculator
Sample lag equals the gas volume in the line, at line pressure, divided by the flow measured at atmospheric pressure.
OpenPressure effect on oxygen reading calculator
A sensor that responds to oxygen partial pressure reads in proportion to absolute pressure.
OpenInerting purge time and volume calculator
With ideal mixing, oxygen falls exponentially as inert gas is added: the number of vessel volumes needed is the natural log of the starting concentration divided by the target.
OpenEvery lesson
Foundations
- What is online gas analysis? A plain introductionOnline gas analysis measures the composition of a process gas continuously, as it flows, and sends the result to a display, an alarm or a control system.Beginner3 min
- Online vs laboratory vs portable gas analysisUse an online analyser when the answer is needed continuously to control or protect a process.Beginner3 min
- How gases behave: pressure, temperature and partial pressureIn a gas mixture each component contributes a partial pressure equal to its fraction times the total pressure.Beginner3 min
- ppm, percent and ppb explained for gas analysisPercent, ppm and ppb are volume fractions: 1 % equals 10,000 ppm, and 1 ppm equals 1000 ppb.Beginner3 min
- ppm to mg/Nm3: volume vs mass concentrationMass concentration in mg per cubic metre equals ppm times molar mass divided by molar volume, and the molar volume depends on temperature and pressure.Intermediate3 min
- Dry basis vs wet basis in gas measurementA wet-basis reading counts water vapour as part of the gas; a dry-basis reading removes it.Intermediate3 min
- Accuracy, precision, repeatability and drift in gas analysersAccuracy is how close a reading is to the true value; precision or repeatability is how closely repeated readings agree; drift is how the reading moves over time.Intermediate3 min
- Response time, T90 and sample lag in gas analysisT90 is the time an analyser takes to reach 90 % of a step change in concentration.Intermediate3 min
- How to read a gas analyser datasheetRead a datasheet in four groups: what it measures (gas, range, minimum range), how well (accuracy, repeatability, drift, response), what it needs (sample conditions, power, utilities, ambient limits) and what it gives (outputs, approvals, options).Intermediate5 min
Measurement principles
- Gas analysis technologies compared: which sensor for which gasEach gas analysis technology works only on gases with the right physical property: electrochemical and paramagnetic sensors for oxygen, zirconia for hot oxygen, NDIR for gases that absorb infrared (CO, CO2, methane), thermal conductivity for binary mixtures such as hydrogen in nitrogen, ceramic or chilled-mirror sensors for moisture.Beginner3 min
- Electrochemical vs paramagnetic oxygen sensorsElectrochemical oxygen cells suit ppm and percent oxygen, are compact, and are consumed over time.Intermediate4 min
- How a zirconia oxygen analyser worksA zirconia analyser measures oxygen with a heated ceramic cell.Intermediate4 min
- What is an NDIR gas analyser?An NDIR (non-dispersive infrared) analyser shines infrared light through a sample and measures how much a target gas absorbs.Beginner4 min
- How thermal conductivity gas analysers (TCD) workA thermal conductivity analyser heats a small element and measures how quickly the surrounding gas carries heat away.Intermediate4 min
- What is dew point and how is it measured?Dew point is the temperature at which moisture in a gas begins to condense.Beginner4 min
- Moisture measurement methods compared for dry gasOnline dry-gas moisture is usually measured with a ceramic or capacitive sensor that responds to adsorbed water.Intermediate3 min
- Cross-Sensitivity and interference in gas analysersCross-sensitivity is a sensor's response to gases other than its target.Intermediate3 min
- Trace oxygen measurement and air ingress: ppm done rightAir is 209,500 ppm oxygen, so a leak of just 0.01 % air adds about 21 ppm to a sample.Intermediate4 min
- Measuring hydrogen: purity, mixtures and safety limitsHydrogen purity and hydrogen in nitrogen or argon are measured by thermal conductivity, because hydrogen conducts heat about seven times better than most gases.Intermediate3 min
- Measuring carbon monoxide and carbon dioxide onlineCarbon dioxide is measured by NDIR over ppm to 100 % ranges.Intermediate3 min
- Measuring methane and biogas composition onlineMethane and CO2 in biogas are measured by NDIR, separately or in one dual-channel analyser, and oxygen is measured to show air ingress.Intermediate3 min
- Extractive vs In-Situ gas analysisIn-situ analysers measure the gas where it flows, with no sample line, and suit hot, dusty ducts.Beginner3 min
- Choosing an oxygen analyser: trace, percent or purityChoose by range.Beginner3 min
- Why ambient pressure affects oxygen purity readingsMany oxygen sensors respond to the partial pressure of oxygen, which falls when barometric pressure falls even if the gas is unchanged.Intermediate4 min
Sampling and conditioning
- How to design a gas sample handling systemA good sample handling system delivers a representative, clean, dry sample at constant pressure and flow.Intermediate4 min
- Choosing the sample point for a gas analyserTake the sample where the gas is well mixed, at process conditions you understand, upstream of any air leak, away from dead zones, and with a tapping that cannot fill with liquid or dust.Intermediate4 min
- Sample lines: materials, length, diameter and lagUse stainless steel tube for trace oxygen, moisture and reactive gases, because plastic lets oxygen and water in and out.Intermediate4 min
- Filters for gas analysers: dust, mist, membraneUse filtration in stages: a coarse filter or probe filter near the tapping for bulk dust, a finer filter after the cooler, a coalescing filter to remove liquid mist and oil aerosol, and a membrane filter as a final guard.Intermediate3 min
- Condensate, moisture and sample cooling for gas analysisWet samples must be cooled to a fixed, low dew point so that water leaves as condensate at a controlled place, not in the analyser.Intermediate3 min
- Sample pressure, flow and fast bypass loopsAn analyser needs a steady sample at its specified pressure and flow.Intermediate3 min
- Probes and blowback for hot, dusty and tarry gasHot, dusty or tarry gas needs a probe that survives the temperature, a filter at the tip that keeps dust out, and a way to clear the filter, usually automatic blowback with clean air or gas.Intermediate3 min
- Sampling cryogenic and liquefied gases: vaporisersA liquid sample must be vaporised completely before it reaches an analyser.Intermediate3 min
- What samples an analyser cannot take: know the limitsStandard ACCURRA-S analysers expect a non-corrosive, non-toxic, non-flammable, non-condensing and dry sample, free of oil, at 2 to 10 psig and about 1 LPM, with dust below 3 micron.Intermediate3 min
- Sample system troubleshooting: symptoms to causesMost analyser faults are sample system faults.Intermediate3 min
Installation and integration
- Where to mount a gas analyser: panel, shelter or fieldMount an analyser as close to the sample point as practical, in a clean, temperature-stable, accessible place, out of vibration, direct sun and weather.Beginner3 min
- Analyser outputs: 4-20 mA, relays and Modbus explainedA 4-20 mA output sends the reading as a current, with 4 mA for the bottom of the range and 20 mA for the top; the live zero lets the system detect a broken wire.Beginner3 min
- Wiring, grounding and electrical noise for gas analysersUse shielded twisted-pair cable for signals, ground the shield at one end, keep signal cables away from power and motor cables, and avoid ground loops by giving the loop a single reference.Intermediate3 min
- Connecting a gas analyser to PLC, DCS and SCADAScale the 4-20 mA input to the analyser range, alarm on out-of-range currents as a fault, and use the analyser's fault and calibration-in-progress signals to hold or flag the value.Intermediate3 min
- Site utilities checklist for gas analysersAn analyser installation needs more than a power connection: a clean, stable electrical supply, instrument air or nitrogen for blowback and purges, a calibration gas supply with regulators, a safe vent, a drain for condensate, and in some cases heating, cooling or ventilation for the enclosure.Beginner3 min
- Commissioning an online gas analyser: A Step-by-Step guideCommission in order: inspect and verify the installation, leak-test the sample system, power up, purge with sample or inert gas, set pressure and flow, calibrate with zero and span gas, test alarms and outputs end to end, run on process gas, and record a baseline of flow, pressure and readings.Intermediate3 min
Calibration and maintenance
- How gas analyser calibration works: zero and spanCalibration compares the analyser's reading with a gas of known concentration and adjusts it to match.Beginner4 min
- Calibration gases: choosing, handling and certificatesChoose span gas near 70 to 90 % of the range in a balance gas that matches the sample, with a certificate stating concentration, uncertainty and expiry.Intermediate3 min
- How often should you calibrate a gas analyser?Set the interval from three things: how much error the process can tolerate, how fast the analyser drifts, and what a wrong reading costs.Intermediate3 min
- Auto-Calibration and validation checks for gas analysersAuto-calibration switches zero and span gas to the analyser by solenoid valves on a timer, keypad or Modbus command, and adjusts or records the result.Intermediate3 min
- Troubleshooting gas analyser readings: high, low, noisy, slowApply known gas at the analyser inlet.Intermediate3 min
- Maintenance schedule and spares for gas analysersMaintain analysers by routine: a daily look at flow, pressure and condensate; a weekly check of filters and drain; a monthly calibration check; and an annual full calibration, sensor check and leak test.Intermediate3 min
- Sensor life, poisoning and replacement in gas analysersElectrochemical cells are consumed by the gas they measure and last months to years depending on exposure.Intermediate3 min
- Records, audit trails and qualification for regulated plantsRegulated plants must show that an analyser was fit for purpose and stayed so: traceable calibration records, alarm and fault history, controlled access to settings, change control, and documented qualification (installation, operation and performance).Advanced3 min
Safety and hazardous areas
- Flammability, explosive limits and gas safety basicsA flammable gas burns only when fuel, oxygen and an ignition source are present and the fuel-air mixture lies between the lower and upper explosive limits (LEL and UEL).Beginner3 min
- What is limiting oxygen concentration (LOC)?The limiting oxygen concentration (LOC) is the oxygen level below which a flammable vapour or dust cannot sustain a flame.Beginner3 min
- Inerting and purging: how they work and how to prove themInerting keeps a vessel's oxygen below the limiting oxygen concentration by filling it with nitrogen or another inert gas.Intermediate3 min
- Hazardous area classification: zones, gas groups, T-ClassesArea classification says how likely an explosive atmosphere is (Zone 0, 1 or 2 for gas), what gas it could be (group IIA, IIB or IIC) and how hot equipment may get (T1 to T6, 450 to 85 C).Intermediate3 min
- Hazardous area gas analysers explainedA hazardous area analyser is built so it cannot ignite flammable gas.Intermediate4 min
- Protection concepts: flameproof, intrinsic safety, purgingHazardous-area equipment avoids ignition in one of three ways: contain an explosion (flameproof, Ex d), prevent sparks and heat (increased safety Ex e, non-sparking Ex n or ec, encapsulation Ex m), or limit energy (intrinsic safety Ex i), or keep the gas out (pressurised or purged enclosures, Ex p).Advanced3 min
- What SIL means for gas analysers, and what it does notA safety integrity level (SIL 1 to 4) states how reliably a safety instrumented function does its job on demand.Advanced3 min
- Oxygen enrichment and oxygen deficiency hazardsToo much oxygen makes materials ignite more easily and burn fiercely; above about 23.5 % is commonly treated as oxygen-enriched.Beginner3 min
Process primers
- Combustion, excess air and flue gas oxygen explainedBurners need slightly more air than the exact amount that burns the fuel, so that combustion is complete.Beginner3 min
- How an air separation plant works and what is measuredAn air separation unit compresses air, removes water and CO2, cools it until it liquefies and distils the liquid into oxygen, nitrogen and argon, which boil at different temperatures.Beginner3 min
- PSA, VPSA and membrane gas generators explainedPSA nitrogen generators use carbon molecular sieve to adsorb oxygen from compressed air, giving nitrogen with ppm to percent residual oxygen.Beginner3 min
- Heat treatment furnace atmospheres explainedA furnace atmosphere protects the metal from oxidation or adds carbon to its surface.Intermediate3 min
- Hydrogen electrolysers and gas analysis: what to measureAn electrolyser splits water into hydrogen at one electrode and oxygen at the other.Intermediate3 min
- Biogas, landfill gas and digesters explainedAnaerobic digestion converts organic waste into biogas: methane, CO2, water vapour and traces of H2S.Beginner3 min
- Controlled-Atmosphere fruit storage explainedFruit keeps breathing after harvest, consuming oxygen and releasing CO2.Beginner3 min
- Cement kiln gas analysis explained: O2, CO and safetyA cement kiln burns fuel to heat raw meal to about 1450 C.Intermediate3 min
- Steel plant gases: coke oven, blast furnace, converterIntegrated steelworks make three main by-product fuel gases: coke oven gas, rich in hydrogen and methane; blast furnace gas, lean and mostly CO, CO2 and nitrogen; and converter gas, rich in CO.Intermediate3 min
- Emission monitoring (CEMS) basics: what a system doesA continuous emission monitoring system measures pollutants and oxygen in stack gas and reports concentrations corrected to a reference oxygen and dry basis.Intermediate3 min
- Compressed air quality and dew point (ISO 8573-1)ISO 8573-1 classifies compressed air purity by particles, water and oil.Beginner3 min
- Inert gas blanketing in pharma and chemical plantsBlanketing fills the space above a liquid or powder with nitrogen so that oxygen stays below the limit at which a flame can spread.Intermediate3 min
- Glove boxes and Controlled-Atmosphere chambers explainedA glove box is a sealed chamber filled with nitrogen or argon in which oxygen and moisture are held at very low levels, often through a circulating purifier that removes both.Intermediate3 min
Choosing and buying
- How to specify a gas analyser: A practical checklistA good specification states the question the number must answer, then the measurement (gas, range, accuracy, response), the sample (composition, pressure, temperature, dust, moisture), the site (area classification, ambient limits, utilities), the outputs and standards, and the support needed.Intermediate4 min
- What accuracy do you really need from a gas analyser?Start from the tolerance the process or product can bear, then build an error budget: sensor accuracy, calibration gas uncertainty, drift between calibrations, sample effects and pressure and temperature effects.Intermediate4 min
- Total cost of ownership of a gas analyserThe analyser is often a third or less of the cost of a working measurement.Intermediate3 min
- How to compare gas analyser quotes like for likePut quotes side by side on the same scope: what is included in the sample system, what range and accuracy basis (span or reading), what response time and at what flow, certificates and documents, calibration gas and spares, commissioning and support, and what is excluded.Intermediate3 min
- Questions to ask a gas analyser supplier before you buyAsk about application experience on streams like yours, what the analyser cannot take, how the sample is handled, what certificates and documents come with it, how calibration works, what spares and service you can count on, and what the supplier does not claim.Intermediate3 min
- The application data sheet: what to send SarveshThe application data sheet collects the pre-order design inputs an engineer needs: the measurement points and gases, ranges, sample pressure, temperature and dew point, dust and corrosives, area classification, utilities, outputs and calibration approach.Beginner3 min
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